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1

Crystallization of calcite on chitin  

NASA Astrophysics Data System (ADS)

Chitin, a nitrogen-containing polysaccharide which is a major component of the exoskeletons of arthropods, was found to be a substrate favouring the deposition of calcite crystals from stable supersaturated solutions at pH 8.50 and at 25°C. The crystallization was studied at constant solution supersaturation thus making it possible for relatively large amounts of the overgrowth to be formed and be identified exclusively as calcite. The induction periods varied markedly with supersaturation. The apparent order found from kinetics data was n = 2, thus suggesting a surface diffusion-controlled mechanism.

Manoli, F.; Koutsopoulos, S.; Dalas, E.

1997-12-01

2

Crystallization of calcite on chitin  

Microsoft Academic Search

Chitin, a nitrogen-containing polysaccharide which is a major component of the exoskeletons of arthropods, was found to be a substrate favouring the deposition of calcite crystals from stable supersaturated solutions at pH 8.50 and at 25°C. The crystallization was studied at constant solution supersaturation thus making it possible for relatively large amounts of the overgrowth to be formed and be

F. Manoli; S. Koutsopoulos; E. Dalas

1997-01-01

3

Pressure effect on magnesian calcite coating calcite and synthetic magnesian calcite seeds added to seawater in a closed system  

Microsoft Academic Search

When pure crystalline calcite seeds are added to supersaturated seawater, precipitation results in a coating which with time equilibrates at atmospheric pressure with seawater and corresponds to a calcite containing probably only 2 or 3% of MgCO 3 (mole fraction). If synthetic crystalline magnesian calcite is added, the surface layer equilibrates not only with respect to seawater but also in

B. Koch; A. Disteche

1984-01-01

4

Crystal growth of calcite from calcium bicarbonate solutions at constant PCO2 and 25??C: a test of a calcite dissolution model  

USGS Publications Warehouse

A highly reproducible seeded growth technique was used to study calcite crystallization from calcium bicarbonate solutions at 25??C and fixed carbon dioxide partial pressures between 0.03 and 0.3 atm. The results are not consistent with empirical crystallization models that have successfully described calcite growth at low PCO2 (< 10-3 atm). Good agreement was found between observed crystallization rates and those calculated from the calcite dissolution rate law and mechanism proposed by Plummer et al. (1978). ?? 1981.

Reddy, M. M.; Plummer, L. N.; Busenberg, E.

1981-01-01

5

Interactions of salicylic acid derivatives with calcite crystals.  

PubMed

Investigation of basic interactions between the active pharmaceutical compounds and calcium carbonates is of great importance because of the possibility to use the carbonates as a mineral carrier in drug delivery systems. In this study the mode and extent of interactions of salicylic acid and its amino acid derivates, chosen as pharmaceutically relevant model compounds, with calcite crystals are described. Therefore, the crystal growth kinetics of well defined rhombohedral calcite seed crystals in the systems containing salicylic acid (SA), 5-amino salicylic acid (5-ASA), N-salicyloil-l-aspartic acid (N-Sal-Asp) or N-salicyloil-l-glutamic acid (N-Sal-Glu), were investigated. The precipitation systems were of relatively low initial supersaturation and of apparently neutral pH. The data on the crystal growth rate reductions in the presence of the applied salicylate molecules were analyzed by means of Cabrera & Vermileya's, and Kubota & Mullin's models of interactions of the dissolved additives and crystal surfaces. The crystal growth kinetic experiments were additionally supported with the appropriate electrokinetic, spectroscopic and adsorption measurements. The Langmuir adsorption constants were determined and they were found to be in a good correlation with values obtained from crystal growth kinetic analyses. The results indicated that salicylate molecules preferentially adsorb along the steps on the growing calcite surfaces. The values of average spacing between the adjacent salicylate adsorption active sites and the average distance between the neighboring adsorbed salicylate molecules were also estimated. PMID:21963207

Ukrainczyk, Marko; Gredi?ak, Matija; Jeri?, Ivanka; Kralj, Damir

2011-09-12

6

Calcite Crystal Growth Rate Inhibition by Polycarboxylic Acids  

Microsoft Academic Search

Calcite crystal growth rates measured in the presence of several polycarboxyclic acids show that tetrahydrofurantetracarboxylic acid (THFTCA) and cyclopentanetetracarboxylic acid (CPTCA) are effective growth rate inhibitors at low solution concentrations (0.01 to 1 mg\\/L). In contrast, linear polycarbocylic acids (citric acid and tricarballylic acid) had no inhibiting effect on calcite growth rates at concentrations up to 10 mg\\/L. Calcite crystal

Michael M. Reddy; Anthony R. Hoch

2001-01-01

7

Growth of Vaterite and Calcite Crystals in Gels.  

National Technical Information Service (NTIS)

Crystals of vaterite and calcite have been simultaneously grown in sodium metasilicate-acetic acid gels. The crystals were characterized by x-ray diffraction and infrared spectroscopy. The growth ratios of vaterite and calcite are functions of gel pH valu...

A. Schwartz D. Eckart J. O'Connell K. Francis

1971-01-01

8

Calcite crystallization in the presence of europium: Preliminary results  

Microsoft Academic Search

Ubiquitous calcite is found in natural abiogenic and biogenic environments. Europium radionuclide is enlisted as a rare Earth element and frequently occurs in natural samples of calcite. In this study two complementary crystallization methods are tested to investigate possible conditions of europium uptake by calcite. Counter-diffusion-reaction of (Ca2+, Eu3+) and (Na+, CO32-) ions through a porous silica gel (Prieto et

Aikaterini Vavouraki; Ángeles Fernández-González; Manuel Prieto

2010-01-01

9

Calcite crystal growth rate inhibition by polycarboxylic acids  

USGS Publications Warehouse

Calcite crystal growth rates measured in the presence of several polycarboxyclic acids show that tetrahydrofurantetracarboxylic acid (THFTCA) and cyclopentanetetracarboxylic acid (CPTCA) are effective growth rate inhibitors at low solution concentrations (0.01 to 1 mg/L). In contrast, linear polycarbocylic acids (citric acid and tricarballylic acid) had no inhibiting effect on calcite growth rates at concentrations up to 10 mg/L. Calcite crystal growth rate inhibition by cyclic polycarboxyclic acids appears to involve blockage of crystal growth sites on the mineral surface by several carboxylate groups. Growth morphology varied for growth in the absence and in the presence of both THFTCA and CPTCA. More effective growth rate reduction by CPTCA relative to THFTCA suggests that inhibitor carboxylate stereochemical orientation controls calcite surface interaction with carboxylate inhibitors. ?? 20O1 Academic Press.

Reddy, M. M.; Hoch, A. R.

2001-01-01

10

Calcite crystallization in the presence of europium: Preliminary results  

NASA Astrophysics Data System (ADS)

Ubiquitous calcite is found in natural abiogenic and biogenic environments. Europium radionuclide is enlisted as a rare Earth element and frequently occurs in natural samples of calcite. In this study two complementary crystallization methods are tested to investigate possible conditions of europium uptake by calcite. Counter-diffusion-reaction of (Ca2+, Eu3+) and (Na+, CO32-) ions through a porous silica gel (Prieto et al., 1997) was used to examine possible Ca-Eu-Na carbonate solid solution formation. A variety of calcium, carbonate and europium aqueous solution compositions were prepared at high supersaturation with respect to calcite. Crystallization in gel media enables to improve crystal size and facilitates with its characterization. Here, polymorphs of calcium carbonate were obtained in the presence of europium in one month duration gel experiments. An alternative quick method of crystallizing calcium carbonate is the method of vapour diffusion on a crystallization mushroom (Hernández-Hernández et al., 2008). Ammonium bicarbonate solution releases NH3(g) and CO2(g) and in a sitting drop of calcium chloride, nucleation of calcium carbonate is observed. In both diffusion processes calcite crystals and its polymorphs of aragonite and vaterite were obtained and analyzed by scanning electron microscopy (SEM-EDS) and microprobe (EMP). Crystals exhibited a wide variety of forms as a function of the europium mole fraction (XEu,aq.) in the aqueous solution. At XEu,aq. = 0.02 trace of europium was detected into solid calcite crystal structure. This study examines calcite as a potential host mineral for radionuclide deposits such as europium. References Prieto, M. et al., 1997. Geochim. Cosmochim. Acta 61, 3383-3397. Hernández-Hernández, A. et al., 2008. J. Crystal Growth 310, 1754-1759.

Vavouraki, Aikaterini; Fernández-González, Ángeles; Prieto, Manuel

2010-05-01

11

Ultrasonic reactivation of phosphonate poisoned calcite during crystal growth.  

PubMed

The effect of ultrasonic irradiation (42,150 Hz, 17 W dm(-3)/7.1 W cm(-2)) on the growth of calcite in the presence of the inhibitor nitrilotris(methylene phosphonic acid) (NTMP) was investigated at constant composition conditions. In seeded growth experiments, it was found that the inhibiting effect of NTMP on crystal growth could be seriously mitigated under influence of ultrasonic irradiation. An approximately twofold increase in volumetric growth rate was achieved during ultrasonic irradiation, and recovery of the growth rate following inhibition was strongly enhanced compared to growth experiments without ultrasonic irradiation. The results could be explained in part by the physical effect of ultrasound that causes breakage and attrition of poisoned crystals, which resulted in an increase in fresh surface area. Mass spectroscopy analysis of sonicated NTMP solutions revealed that there is also a chemical effect of ultrasound that plays an important role. Several breakdown products were identified, which showed that ultrasound caused the progressive loss of phosphonate groups from NTMP, probably by means of physicochemically generated free radicals and/or pyrolysis in the hot bubble-bulk interface. PMID:21463963

Boels, L; Wagterveld, R M; Witkamp, G J

2011-04-03

12

Nanoripples formation in calcite and indium phosphide (InP) single crystals  

Microsoft Academic Search

In this project we studied the formation of nanoripples in calcite and InP single crystals by continuous scanning using the nanoindenter in the ambient environment and by Argon ion irradiation under ultra high vacuum conditions, respectively. Formation of tip induced nanowear ripples is studied on a freshly cleaved calcite single crystal as a function of scanning frequency and contact load

Ramakrishna Gunda

2007-01-01

13

Laser Seeding for Biomolecular Crystallization  

NASA Astrophysics Data System (ADS)

Seeding is a powerful tool for biomolecular crystal growth because it permits the decoupling of crystal nucleation and growth. However, its usefulness is limited by the inability of current techniques to manipulate single seeds and it is often considered as a last resort in protein crystal growth trials. Here, a novel seeding technique is described in which a laser beam is used to select and transfer microscopic seed crystals in a growth solution. The technique permits the non-mechanical, in situ manipulation of individual seed crystals as small as one micron. Coupled with a conventional microscope the laser seeding procedure is monitored in real time under high magnification. Crystals of tomato bushy stunt virus have been successfully seeded and grown in test trials.

Bancel, P. A.; Cajipe, V. B.; Rodier, F.; Witz, J.

1998-03-01

14

Evaluation of strengthening mechanisms in calcite single crystals from mollusk shells.  

PubMed

Biogenic single-crystal calcite is often reported to be harder and tougher than geologic calcite in the form of Iceland spar. However, the mechanistic origins of the superior mechanical properties of the biogenic materials are still debated. We investigate the hardness and modulus of biogenic calcite from the prismatic layer of the mollusk Atrina rigida compared with a pure geologic calcite, Iceland spar. On the {001} face, biogenic calcite is found to be 50-70% harder than geologic calcite. This range is due to the fact that changes in azimuthal angle of the indenter tip lead to a hardness variation of ?20% in A. rigida but only ?7% in Iceland spar. The higher hardness and increased anisotropy of biogenic calcite could be accounted for by hardening mechanisms based on hindered dislocation motion rather than crack deflection. PMID:23036948

Kunitake, Miki E; Mangano, Lauren M; Peloquin, John M; Baker, Shefford P; Estroff, Lara A

2012-10-02

15

Geobacillus thermoglucosidasius endospores function as nuclei for the formation of single calcite crystals.  

PubMed

Geobacillus thermoglucosidasius colonies were placed on an agar hydrogel containing acetate, calcium ions, and magnesium ions, resulting in the formation of single calcite crystals (calcites) within and peripheral to the plating area or parent colony. Microscopic observation of purified calcites placed on the surface of soybean casein digest (SCD) nutrient medium revealed interior crevices from which bacterial colonies originated. Calcites formed on the gel contained [1-(13)C]- and [2-(13)C]acetate, demonstrating that G. thermoglucosidasius utilizes carbon derived from acetate for calcite formation. During calcite formation, vegetative cells swam away from the parent colony in the hydrogel. Hard-agar hydrogel inhibited the formation of calcites peripheral to the parent colony. The calcite dissolved completely in 1 M HCl, with production of bubbles, and the remaining endospore-like particles were easily stained with Brilliant green dye. The presence of DNA and protein in calcites was demonstrated by electrophoresis. We propose that endospores initiate the nucleation of calcites. Endospores of G. thermoglucosidasius remain alive and encapsulated in calcites. PMID:23455343

Murai, Rie; Yoshida, Naoto

2013-03-01

16

Geobacillus thermoglucosidasius Endospores Function as Nuclei for the Formation of Single Calcite Crystals  

PubMed Central

Geobacillus thermoglucosidasius colonies were placed on an agar hydrogel containing acetate, calcium ions, and magnesium ions, resulting in the formation of single calcite crystals (calcites) within and peripheral to the plating area or parent colony. Microscopic observation of purified calcites placed on the surface of soybean casein digest (SCD) nutrient medium revealed interior crevices from which bacterial colonies originated. Calcites formed on the gel contained [1-13C]- and [2-13C]acetate, demonstrating that G. thermoglucosidasius utilizes carbon derived from acetate for calcite formation. During calcite formation, vegetative cells swam away from the parent colony in the hydrogel. Hard-agar hydrogel inhibited the formation of calcites peripheral to the parent colony. The calcite dissolved completely in 1 M HCl, with production of bubbles, and the remaining endospore-like particles were easily stained with Brilliant green dye. The presence of DNA and protein in calcites was demonstrated by electrophoresis. We propose that endospores initiate the nucleation of calcites. Endospores of G. thermoglucosidasius remain alive and encapsulated in calcites.

Murai, Rie

2013-01-01

17

Origin of platy calcite crystals in hot-spring deposits in the Kenya Rift Valley  

SciTech Connect

Platy calcite crystals, which have their c axis parallel to their shortest length axis, are common components of travertine deposits found around some hot springs in the Kenya Rift Valley. They are composite crystals formed of numerous paper-thin subcrystals. Individual plates allowed to grow without obstruction develop a hexagonal motif. The Kenyan crystals typically form in hot (>75 C) waters that have a low Ca content (<10 mg/l), a high CO{sub 2} content, and a high rate of CO{sub 2} degassing. At Chemurkeu, aggregates of numerous small platy crystals collectively form lattice crystals that superficially resemble ray crystals. The walls of the lattice crystals are formed of large platy crystals that have their long and intermediate length axes aligned parallel to the plane of the long axis of the lattice crystal. Internally, the lattice crystals are formed of small platy calcite crystals arranged in a boxlike pattern that creates the appearance of a lattice when viewed in thin section. Lattice crystals are highly porous, with each pore being enclosed by platy crystals. At Lorusio, travertines are mainly formed of pseudodentrites that are constructed by numerous small platy crystals attached to a main stem which is a large platy crystal that commonly curves along its long axis. The pseudodentrites are the main construction blocks in ledges and lilypads that form in the vent pool and spring outflow channels, where the water is too hot for microbes other than hyperthermophiles. The platy calcite crystals in the Kenyan travertines are morphologically similar to platy calcite crystals that form as scale in pipes in the geothermal fields of New Zealand and hydrothermal angel wing calcite from the La Fe mine in Mexico. Comparison of the Kenyan and New Zealand crystals indicates that platy calcite crystals form from waters with a low Ca{sup 2+} content and a high CO{sub 3}/Ca ratio due to rapid rates of CO{sub 2} degassing.

Jones, B. [Univ. of Alberta, Edmonton, Alberta (Canada). Dept. of Earth and Atmospheric Sciences; Renault, R.W. [Univ. of Saskatchewan, Saskatoon, Saskatchewan (Canada). Dept. of Geological Sciences

1998-09-01

18

Initial formation of calcite crystals in the thin prismatic layer with the periostracum of Pinctada fucata.  

PubMed

Although the formation mechanism of calcite crystals in the prismatic layer has been studied well in many previous works, the initial state of calcite formation has not been observed in detail using electron microscopes. In this study, we report that the soft prismatic layer with transparent color (the thin prismatic layer) in the tip of the fresh shell of Pinctada fucata was picked up to observe the early calcification phase. A scanning electron microscope (SEM) image showed that the growth tip of the thin prismatic layer was covered by the periostracum, which was also where the initial formation of calcite crystals began. A cross-section containing the thin calcite crystals in the thin prismatic layer with the periostracum was made using a focused ion beam (FIB) system. In a transmission electron microscope (TEM) observation, the thin calcite crystal (thickness is about 1?m) on the periostracum was found to be a single crystal with the c-axis oriented perpendicular to the shell surface. On the other hand, many aggregated small particles consisting of bassanite crystals were observed in the periostracum suggesting the possibility that not only organic sulfate but also inorganic sulfates exist in the prismatic layer. These discoveries in the early calcification phase of the thin prismatic layer may help to clarify the mechanism of regulating the nucleation and orientation of the calcite crystal in the shell. PMID:23176816

Suzuki, Michio; Nakayama, Seiji; Nagasawa, Hiromichi; Kogure, Toshihiro

2012-10-29

19

An artificial biomineral formed by incorporation of copolymer micelles in calcite crystals.  

PubMed

Biominerals exhibit morphologies, hierarchical ordering and properties that invariably surpass those of their synthetic counterparts. A key feature of these materials, which sets them apart from synthetic crystals, is their nanocomposite structure, which derives from intimate association of organic molecules with the mineral host. We here demonstrate the production of artificial biominerals where single crystals of calcite occlude a remarkable 13 wt% of 20 nm anionic diblock copolymer micelles, which act as 'pseudo-proteins'. The synthetic crystals exhibit analogous texture and defect structures to biogenic calcite crystals and are harder than pure calcite. Further, the micelles are specifically adsorbed on {104} faces and undergo a change in shape on incorporation within the crystal lattice. This system provides a unique model for understanding biomineral formation, giving insight into both the mechanism of occlusion of biomacromolecules within single crystals, and the relationship between the macroscopic mechanical properties of a crystal and its microscopic structure. PMID:21892179

Kim, Yi-Yeoun; Ganesan, Kathirvel; Yang, Pengcheng; Kulak, Alexander N; Borukhin, Shirly; Pechook, Sasha; Ribeiro, Luis; Kröger, Roland; Eichhorn, Stephen J; Armes, Steven P; Pokroy, Boaz; Meldrum, Fiona C

2011-09-04

20

Skeletal crystals of calcite and trona from hot-spring deposits in Kenya and New Zealand  

SciTech Connect

Skeletal crystals are hollow crystals that develop because their outer walls grow before their cores. The presence of skeletal crystals of calcite (three types--trigonal prisms, hexagonal prisms, and plates) and trona in hot (> 90 C) spring deposits in New Zealand (Waikite Springs and Ohaaki Pool) and Kenya (Lorusio hot springs) shows that they can form in natural sedimentary regimes. Analysis of samples from these deposits shows that this crystal morphology develops under disequilibrium conditions that are unrelated to a specific environmental or diagenetic setting. Skeletal crystals transform into solid crystals when subsequent precipitation fills their hollow cores. In some cases, this may involve precipitation of crystalline material that has a sieve-like texture. In other examples, the skeletal crystal provides a framework upon which other materials can be precipitated. Walls in the skeletal trigonal calcite prisms from Waikite Springs are formed of subcrystals that mimic the shape of the parent crystal. Similarly, plate-like skeletal crystals from Lorusio are formed of densely packed subcrystals that are < 0.5 {micro}m long. Conversely, the walls of the skeletal hexagonal calcite crystals from Ohaaki Pool and the skeletal trona crystals from Lorusio are not formed of subcrystals. Recognition of skeletal crystals is important because they represent growth that follows the reverse pattern of normal growth. Failure to recognize that crystal growth followed the skeletal motif may lead to false interpretations concerning the growth of a crystal.

Jones, B. [Univ. of Alberta, Edmonton, Alberta (Canada). Dept. of Earth and Atmospheric Sciences; Renaut, R.W. [Univ. of Saskatchewan, Saskatoon, Saskatchewan (Canada). Dept. of Geological Sciences

1996-01-01

21

Amelogenin processing by MMP-20 prevents protein occlusion inside calcite crystals  

PubMed Central

Calcite crystals were grown in the presence of full-length amelogenin and during its proteolysis by recombinant human matrix metalloproteinase 20 (rhMMP-20). Recombinant porcine amelogenin (rP172) altered the shape of calcite crystals by inhibiting the growth of steps on the {104} faces and became occluded inside the crystals. Upon co-addition of rhMMP-20, the majority of the protein was digested resulting in a truncated amelogenin lacking the C-terminal segment. In rP172-rhMMP-20 samples, the occlusion of amelogenin into the calcite crystals was drastically decreased. Truncated amelogenin (rP147) and the 25-residue C-terminal domain produced crystals with regular shape and less occluded organic material. Removal of the C-terminal diminished the affinity of amelogenin to the crystals and therefore prevented occlusion. We hypothesize that HAP and calcite interact with amelogenin in a similar manner. In the case of each material, full-length amelogenin binds most strongly, truncated amelogenin binds weakly and the C-terminus alone has the weakest interaction. Regarding enamel crystal growth, the prevention of occlusion into maturing enamel crystals might be a major benefit resulting from the selective cleavage of amelogenin at the C-terminus by MMP-20. Our data have important implications for understanding the hypomineralized enamel phenotype in cases of amelogenesis imperfecta resulting from MMP-20 mutations and will contribute to the design of enamel inspired biomaterials.

Bromley, Keith M.; Lakshminarayanan, Rajamani; Thompson, Mitchell; Lokappa, Sowmya B.; Gallon, Victoria A.; Cho, Kang R.; Qiu, S. Roger; Moradian-Oldak, Janet

2012-01-01

22

In situ AFM crystal growth and dissolution study of calcite in the presence of aqueous fluoride  

NASA Astrophysics Data System (ADS)

Fluoride is naturally abundant, encountered in rocks, soil and fresh and ocean water. Calcite crystals, during crystal growth may incorporate fluoride ions into their lattice (Okumura et al., 1983). In situ atomic force microscopy (AFM) has been used to study the growth and dissolution of calcite {104} surfaces in aqueous solutions in the presence of fluoride, using a fluid cell in which the supersaturated and the understaturated solutions respectively, flow over a freshly cleaved calcite crystal. For growth experiments, supersaturation index (S.I.) with respect to calcite was equal to 0.89 and the initial solution pH 10.2. The crystal growth rates were measured from the closure of the rhombohedral etch pits along the [010] direction induced by an initial dissolution step using pure water. The spreading rate of 2-dimensional nuclei was also measured along the same direction. In the presence of low fluoride concentrations (?0.33 mM), the crystal growth rate of calcite was unaffected. At higher concentrations (up to 5 mM) growth rate decreased substantially to 50% of the rate in the absence of fluoride. Potential fluoride sorption over the calcite surface may ascribe the decrease of growth rates. Dissolution experiments were conducted at pH= 7.2 and dissolution rates of calcite were measured from the spreading of rhombohedral etch pits along both [010] and [42] directions. The presence of low concentrations of fluoride (?1.1 mM) in the undersaturated solutions enhanced the dissolution rate along the [42] direction by 50% in comparison with pure water. The morphology of rhombohedral etch pits changed to hexagonal in the presence of fluoride in the undersaturated solutions. The AFM dissolution experiments suggested that the fluoride ions adsorbed onto the calcite surface. Further increase of fluoride concentrations (up to 1.6 mM) resulted in the decrease of the calcite dissolution rate by 60% in both [010] and [42] directions. Reference: Okumura, M, Kitano, Y., Idogaki, M., 1983. Inorporation of fluoride ions into calcite - Effect of organic materials and magnesium ions in a parent solution. Geochemical Journal 17, 257-263.

Vavouraki, A.; Putnis, C. V.; Putnis, A.; Koutsoukos, P. G.

2009-04-01

23

Anisotropic surface coupling while sliding on dolomite and calcite crystals  

NASA Astrophysics Data System (ADS)

High-resolution friction force microscopy has been performed on the (104) surfaces of dolomite and calcite in deionized water. The two rows of oxygen atoms alternating in a zigzag way on top of both surfaces are resolved with similar contrast while scanning along the [421¯] direction. Along the [010] direction, only one row is resolved, provided that the normal loading is large enough. The direction-dependent interaction between the probing tip and surface atoms is explained by numeric calculations based on the Prandtl-Tomlinson model.

Pina, Carlos M.; Miranda, Rodolfo; Gnecco, Enrico

2012-02-01

24

Sector zoning in calcite cement crystals: Implications for trace element distributions in carbonates  

Microsoft Academic Search

Compositional sector zoning in calcite cement crystals from a sedimentary limestone is represented by differences of Mg content of over 1300 ppm, or 95%, and of Sr and Mn contents of up to 400 ppm, or roughly 50% and 80% respectively, between crystallographically nonequivalent, time-synchronous growth sectors. {10 1} sectors are enriched with both Mg and Mn relative to {10

Richard J. Reeder; John C. Grams

1987-01-01

25

Single Crystal Growth of Calcium Carbonate (Synthetic Calcite). Phase 1, Final Report.  

National Technical Information Service (NTIS)

The feasibility of calcite single crystal growth from a Li2CO3 solvent using the Heat Exchanger Method (HEM) was demonstrated. It was observed that a CO2 atmosphere is necessary to prevent decomposition of calcium carbonate (CaCO3). Under flowing CO2 at 1...

F. Schmid

1987-01-01

26

Atomic layer wear of single-crystal calcite in aqueous solution using scanning force microscopy  

Microsoft Academic Search

Scanning force microscopy (SFM) is employed to study nm-scale wear of single-crystal calcite in an aqueous solution. When the SFM tip is drawn back and forth in a linear fashion across the edge of a preexisting single atomic layer etch pit, dissolution is strongly enhanced at the point where the tip crosses the step. The wear rate as a function

Nam-Seok Park; Myoung-Won Kim; S. C. Langford; J. T. Dickinson

1996-01-01

27

Trigonal dendritic calcite crystals forming from hot spring waters at Waikite, North Island, New Zealand  

SciTech Connect

Amorphous silica and calcite form the deposits in the vent and on the discharge apron of Waikite Spring 100 (WS-100), which is located in the Waikite Geothermal area on North Island, New Zealand. These precipitates formed from spring water that has a temperature of >90 C and a pH of 8.1--8.8. The opaline silica is restricted to areas around the vent where cooling and evaporation of the spring water triggered precipitation. The calcite deposits in the spring vent and on the discharge apron are formed of large (up to 15 cm long) asymmetrical dendrite crystals that are characterized by multiple levels of branching. Branches grew preferentially from the downflow side of their parent branch. All branches have a trigonal transverse cross section except in areas where competition for growth space induced merger of neighboring crystals. The primary branches of the dendrite crystals are (sub)perpendicular to the substrate even in areas where the discharge apron slopes at a high angle (up to 80{degree}). On the steeper parts of the discharge apron, the plate-like primary branches form the floors of the small terrace pools whereas their distal edges form the rims of the pools. Growth of these dendrite crystals is attributed to abiotic processes. High levels of saturation with respect to calcite were caused by rapid CO{sub 2} degassing of the sheets of spring water that flowed down the steep discharge apron. Calcite crystals with different crystal morphologies characterize other springs near this spring. The variation in crystal morphologies from spring to spring is attributed to different levels of saturation that are related to the initial PCO{sub 2} of the spring water upon discharge and the rate of CO{sub 2} degassing at each spring.

Jones, B.; Renault, R.W.; Rosen, M.R.

2000-05-01

28

Patterns of distribution of calcite crystals in soft corals sclerites.  

PubMed

The gross morphology of soft coral surface sclerites has been studied for taxonomic purposes for over a century. In contrast, sclerites located deep in the core of colonies have not received attention. Some soft coral groups develop massive colonies, in these organisms tissue depth can limit light penetration and circulation of internal fluids affecting the physiology of coral tissues and their symbiotic algae; such conditions have the potential to create contrasting calcifying conditions. To test this idea, we analyzed the crystal structure of sclerites extracted from different colony regions in selected specimens of zooxanthellate and azooxanthellate soft corals with different colony morphologies, these were: Sarcophyton mililatensis, Sinularia capillosa, Sinularia flexibilis, Dendronephthya sp. and Ceeceenus levis. We found that the crystals that constitute polyp sclerites differ from those forming stalk sclerites. We also observed different crystals in sclerites located at various depths in the stalk including signs of sclerite breakdown in the stalk core region. These results indicate different modes of calcification within each colonial organism analyzed and illustrate the complexity of organisms usually regarded as repetitive morphological and functional units. Our study indicates that soft corals are ideal material to study natural gradients of calcification conditions. PMID:21433054

Tentori, Ernestina; van Ofwegen, Leen P

2011-03-23

29

Sector zoning in calcite cement crystals: Implications for trace element distributions in carbonates  

NASA Astrophysics Data System (ADS)

Compositional sector zoning in calcite cement crystals from a sedimentary limestone is represented by differences of Mg content of over 1300 ppm, or 95%, and of Sr and Mn contents of up to 400 ppm, or roughly 50% and 80% respectively, between crystallographically nonequivalent, time-synchronous growth sectors. {10 1} sectors are enriched with both Mg and Mn relative to {10 4} sectors, and other sectors show both greater and lesser enrichment. This systematic zoning pattern requires effective distribution coefficients for these trace elements to have been different for growth on different faces. Differential growth rate effects for nonequivalent faces are shown to be unimportant relative to differential growth face / element preferences. Surface crystal structure and differing coordination environments are regarded as the principal factors influencing sector-related elemental partitioning differences in calcite and dolomite.

Reeder, Richard J.; Grams, John C.

1987-02-01

30

Morphology of calcite crystals in clast coatings from four soils in the Mojave desert region  

NASA Astrophysics Data System (ADS)

Pedogenic calcite-crystal coatings on clasts were examined in four soils along an altitudinal gradient on Kyle Canyon alluvium in southern Nevada. Clast coatings were studied rather than matrix carbonate to avoid the effects of soil matrix on crystallization. Six crystal sizes and shapes were recognized and distinguished. Equant micrite was the dominant crystal form with similar abundance at all elevations. The distributions of five categories of spar and microspar appear to be influenced by altitudinally induced changes in effective moisture. In the drier, lower elevation soils, crystals were equant or parallel prismatic with irregular, interlocking boundaries while in the more moist, higher elevation soils they were randomly oriented, euhedral, prismatic, and fibrous. There was little support for the supposition that Mg(+2) substitution or increased (Mg + Ca)/HCO3 ratios in the precipitating solution produced crystal elongation.

Chadwick, Oliver A.; Sowers, Janet M.; Amundson, Ronald G.

1989-02-01

31

Swift heavy ion irradiation induced phase transformation in calcite single crystals  

Microsoft Academic Search

Ion irradiation induced phase transformation in calcite single crystals have been studied by means of Raman and infrared spectroscopy using 120 MeV Au9+ ions. The observed bands have been assigned according to group theory analysis. For higher fluence of 5×1012 ion\\/cm2, an extra peak on either side of the 713 cm?1 peak and an increase in the intensity of 1085 cm?1 peak were observed in Raman

H. Nagabhushana; B. M. Nagabhushana; B. N. Lakshminarasappa; Fouran Singh; R. P. S. Chakradhar

2009-01-01

32

Electron spin resonance absorption spectrum of CO2- molecule-ions in single crystal calcite  

Microsoft Academic Search

An electron spin resonance absorption spectrum due to CO2- molecule-ions has been observed at 3 cm wavelength in single crystals of optical grade calcite. The spectrum exhibits orthorhombic symmetry with both the 12C and 13C isotopically substituted CO2- molecule-ions having the same g-tensor components given by gzz = 2·00161, gxx = 2·00320 and gyy = 1·99727, where z refers to

S. A. Marshall; A. R. Reinberg; R. A. Serway; J. A. Hodges

1964-01-01

33

Calcite crystal growth inhibition by humic substances with emphasis on hydrophobic acids from the Florida Everglades  

USGS Publications Warehouse

The crystallization of calcium carbonate minerals plays an integral role in the water chemistry of terrestrial ecosystems. Humic substances, which are ubiquitous in natural waters, have been shown to reduce or inhibit calcite crystal growth in experiments. The purpose of this study is to quantify and understand the kinetic effects of hydrophobic organic acids isolated from the Florida Everglades and a fulvic acid from Lake Fryxell, Antarctica, on the crystal growth of calcite (CaCO3). Highly reproducible calcite growth experiments were performed in a sealed reactor at constant pH, temperature, supersaturation (?? = 4.5), P(CO2) (10-3.5atm), and ionic strength (0.1 M) with various concentrations of organic acids. Higher plant-derived aquatic hydrophobic acids from the Everglades were more effective growth inhibitors than microbially derived fulvic acid from Lake Fryxell. Organic acid aromaticity correlated strongly with growth inhibition. Molecular weight and heteroatom content correlated well with growth inhibition, whereas carboxyl content and aliphatic nature did not. Copyright (C) 1999 Elsevier Science Ltd.

Hoch, A. R.; Reddy, M. M.; Aiken, G. R.

2000-01-01

34

Morphogenetic control of calcite crystal growth in sulfonic acid based hydrogels.  

PubMed

In this paper the mineralization of CaCO(3) in various hydrogel matrices is presented. Sulfonic acid based hydrogels were prepared by introduction of sulfonate-containing monomers into a polyacrylamide network. The sulfonate content of polyacrylamide-co-vinylsulfonate and polyacrylamide-co-allylsulfonate decreases during elution of the copolymers in demineralized water, indicating insufficient linking of the sulfonate-bearing monomers within the hydrogel. In contrast to this, acrylamidomethylpropanesulfonate (AMPS) effectively copolymerizes with acrylamide (AAm) monomers. To study the influence of spatial arrangement of ionic functional groups within hydrogel networks on the mineralization of CaCO(3), AMPS copolymers with different degrees of AMPS cross-linking were synthesized. For the mineralization experiments the copolymers were placed into a double-diffusion arrangement. Calcite as the thermodynamically stable modification of CaCO(3) was obtained with a particular morphology. The pseudocubic habitus resembles aggregates obtained by mineralization in pure polyacrylamide. However, closer examination of the aggregates by scanning electron microscopy (SEM) shows that the crystal growth in the AMPS copolymers is different from that observed in polyacrylamide. Whereas the morphology of the calcite aggregates could be fine-tuned by using copolymers with different sulfonate content, the spatial distribution of the ionic functional groups alters the course of crystallization. Calcium ions are locally accumulated due to the heterogeneous distribution of functional sulfonate groups within the copolymer network. Thereby the nucleation of calcite is triggered, resulting in enhanced mineralization. PMID:12645020

Grassmann, Olaf; Löbmann, Peer

2003-03-17

35

New type of compositional zoning in calcite: Insights into crystal-growth mechanisms  

NASA Astrophysics Data System (ADS)

Compositional zoning of trace elements is observed in time-equivalent regions within 1014 growth sectors of natural and synthetic calcite single crystals, and has been called "intrasectoral zoning" to distinguish it from conventional sectoral zoning. Differential interference contrast microscopy shows that {10114} faces grow by the spiral mechanism. Cathodoluminescence microscopy and electron probe microanalyses reveal that trace element partitioning differences on {10114} faces correspond exactly to crystallographically controlled orientations of growth steps. Effective partitioning of Mg and Mn varies up to a factor of four between time-correlative growth steps spreading in different directions. Effective Sr partitioning varies by approximately 20% and is inversely correlated with enrichment trends of Mg and Mn. Preferential incorporation of different aqueous species at different growth steps may provide an explanation for morphological and chemical variation in natural calcites.

Paquette, Jeanne; Reeder, Richard J.

1990-12-01

36

Precipitation of different calcite crystal morphologies in the presence of sodium stearate.  

PubMed

The influence of sodium stearate (NaSt) on the precipitation of calcium carbonate during the semicontinuous process of slaked lime carbonation was studied in the systems in which process parameters, like concentration of total dissolved calcium, temperature, CO(2) flow rate and initial addition rate of slaked lime, were controlled. It was found that calcite was the only calcium carbonate polymorph that appeared under the investigated experimental conditions, while FT-IR spectroscopy and thermogravimetric analysis of samples confirmed the presence of stearate on the surface of precipitated calcium carbonate (PCC). Specific surface area of PCC increased with increasing stearate content: the highest value, s = 52.8 m(2) g(-1), was obtained at t = 20 degrees C, c(tot) = 17.0 mmol dm(-3) and the stearate content of m(NaSt)/m(CaO) = 0.03. It was also found that hydrophobic calcite crystals in the form of rhombohedral and scalenohedral morphology can be produced at m(NaSt)/m(CaO) > 0.01. The exception is the case of nanosized PCC production, when much higher concentration of NaSt is needed, m(NaSt)/m(CaO) = 0.22. Minimal amount of stearate necessary to build up the monolayer and corresponding cross sectional area of one stearate molecule were estimated for the obtained calcite morphologies. PMID:18848702

Ukrainczyk, Marko; Kontrec, Jasminka; Kralj, Damir

2008-09-23

37

In vitro effects of recombinant Otoconin 90 upon calcite crystal growth. Significance of tertiary structure  

PubMed Central

Otoconia are biomineral particles of microscopic size essential for perception of gravity and maintenance of balance. Millions of older Americans are affected in their mobility, quality of life and in their health by progressive demineralization of otoconia. Currently, no effective means to prevent or counteract this process are available. Because of prohibitive anatomical and biological constraints, otoconial research is lagging far behind other systems such as bone and teeth. We have overcome these obstacles by generating otoconial matrix proteins by recombinant techniques. In the present study, we evaluated the effects of recombinant Otoconin 90 (OC90), the principal soluble matrix protein upon calcite crystal growth patterns in vitro. Our findings highlight multiple effects, including facilitation of nucleation, and inhibition of crystal growth in a concentration-dependent manner. Moreover, OC90 induces morphologic changes characteristic of native otoconia. OC90 is considerably less acidic than the prototypical invertebrate CaCO3 –associated protein, but is nevertheless an effective modulator of calcite crystal growth. Based on homology modeling of the sPLA2-like domains of OC90, we propose that the lower density of acidic residues of the primary sequence is compensated by formation of major anionic surface clusters upon folding into tertiary conformation.

Lu, Wenfu; Zhou, Dan; Freeman, John J.; Thalmann, Isolde; Ornitz, David M.; Thalmann, Ruediger

2010-01-01

38

Modification of calcite crystal growth by abalone shell proteins: an atomic force microscope study.  

PubMed Central

A family of soluble proteins from the shell of Haliotis rufescens was introduced over a growing calcite crystal being scanned in situ by an atomic force microscope (AFM). Atomic step edges on the crystal surface were altered in shape and speed of growth by the proteins. Proteins attached nonuniformly to the surface, indicating different interactions with crystallographically different step edges. The observed changes were consistent with the habit modification induced by this family of proteins, as previously observed by optical microscopy. To facilitate further studies in this area, AFM techniques and certain AFM imaging artifacts are discussed in detail. Images FIGURE 1 FIGURE 3 FIGURE 4 FIGURE 5 FIGURE 6 FIGURE 8 FIGURE 9

Walters, D A; Smith, B L; Belcher, A M; Paloczi, G T; Stucky, G D; Morse, D E; Hansma, P K

1997-01-01

39

Formation of star-shaped calcite crystals with Mg{sup 2+} inorganic mineralizer without organic template  

SciTech Connect

Star-shaped calcite crystals with 3-bar m symmetry were obtained in the mixed solvent of ethanol and H{sub 2}O (4:1vol%) using Mg{sup 2+} as grow mineralizer without any organic template under the solvothermal condition. The crystals branched to the six directions perpendicular to the c-axis. In the process, Mg{sup 2+} takes an important influence on such novel morphology via entering the crystal lattice of calcite to absorb the special plane and change the general growth habit. The aqueous solvent is favorable to form aragonite, while the presence of alcohol promotes the formation of calcite, the thermodynamically stable phase. The products were characterized by the techniques of XRD, SEM, SAED, IR and ICP. The formation process was also primarily studied.

Zhu Liying [School of Chemical and Materials Engineering, Southern Yangtze University, Wuxi, Jiangsu 214036 (China); Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026 (China); Zhao Qingrui [Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026 (China); Zheng Xiuwen [Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026 (China); Xie Yi [School of Chemical and Materials Engineering, Southern Yangtze University, Wuxi, Jiangsu 214036 (China) and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026 (China)]. E-mail: yxielab@ustc.edu.cn

2006-04-15

40

Corrosion of calcite crystals by metal-rich mud in caves: Study case in Crovassa Ricchi in Argento Cave (SW Sardinia, Italy)  

NASA Astrophysics Data System (ADS)

Unusual orange ochre crusts were recently discovered in Crovassa Ricchi in Argento Cave (San Giovanni Mine, SW Sardinia). These speleothems appear covering the cave walls on hydrothermal calcite spars as well as filling widened spaces between calcite crystals. Planar crusts display geometrical forms following the boundaries between the calcite spars. EDX–SEM microanalyses reveal that these deposits comprise substances of Fe, Mn, Pb, Zn and O that occur as solid inclusions in pits on the surface of altered calcite microcrystals. Micro-Raman spectroscopy analyses suggest the presence of calcite and ferromanganese oxides with a low degree of crystallinity. The genetic mechanism proposed for these speleothems describes an initial stage of precipitation of euhedral calcite crystals from warm water under subaqueous conditions. The crystal surfaces were eroded and corroded by colder aggressive water that smoothed the surfaces of the crystals and slightly widened the spaces between calcite spars. Metal-rich mud coming from alteration of bedrock and ore bodies filled the cave, also penetrating along the spaces between the calcite spars. When the water table fell below the cave level, part of the sediments was eroded but the cave walls remained covered with metal-rich clayey sediments. Under aerobic conditions, metals – which were reduced in previous stages – oxidized to oxides, lowering the pH and thus the crystal surface and the calcite planes between the spars were corroded. Subsequently, the polymetallic crusts became harder through evaporation within the cave, "fossilizing" the products of this process within the planes between spars. Finally, the exposed calcite surfaces continued to be altered due to CO2 diffusion into condensation water, while the boundaries between crystals were preserved against corrosion thanks to the crust coating. As a result, the external crystal edges protrude by several centimeters from the current cave wall, while the crystal surfaces are depressed, giving rise to calcite "ghosts".

Gázquez, Fernando; Calaforra, José-María; Forti, Paolo; De Waele, Jo; Sanna, Laura; Rull, Fernando; Sanz, Aurelio

2013-09-01

41

Viewing the seeds of crystallization.  

SciTech Connect

All materials search for the lowest accessible energy state. As temperature is increased in disordered materials, atoms diffuse and explore different chemical and structural configurations. Crystalline phases may be favored, but a very small crystal is unstable, so there is a 'nucleation barrier' to overcome; only after reaching a critical size can the nucleus grow. Although we understand the thermodynamics of the nucleation process [e.g., (1)], observation of the actual atomic-scale complexity during nucleation has remained elusive, despite its importance to the properties of materials. Taking nucleation out of the 'black box' is one of the grand challenges to 'materials by design' that is seen as the future solution to major societal problems such as sustainable energy (2). On page 980 of this issue, Lee et al. (3) use fluctuation electron microscopy to image subcritical nuclei in a solid material, observing metastable structural states that facilitate later nucleation in amorphous films. Their study is applied to a technologically important case of 'phase-change memory' and therefore may facilitate efforts to design faster higher-density nonvolatile memory.

Gibson, J. M.

2009-11-13

42

Growth of Seeded Copper Crystals of Perfection Comparable to Unseeded Crystals  

Microsoft Academic Search

A graphite mold of a new design was made for growing better seeded crystals from the melt. The main features were (1) a narrow seed constriction, (2) a split mold, (3) high density graphite, (4) equal cross sections of seed and crystal, and (5) high ratio of sample to mass crucible mass.The resulting crystals were compared by two complementary techniques

Lawrence D. Dyer

1965-01-01

43

Effect of cooling mode on product crystal size in seeded batch crystallization of potassium alum  

Microsoft Academic Search

Potassium alum was crystallized by seeding in a batch crystallizer under controlled and natural cooling modes. Regardless of the cooling mode, the product crystal size distribution (CSD) became bi-modal at low seed concentrations because of enormous secondary nucleation. The mean mass size of the product was smaller for the natural cooling mode compared to that for the controlled cooling mode

Norihito Doki; Noriaki Kubota; Akira Sato; Masaaki Yokota

2001-01-01

44

Kinetic and thermodynamic factors controlling the distribution of SO32- and Na+ in calcites and selected aragonites  

USGS Publications Warehouse

Significant amounts of SO42-, Na+, and OH- are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO42-. Aragonites and most biogenic low Mg-calcites contain significant amounts of Na+, but very low concentrations of SO42-. The SO42- content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na+ and SO42- increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO42- beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO42- is the same as that of aragonite. Na+ appears to have very little effect on the solubility product of calcites. The amounts of Na+ and SO42- incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient (D) of SO42- in calcite at 25.0??C and 0.50 molal NaCl is described by the equation D = k0 + k1R where k0 and k1 are constants equal to 6.16 ?? 10-6 and 3.941 ?? 10-6, respectively, and R is the rate of crystal growth of calcite in mg??min-1??g-1 of seed. The data on Na+ are consistent with the hypothesis that a significant amount of Na+ occupies interstitial positions in the calcite structure. The distribution of Na+ follows a Freundlich isotherm and not the Berthelot-Nernst distribution law. The numerical value of the Na+ distribution coefficient in calcite is probably dependent on the number of defects in the calcite structure. The Na+ contents of calcites are not very accurate indicators of environmental salinities. ?? 1985.

Busenberg, E.; Niel, Plummer, L.

1985-01-01

45

A conceptual model for near-surface kinetic controls on the trace-element and stable isotope composition of abiogenic calcite crystals  

NASA Astrophysics Data System (ADS)

The surface of a crystal in equilibrium with solute-bearing fluid generally has a composition that differs from that of the bulk crystal. If the crystal is growing, the surface composition may be "captured" by the newly formed lattice to a degree that depends upon the growth rate and the mobility of atoms in the near-surface region: rapid growth promotes this growth "entrapment," high near-surface mobility works against it. Natural calcites may be particularly susceptible to this kind of kinetic disequilibrium, because their precipitation rates from aqueous solution can be relatively high even at near-ambient temperatures, where ion mobility in the critical near-surface region may be limited. Existing laboratory data on trace-element uptake as a function of calcite growth rate are examined here in the context of recent discoveries concerning the structure, chemistry and kinetics of the near-surface region of calcite crystals. Recent demonstrations that ions can be mobile in the outermost few nanometers of the calcite lattice even at room temperature have the greatest potential to affect growth entrapment. The model of Watson and Liang (1995)—which quantifies entrapment efficiency in terms of growth rate, diffusivity and surface-layer thickness—is modified to include a depth-dependent diffusivity and possible depletion (as well as enrichment) of some elements in the near-surface region. With these changes, the model is shown to be qualitatively consistent with the body of experimental data on trace element uptake during calcite precipitation. This apparent success of the model invites application to stable isotopes. Constraining data are few, but available information on oxygen isotope fractionation can be used to show that growth entrapment at ambient temperatures may (depending on model assumptions) produce deviations from calcite/H 2O equilibrium of up to several ‰. The preferred choice of 18O/ 16O for the surface layer is lighter than the lattice equilibrium value, and leads to a reduction in 18O/ 16O of crystals grown at higher growth rates, mimicking "vital effects."

Watson, E. Bruce

2004-04-01

46

Seeded Growth of AlN Crystals on Non-Polar Seeds Via Physical Vapor Transport  

SciTech Connect

Seeded growth of AlN single crystals was demonstrated in an induction-heated, high-temperature reactor via a physical vapor transport (PVT) process. AlN seeds were prepared from a self-seeded boule containing large single-crystalline grains. Seeded growth was interrupted several times in order to refill the AlN powder source, and a dedicated process scheme was used to ensure epitaxial growth on the seed surface, after prior exposure to air. The growth temperatures were in the range of 2200-2300 C, and the reactor pressure was in the range of 500-900 torr of UHP-grade nitrogen during each growth run. Under these growth conditions, a seed (10 mm diameter) expanded at an angle of 45{sup o}, and a larger single crystal up to 18 mm in diameter was obtained. The as-grown surface had three facets, of which facet (1120) was smooth and featureless while the other two, (4150) and (2570), showed serrated morphologies. The double-crystal x-ray rocking curve and glow discharge mass spectroscopy analysis confirmed that the grown crystal was of high crystalline quality with low impurity incorporation.

Zhuang,D.; Herro, Z.; Schlesser, R.; Ragothamachar, B.; Dudley, M.; Sitar, Z.

2006-01-01

47

Investigation of the transition range in seeded vapour grown ZnSe crystals  

Microsoft Academic Search

Prerequisite for the growth of large area single crystals are exactly (111)Se orientated seeds. For both used growth methods, seeded physical vapour transport (SPVT) and seeded chemical vapour transport (SCVT), an improved crystal quality can be achieved. In SCVT crystals, optically active deep level (DL) defects dominate mainly due to the self activated (SA)-centre. The transition range width was more

U. Geissler; H. Hartmann; E. Krause; D. Siche

1996-01-01

48

Carbon nanotube seeded silicon crystal growth  

NASA Astrophysics Data System (ADS)

Crystal growth of liquid silicon on the heterogeneous surfaces of carbon nanotubes (CNTs) is simulated. Silicon atoms are concentrated to form perfect coaxial cylindrical configurations around CNT cores. The heredity effect makes silicon imprint cylindrical shapes of CNTs during the solidification. The CNTs have a great influence on the stacking sequence of silicon atoms. Growth competition between CNT cores is clearly observed, which results in defects at the shared interface. The internal potential field around CNTs is responsible for growth competition and the heredity effect.

Li, Y. F.; Li, H.; Sun, F. W.; Zhang, X. Q.; Liew, K. M.

2011-07-01

49

Isolation and thermal characterization of high-melting seed crystals formed during cocoa butter solidification  

Microsoft Academic Search

Seed crystals which formed during early stages of cocoa butter solidification have been isolated and determined to have extremely\\u000a high melting points. The melting points of the seed crystals generally exceeded 60C, in contrast to cocoa butter, which melts\\u000a between 30–35C. In addition, the melting point of the seed crystals decreased as a function of crystal growth time. Evidence\\u000a suggests

Thomas R. Davis; Paul S. Dimick

1989-01-01

50

Phospholipid composition of lipid seed crystal isolates from ivory coast cocoa butter  

Microsoft Academic Search

Seed crystals isolated from Ivory Coast cocoa butter were shown to differ in chemical and thermal characteristics from solidified\\u000a Ivory Coast butter. Higher concentrations of complex lipids in the seed crystals have led to speculation on the role these\\u000a polar molecules play in lipid crystallization events. Phospholipids separated from lipid seed crystal isolates were twelve-fold\\u000a more concentrated than the original

Doris H. Arruda; Paul S. Dimick

1991-01-01

51

Why calcite can be stronger than quartz  

Microsoft Academic Search

In the Neves area (Eastern Alps), calcite forms asymmetric centimeter-scale single-crystal porphyroclasts in quartz mylonites developed during hydrous amphibolite facies metamorphism at ?550°C. Under these conditions, coarse calcite was clearly stronger than the surrounding polycrystalline, dynamically recrystallized, quartz matrix. Experimental results indicate that coarse calcite is less strain rate sensitive than wet quartzite, consistent with an inversion in strength on

Neil S. Mancktelow; Giorgio Pennacchioni

2010-01-01

52

Isothermal Crystallization Kinetics of Mango (Mangifera indica) Almond Seed Fat  

NASA Astrophysics Data System (ADS)

In this study, the kinetics of isothermal crystallization of mango (Mangifera indica) almond seed fat var. Manila (MAF) was analyzed, within the theoretical context of the Sestak-Berggren model, the Avrami Equation and its modification by Khanna and Taylor. The results showed that the induction times for the formation of crystalline nuclei increased with the crystallization temperature (3.3 min at 8°C and 10.9 min at 12°C). The supercooling level notably influenced the MAF crystallization rate, since the global constant of crystallization rate, Z, grew 3.3 times from 12 to 8°C (for fractions of fat solids between 0.25 and 0.75, Z was 0.2904, 0.1584 and 0.0879 min-1 at 8, 10 and 12°C, respectively) and the Avrami parameter r was higher than 4; this demonstrates the effect of fat system complexity due to its multi-component nature and the heterogeneous character of this crystallization process, which includes additional participation of nucleation sites. The modified model by Khanna and Taylor provided better parametral values than the other two studied for explaining MAF crystallization kinetic.

Solis-Fuentes, J. A.; Hernandez-Medel, M. R.; Duran-de-Bazua, M. C.

53

Effect of baffle geometries on crystal size distribution of aluminum potassium sulfate in a seeded batch crystallizer  

Microsoft Academic Search

Effects of geometrical configurations of crystallizers upon the crystal size distribution have been quantitatively investigated using four types of baffles with different geometries, in batch crystallization of aluminum potassium sulfate in a seeded batch crystallizer agitated by a 6-bladed flat paddle. Two evident maxima in size distribution were obtained for a vessel with normal wall baffles, whereas the maximum at

Kenji Shimizu; Koji Takahashi; Eiichi Suzuki; Toshio Nomura

1999-01-01

54

Lipid composition of high-melting seed crystals formed during cocoa butter solidification  

Microsoft Academic Search

High-melting seed crystals which form during the early stages of cocoa butter solidification possess a lipid composition different\\u000a than the cocoa butter from which the seed crystals were grown. Significantly large quantities of glycolipids, 11.1%, and phospholipids,\\u000a 6.6–8.1%, were found in the high-melting seed crystals along with a dramatic decrease in the simple lipid class. The fatty\\u000a acids comprising the

Thomas R. Davis; Paul S. Dimick

1989-01-01

55

High-energy seeded optical parametric generation with elliptical beams in quasi-phase-matched crystals  

NASA Astrophysics Data System (ADS)

Large-aperture and high-energy seeded operation of optical parametric generators (OPGs) based on quasi-phase-matched crystals is presented. High-quality seeded outputs as evidenced by narrow bandwidth and low divergence are demonstrated with energies up to 12.5 mJ in the signal and the idler. The results of OPG devices operating in a partially seeded regime are also presented to give a general prescription for successful seeding at high energies.

Powers, Peter E.; Bojja, Prasanth K.; Alkuwari, Rashid; Vershure, Eric; Schepler, Kenneth L.

2005-10-01

56

Calcium oxalate crystals in the mature seeds of Norway spruce, Picea abies (L.) Karst  

Microsoft Academic Search

The morphology and location of crystals encountered in the mature seeds of Norway spruce, Picea abies (L.) Karst., were examined using light and field emission scanning electron microscopy (FESEM). Crystals of various forms\\u000a and sizes were discovered in different regions and tissues of seeds, particularly in the testa and the nucellus. Both solitary\\u000a crystals and druses were occasionally enveloped by

E. Tillman-Sutela; Anneli Kauppi

1999-01-01

57

Crystallization during volume reduction of solutions with a composition corresponding to that in the collecting duct: the influence of hydroxyapatite seed crystals and urinary macromolecules  

Microsoft Academic Search

To examine the effect of hydroxyapatite (HAP) seed crystals and urinary macromolecules on the crystallization under conditions\\u000a similar to those in the collecting duct, we evaporated 100?ml samples of salt solutions with an ion composition assumed to\\u000a correspond to that in the collecting duct without and with HAP seed crystals. The crystallization in seeded solutions was\\u000a assessed both with and

Inge Højgaard; Anne-Marie Fornander; Mari-Anne Nilsson; Hans-Göran Tiselius

1999-01-01

58

Stray grain formation in the seed region of single-crystal turbine blades  

NASA Astrophysics Data System (ADS)

Seed crystals are frequently used to provide an off-axial <001> crystallographic orientation to investment cast single-crystal, nickel-based superalloy turbine blades. However, stray grain defects can form during the melt-back of the seed crystal, requiring the use of a helical grain selector between the seed and the blade to remove them. Using meso-scale numerical simulations, the formation mechanisms of these stray grain defects have been investigated. Also investigated was the influence of the seed’s crystallographic orientation relative to blade axis. The model is first validated by comparison to experimental observations and then by its application to a range of casting situations. The results show that initiation of these defects is difficult to avoid. Instead, the impact of stray grains should be controlled during their growth.

Yang, X. L.; Lee, P. D.; D'Souza, N.

2005-05-01

59

Hydrothermal calcite in the Elephant Moraine  

SciTech Connect

In the course of geologic mapping of the Elephant Moraine on the east antarctic ice sheet, Faure and Taylor (1985) collected several specimens of black botryoidal calcite, composed of radiating acicular crystals that resemble stromatolites. Calcite from this and other specimens is significantly enriched in strontium-87 (the strontium-87/strontium-86 ratio equals 0.71417 +/- 0.00002), carbon-12 (delta carbon-13 equals -22.9 parts per thousand, PDB standard) and oxygen-16 (delta oxygen-18 equals -21.1 parts per thousand, standard mean ocean water) compared with calcite of marine origin. The enrichment in carbon-12 is similar to that of calcite associated with coal in the Allan Hills. The enrichment in oxygen-16 indicates that the calcite from the Elephant Moraine could only have precipitated in isotopic equilibrium with glacial melt water. Therefore, the temperature at which the black calcite precipitated from water of that isotope composition was about 85/sup 0/C. A temperature of this magnitude implies that the black calcite formed as a result of volcanic activity under the east antarctic ice sheet. The enrichment of the black calcite in carbon-12 suggests that it formed in part from carbon dioxide derived from the coal seams of the Weller Formation in the Beacon Supergroup. The isotopic composition of strontium in the black calcite is similar to that of carbonate beds and concretions in the Beacon rocks of southern Victoria Land. A volcanic-hydrothermal origin is also consistent with the very low total organic carbon content of 0.15% in the calcite.

Faure, G.; Taylor, K.S.; Jones, L.M.

1986-01-01

60

Why calcite can be stronger than quartz  

NASA Astrophysics Data System (ADS)

Calcite and quartz are two of the most common minerals in the continental crust and it is therefore not surprising that these minerals have been extensively studied since the very beginning of laboratory rock mechanics experiments. Extending such laboratory data to geological deformation rates around 10-14 s-1 requires an extrapolation of more than 7 orders of magnitude, with correspondingly large uncertainties. Extrapolation is based on the assumption that flow parameters are constant with changing conditions and that parameters not included in the flow law have a negligible influence on the creep properties. The validity of this extrapolation can only be tested, at least semi-quantitatively, by comparison with naturally deformed rocks. Observations generally indicate that quartz is significantly stronger than calcite in natural rocks, with quartz forming porphyroclasts in calcite marble mylonites, or with quartzite layers being folded or boudinaged within a weaker calcite marble matrix. However, in the Neves area (Tauern Window, Eastern Alps), shearing of Alpine coarse grained quartz-calcite veins under hydrous amphibolite facies conditions (ca. 550Ë? C) produced quartz mylonites containing asymmetric cm-scale single crystal calcite porphyroclasts. Under these conditions, coarse calcite is clearly stronger than the surrounding polycrystalline, dynamically recrystallized, quartz matrix. The important parameter controlling this difference in observed natural behaviour is the grain size of the calcite. Although there is considerable variation, uncertainty and even contradiction in the published experimental results, we show that extrapolation of laboratory creep data on calcite single crystals and coarse marbles, together with the corresponding data for wet quartzites, is indeed consistent with these natural observations. Extrapolation indicates an inversion in the relative strength of coarse calcite and quartz at a strain rate around 10-11 s-1, corresponding to a differential flow stress of ca. 50 MPa. At lower strain rates and stresses, wet quartz should be weaker than coarse calcite crystals. Field evidence (flow of quartz-rich layers even for orientations with very low resolved shear stress) and the preserved microstructure (lack of recrystallized or bulged twins in the calcite porphyroclasts) in the Neves area are also consistent with flow stresses of less than ca. 50 MPa. These low values during deformation under water-rich amphibolite facies conditions are in marked contrast to the much higher differential stresses reported for the flow (and fracture) of quartz-rich rocks under dry conditions in the middle to lower crust.

Mancktelow, N. S.; Pennacchioni, G.

2009-04-01

61

Induction of crystal growth in alkali-halide aggregates by means of internal seeding  

NASA Astrophysics Data System (ADS)

Following our previous work, new results for seed induced nucleation and crystal growth in nano droplets are presented. A new method that allows to place the seed inside the droplet is presented. The new results show a considerable independence of the processes of nucleation and crystal growth regarding the size of the host droplet. The crystal growth model also previously reported was applied giving good predictions. It supports more insights into the growth trends for different seed placements. Based on its deviation from the ideal condition considered in the crystal growth model development, a reinterpretation for the outside seeding behavior is presented. Additionally, the present results strongly sustain a non-self-wetting behavior for ionic salts aggregates.

Rodrigues, P. C. R.; Silva Fernandes, F. M. S.

2012-06-01

62

High-energy seeded optical parametric generation with elliptical beams in quasi-phase-matched crystals  

Microsoft Academic Search

Large-aperture and high-energy seeded operation of optical parametric generators (OPGs) based on quasi-phase-matched crystals is presented. High-quality seeded outputs as evidenced by narrow bandwidth and low divergence are demonstrated with energies up to 12.5 mJ in the signal and the idler. The results of OPG devices operating in a partially seeded regime are also presented to give a general prescription

Peter E. Powers; Prasanth K. Bojja; Rashid Alkuwari; Eric Vershure; Kenneth L. Schepler

2005-01-01

63

Adsorption of lanthanoid ions on calcite.  

PubMed

The adsorption of 14 trivalent lanthanoid ions and yttrium ion (denoted by Ln3+) on calcite surfaces was investigated under various solution conditions of pH (pH = 6.8-7.8) and calcium ion concentration (pCa = -log[Ca2+]= 2.0 and 3.0), and different surface conditions of calcite crystals (well-developed and rough surfaces). The lanthanoid ions were equilibrated in a solution of ionic strength 0.1 mol dm-3(NaCl) saturated with calcite at 25.0 degrees C using excess (solid) calcite crystals suspended in solution. The concentrations of the lanthanoid ions on the calcite crystals (C(cry)/mol kg-1) and in solution (C(soln)/mol dm-3) were determined by means of inductively coupled plasma-mass spectrometry (ICP-MS). It is found that the distribution ratio (D=C(cry)/C(soln) decreases as the atomic number of the lanthanoid increases showing the so called Tetrad Effect. D values increase with increasing pH, whereas they are independent of the calcium ion concentration (i.e., carbonate ion concentration). These results indicate that lanthanoid ions are adsorbed on the calcite surface together with hydroxide ions, i.e., the adsorption of hydroxo-complexes. The heavy lanthanoid ions (Er3+ to Lu3+) are adsorbed as monohydroxo-complexes, (Ln(OH)2+), whereas those of the light lanthanoids are predominantly adsorbed as dihydroxo-complexes (Ln(OH)2+). Other lanthanoids show competitive adsorption reactions of mono- and dihydroxo complexes. Both successive adsorption constants of hydroxo complexes increase with decreasing atomic number of the lanthanoid. The rough surface of calcite is quite active and the distribution ratio of the lanthanoid ions on the rough surface is much higher than that on the well-developed crystalline surface. Rates of adsorption of lanthanide ions were measured and mechanisms are being discussed PMID:16279010

Sawada, K; Abdel-Aal, K; Tan, K; Satoh, K

2005-10-21

64

Growth of lead magnesium niobate-lead titanate single crystals by seeded polycrystal conversion  

NASA Astrophysics Data System (ADS)

Relaxor-based ferroelectric single crystals of Pb(Mg1/3Nb 2/3)O3-PbTiO3 [PMN-PT] have been produced via the Seeded Polycrystal Conversion [SPC] technique. Polycrystalline precursors of PMN-PT are converted to single crystals by inducing the boundary of a seed crystal to migrate through a polycrystalline matrix. The quality of PMN-PT single crystals grown by SPC is directly influenced by the microstructure of the polycrystalline precursor. The goal of this work was to examine the factors that controlled the final microstructure of the matrix and grown single crystals and to characterize their effects on properties. Sintering in oxygen was necessary to obtain fully dense matrix microstructures, which, in turn, produced pore-free single crystals. The primary role of oxygen was to remove insoluble gases before densification began, then to reduce the internal pressure by diffusing out and allowing the pores to close from pressures derived from their own curvatures. The presence of a PbO liquid phase assisted this process by enhancing the densification rate in the early stages of sintering. In contrast, fully-dense, hot-pressed samples of PMN-35PT with excess PbO underwent a de-densification process during the annealing treatments which was attributed to pore formation from internally evolved gases. Transparent single crystals with maximum strain values of 0.72% at 46 kV/cm, d33 ˜2180 pC/N, and a room temperature dielectric constant of ˜5300 were obtained for poled <001> oriented crystals of PMN-30mol.%PT. The dielectric behavior of {001} oriented single crystals was found to be highly dependent on the initial orientation of the seed crystal. For example, {111} seeded crystals grown in oxygen yielded Kmax values of ˜33000, while {001} seeded crystals grown in oxygen had Kmax values of ˜25000. In addition, the Tmax values were 3 to 5°C higher for crystals grown from {001} seeds, compared to {111} seeds.

Scotch, Adam Matthew

65

Cryogenic cave calcite from several Central European caves: age, carbon and oxygen isotopes and a genetic model  

Microsoft Academic Search

Cryogenic cave calcite (CCC), formed by segregation of solutes during water freezing, was found in three Central European caves. This calcite type forms accumulations of loose calcite grains on cave floor. The calcite grains are of highly variable crystal morphology, and of sizes ranging from less than 1 mm to over 1 cm. The most typical feature is their accumulation

Karel Žák; Jan Urban; Václav C??lek; Helena Hercman

2004-01-01

66

Atomistic simulation of the differences between calcite and dolomite surfaces  

Microsoft Academic Search

Atomistic simulation methods have been used to calculate and compare the surface structures and energies of the {1014}, {0001}, {1010}, {1120} and {1011} surfaces of calcite and dolomite and to evaluate their equilibrium morphologies. The calcite {1014} and the dolomite {1010} and {1120} surfaces are the most stable crystal planes. Investigation of the segregation of Mg and Ca ions in

J. O. Titiloye; N. H. de Leeuw; S. C. Parker

1998-01-01

67

Dislocation Creep in Magnesium Calcite  

Microsoft Academic Search

To investigate the effect of dissolved Mg on plastic deformation of calcite, we performed triaxial deformation experiments on synthetic calcite with varying amount of Mg content. Mixtures of powders of calcite and dolomite were isostatically hot pressed (HIP) at 850° C and 300 MPa confining pressure for different intervals (2 to 20hrs) resulting in homogeneous aggregates of high-magnesium calcite; Mg

L. Xu; X. Xiao; B. J. Evans

2003-01-01

68

Deep-Sea Authigenic Calcite and Dolomite.  

National Technical Information Service (NTIS)

Crystals of calcite and, in one case, of dolomite up to 90 millimicron in size are dispersed in pelagic red clay at several sites on the floor of the South Pacific Ocean. They were analyzed by microscopic, x-ray diffraction, electron x-ray microprobe, and...

E. Bonatti

1966-01-01

69

Deep-Sea Authigenic Calcite and Dolomite  

Microsoft Academic Search

Crystals of calcite and, in one case, of dolomite up to 90 mu m in size are dispersed in pelagic red clay at several sites on the floor of the South Pacific Ocean. They were analyzed by microscopic, x-ray diffraction, electron x-ray microprobe, and oxygen isotopic techniques. These carbonates are authigenic and were probably precipitated from hydrothermal solutions connected with

Enrico Bonatti

1966-01-01

70

Mineralogcial Biosignature of High-Magnesian Calcite  

NASA Astrophysics Data System (ADS)

Some dolomite formations are associated with sediments where sulfate reduction is active. In general, sulfate has been considered to inhibit dolomite formation. However, it was also suggested that the reaction products of sulfate reduction, such as increased alkalinity, may play a more important role in increasing the formation rate of dolomite than sulfate does in inhibiting it. We have investigated the effect of sulfide on Mg-Ca carbonate precipitation. Our results show that high-magnesian calcite which contains as much as 34 mol% magnesite can precipitate from solutions with the presence of certain amount of dissolved sulfide and pyrite crystals. These high-magnesian calcites form unusual micro-scale spheres with cracks on them. High- resolution SEM and TEM results show that these spheres are actually composed of a great number of nano- crystals (~50 nm) stacking together with low-angle grain boundaries. Both synthesized and natural high-magnesian calcites indicate that dissolved and solid sulfides can catalyze the Mg incorporation into calcite lattice. We hypothesize that S2- (or HS-) ions and >S2 on pyrite surface will decrease the hydration / slovation energy of magnesium ions and thus promote nucleation and crystallization of the high-magnesian calcite. In nature environment, the sulfide could be related to microbe-induced sulfate reduction at low temperature using organic carbon and methane as electron donors. Such kind of high-magnesium calcite that is rich in light carbon also indicates involvement of sulfate reducing bacteria. This work is supported by NASA Astrobiology Institute (N07-5489).

Zhang, F.; Konishi, H.; Xu, H.; Roden, E. E.

2008-12-01

71

Paleotransport of lanthanides and strontium recorded in calcite compositions from tuffs at Yucca Mountain, Nevada, USA  

Microsoft Academic Search

Secondary calcite occurs in both saturated and unsaturated hydrologic zones (SZ and UZ, respectively) in the tuffs at Yucca Mountain, Nevada, USA. In the upper UZ, the major constituents of the calcite crystal structure (C, O) have surface origins. At greater depth there is a “barren zone,” straddling the water table, where calcite is rare and mixing of surface and

David T. Vaniman; Steve J. Chipera

1996-01-01

72

Optical investigations of ZnSe crystals grown by the seeded vapour phase transport technique  

Microsoft Academic Search

The optical properties of ZnSe bulk crystals grown by the seeded physical vapour phase transport technique (SPVT) were studied by low-temperature photoluminescence and room-temperature stimulated emission measurements. Large strain-free ZnSe single crystals with a diameter as large as 5 cm have been grown uniformly without twinning by the SPVT technique. A variety of ZnSe samples, the as-grown and post-growth treated

J. M. Hays; W. Shan; X. H. Yang; J. J. Song; E. Cantwell

1992-01-01

73

Dimeric Crystal Structure of a Bowman-Birk Protease Inhibitor from Pea Seeds  

Microsoft Academic Search

The trypsin\\/chymotrypsin inhibitors from winter pea seeds (PsTI) are members of the Bowman-Birk protease inhibitor (BBPI) family. The crystal structure of the isoform PsTI-IVb was determined by molecular replacement at 2.7Å resolution using the X-ray co-ordinates of the soybean inhibitor as a search model. The inhibitor crystallized with a nearly perfect 2-fold symmetric dimer in the asymmetric unit. Although the

Inés Li de la Sierra; Laurence Quillien; Peter Flecker; Jacques Gueguen; Simone Brunie

1999-01-01

74

GaN crystallization by the high-pressure solution growth method on HVPE bulk seed  

Microsoft Academic Search

Recent results of the bulk GaN crystallization by the high-pressure solution (HPS) growth method are presented. The new experimental configurations for seeded growth on HVPE free-standing GaN crystals are proposed. The growth rate, morphology and basic physical properties of the pressure-grown materials are reported. The finite element calculation is used for modeling the convective transport in the solution. The convectional

M. Bockowski; P. Strak; I. Grzegory; B. Lucznik; S. Porowski

2008-01-01

75

Controls on the precipitation of barite (BaSO 4) crystals in calcite travertine at Twitya Spring, a warm sulphur spring in Canada's Northwest Territories  

Microsoft Academic Search

Twitya Spring discharges warm (24 °C), anoxic, sulphide-, calcium- (65 ppm) and barium- (?0.78 ppm) rich spring water to a steep flow path that is inhabited by streamer and mat-forming microbes (Thiothrix, Beggiatoa, Oscillatoria, Spirulina, diatoms, rod shaped bacteria). Oxidation and CO2 degassing drive precipitation of elemental sulphur, barite, opaline silica, and calcite. A mound of travertine at the base of the flow

Sandy M. Bonny; Brian Jones

2008-01-01

76

Massive cellular disruption occurs during early imbibition of Cuphea seeds containing crystallized triacylglycerols.  

PubMed

The transition from anhydrobiotic to hydrated state occurs during early imbibition of seeds and is lethal if lipid reserves in seeds are crystalline. Low temperatures crystallize lipids during seed storage. We examine the nature of cellular damage observed in seeds of Cuphea wrightii and C. lanceolata that differ in triacylglycerol composition and phase behavior. Intracellular structure, observed using transmission electron microscopy, is profoundly and irreversibly perturbed if seeds with crystalline triacylglycerols are imbibed briefly. A brief heat treatment that melts triacylglycerols before imbibition prevents the loss of cell integrity; however, residual effects of cold treatments in C. wrightii cells are reflected by the apparent coalescence of protein and oil bodies. The timing and temperature dependence of cellular changes suggest that damage arises via a physical mechanism, perhaps as a result of shifts in hydrophobic and hydrophilic interactions when triacylglycerols undergo phase changes. Stabilizers of oil body structure such as oleosins that rely on a balance of physical forces may become ineffective when triacylglycerols crystallize. Recent observations linking poor oil body stability and poor seed storage behavior are potentially explained by the phase behavior of the storage lipids. These findings directly impact the feasibility of preserving genetic resources from some tropical and subtropical species. PMID:16779553

Volk, Gayle M; Crane, Jennifer; Caspersen, Ann M; Hill, Lisa M; Gardner, Candice; Walters, Christina

2006-06-09

77

Catalytic Biomineralization of Fluorescent Calcite by the Thermophilic Bacterium Geobacillus thermoglucosidasius?  

PubMed Central

The thermophilic Geobacillus bacterium catalyzed the formation of 100-?m hexagonal crystals at 60°C in a hydrogel containing sodium acetate, calcium chloride, and magnesium sulfate. Under fluorescence microscopy, crystals fluoresced upon excitation at 365 ± 5, 480 ± 20, or 545 ± 15 nm. X-ray diffraction indicated that the crystals were magnesium-calcite in calcite-type calcium carbonate.

Yoshida, Naoto; Higashimura, Eiji; Saeki, Yuichi

2010-01-01

78

Nucleation studies and characterization of potassium thiocyanate added KDP crystals grown by seed rotation technique  

NASA Astrophysics Data System (ADS)

The effect of the addition of potassium thiocyanate on potassium dihydrogen phosphate (KDP) crystals, grown from aqueous solution by the temperature lowering method using a microcontroller based seed rotation technique has been studied. As part of nucleation studies, metastable zone width, induction period and crystal growth rate of additive added KDP are determined and analyzed with the pure system. Dielectric measurements were carried out on pure and doped crystals at various temperatures ranging from 313 to 423 K and compared. The crystalline perfection of the grown crystal was studied by the high resolution X-ray diffraction analysis. The crystal grown from additive added solution was subjected to structural, optical transmission, second harmonic generation and hardness studies and the effect of additive on pure system is investigated.

Dhanaraj, P. V.; Rajesh, N. P.; Mahadevan, C. K.; Bhagavannarayana, G.

2009-08-01

79

Transformation of echinoid Mg calcite skeletons by heating  

NASA Astrophysics Data System (ADS)

Interambularcral plates of echinoid Heterocentrotus trigonarius, composed of Mg calcite 1 (?14 mol% MgCO 3), were heated in three timed series of experiments at 300°C. Dried plate fragments and fragments with added water were heated separately in pressurized bombs. X-ray powder diffractometry, unit cell dimensions, and phase compositions are used to monitor reaction progress. After 10 h heating in the bombs dolomite (43.5 mol% MgCO 3) and Mg calcite appear (4-7 mol% MgCO 3); by 20 h all Mg calcite 1 is consumed, and at 120 h dolomite composition has evolved to ?47 mol% MgCO 3 and calcite to ?2 mol% MgCO 3. Whole plates heated at 300°C in an open muffle furnace develop dolomite (?42 mol% MgCO 3) and Mg calcite 2 (?6 mol% MgCO 3) after 10 h and remain compositionally invariant throughout subsequent heating to 620 h. Limited skeletal water catalyzes the early reaction but escapes from the open furnace and consequently reaction ceases after ?10 h. The experimentally produced dolomite has relative Mg-Ca ordering of 75% to 79%. The stabilization of echinoid Mg calcite by heating at 300°C to a mixture of dolomite and calcite occurs through a dissolution/precipitation reaction. The alteration fabric produced within the stereom consists of irregularly shaped, branched dolomite crystals > 5 ?m homoaxially set in a calcite 2 (bomb) or Mg calcite 2 (furnace) matrix. Round and tubular pores 1 to 5 ?m are randomly distributed throughout this fabric. The stereom pore system remains intact during furnace heating but is destroyed during heating in bombs. The texture of experimentally stabilized echinoid skeletons is different from that of fossil echinoderms that are composed of microrhomic dolomite homoaxially set in a single calcite crystal.

Dickson, J. A. D.

2001-02-01

80

A novel calcium oxalate crystal growth inhibitory protein from the seeds of Dolichos biflorus (L.).  

PubMed

Recurrence and persistent side effects of present day treatment for urolithiasis restrict their use, so an alternate, using phytotherapy is being sought. Dolichos biflorus seeds, which are used as dietary food in India, possess antilithiatic properties. In the present study, a novel dimeric antilithiatic protein (98 kDa) from its seeds was purified based on its ability to inhibit calcium oxalate crystallization in vitro. Amino acid analysis of Dolichos biflorus antilithiatic protein showed abundant acidic amino acids. The mascot search engine presented sequence similarity with a calcium binding protein, calnexin of Pisum sativum from the m/z data obtained by MALDI TOF mass spectrometer. Above results demonstrate the anticalcifying/antilithiatic nature of a novel protein from the seeds of Dolichos biflorus and thus open new vistas for using plant proteins as therapeutic agents to treat urolithiasis. PMID:19488841

Bijarnia, Rakesh K; Kaur, Tanzeer; Singla, Surinder K; Tandon, Chanderdeep

2009-05-01

81

Multi feed seed (MFS) high pressure crystallization of 1–2 in GaN  

NASA Astrophysics Data System (ADS)

The growth and physical properties of GaN crystallized in a multi feed-seed (MFS) configuration by High Nitrogen Pressure Solution (HNPS) growth method are presented in detail. The conversion of free standing HVPE-GaN crystals to free standing HNPS-GaN is the basis of the MFS configuration. The influence of the experimental conditions (i.e. growth temperature, temperature gradient, etc.), the c-plane bowing of the initial substrate, the electrical properties of HNPS-GaN, and the rate and mode of growth from solution are analyzed. We show that the HNPS-GaN crystals have better structural quality than their HVPE-GaN seeds. The defect density decreases with increasing growth temperature, reaching 5×105 cm?2 for crystals grown at 1420 °C or higher. In contrast, the free carrier concentration in HNPS-GaN increases with increasing growth temperature, reaching 7×1019 cm?3 for samples crystallized at 1440 °C. Thus the possibility to obtain good quality plasmonic GaN substrates for laser diodes can be realized.

Bockowski, M.; Grzegory, I.; Lucznik, B.; Sochacki, T.; Nowak, G.; Sadovyi, B.; Strak, P.; Kamler, G.; Litwin-Staszewska, E.; Porowski, S.

2012-07-01

82

Novel Fabrication Method for Transparent Conducting Oxide Films Utilizing Solid-Phase Crystallized Seed Layers  

NASA Astrophysics Data System (ADS)

High quality ZnO:Al (AZO) films have been obtained by our novel fabrication method utilizing solid-phase crystallized (SPC) seed layers. The crystal grain size of AZO films with seed layers is 5 times larger than that of conventional films, which is due to the low nuclei density of seed layers. As a result, the resistivity of AZO films of 18 nm in total film thickness is drastically reduced from 40×10-4 ? cm for the conventional films to 5×10-4 ? cm for our films. No differences in transparency hve been observed between AZO films with and without seed layers and their optical transmittance is higher than 80% in a wide wavelength range of 400-2500 nm. These results reveal that our method is very promising for fabrication of transparent conducting oxide films. Our method opens up a new avenue for oxide fabrications of nm thick films with excellent crystallinity, because it can be easily extended to other oxides whose amorphous phase is difficult to obtain.

Itagaki, Naho; Kuwahara, Kazunari; Nakahara, Kenta

2011-12-01

83

SEM observations on the replacement of Bahaman aragonitic mud by calcite  

NASA Astrophysics Data System (ADS)

Conversion of Holocene aragonitic mud to calcite microspar occurs in fresh pore waters beneath hammocks on the tidal flats of west Andros Island, Bahamas, to a subsurface depth of 2.7 m. The conversion process involves both dissolution of aragonite and precipitation of calcite as a cement in pre-existing pores and displacive calcite crystallization. Neomorphism of aragonite to calcite with preservation of grain shape has not been observed. Resulting diagenetic products are lithified nodules, composed of well-sorted interlocking rhombs of microspar and of stiff, unlithified calcitic aragonite mud.

Steinen, Randolph P.

1982-09-01

84

Crystallization and preliminary X-ray diffraction analysis of a lectin from Canavalia maritima seeds  

PubMed Central

A lectin from Canavalia maritima seeds (ConM) was purified and submitted to crystallization experiments. The best crystals were obtained using the vapour-diffusion method at a constant temperature of 293?K and grew in 7?d. A complete structural data set was collected to 2.1?Å resolution using a synchrotron-radiation source. The ConM crystal belongs to the orthorhombic space group P21212, with unit-cell parameters a = 67.15, b = 70.90, c = 97.37?Å. A molecular-replacement search found a solution with a correlation coefficient of 69.2% and an R factor of 42.5%. Crystallographic refinement is under way.

de Almeida Gadelha, Carlos Alberto; Moreno, Frederico Bruno Mendes Batista; Santi-Gadelha, Tatiane; Cajazeiras, Joao Batista; da Rocha, Bruno Anderson M.; Rustiguel, Joane Kathelen Rodrigues; Freitas, Beatriz Tupinamba; Canduri, Fernanda; Delatorre, Plinio; de Azevedo, Walter Filgueira; Cavada, Benildo S.

2005-01-01

85

SIMULATION OF ROCKET SEEDING OF CONVECTIVE CLOUDS WITH COARSE HYGROSCOPIC AEROSOL. 1. CONDENSATION GROWTH OF CLOUD DROPLETS ON SALT CRYSTALS  

Microsoft Academic Search

Laws of aerosol particle distribution at hygroscopic seeding of convective clouds by means of antihail rockets are considered. Influence of salt particle dispersion degree on inten- sity of condensation growth of separate cloud droplets on these particles is investigated. Ana- lytical formulae describing variation of radius of NaCl crystal and cloud droplet growing on the salt crystal until its entire

M. T. Abshaev; A. M. Abshaev; M. K. Zhekamukhov; E. I. Potapov; I. A. Garaba; E. A. Zasavitsky

86

Calcite twin analysis in syntectonic calcite, Cape Fold Belt, South Africa: Implications for fold and cleavage formation within a shallow thrust front  

Microsoft Academic Search

Lithologies of two formations in the frontal ranges of the Cape Fold Belt, South Africa, host synorogenic calcite precipitated at low metamorphic grade during the end-Permian deformation of this Gondwana-wide orogen. The calcite crystals are mechanically twinned and this deformation is linked to both folding and the formation of an axial planar cleavage with associated extensional veins. Twinning strain analysis

John P. Craddock; Alex W. McKiernan; Maarten J. de Wit

2007-01-01

87

Sulfated Macromolecules as Templates for Calcite Nucleation and Growth  

NASA Astrophysics Data System (ADS)

Mineralization of egg and seashells is controlled by an intimate association of inorganic materials with organic macromolecules. Among them, particular polyanionic sulfated macromolecules referred to as proteoglycans have been described to be involved in the calcification of these biominerals. The sulfated moieties of the proteoglycans are part of polymer chains constituted of building-blocks of disaccharide units, referred to as sulfated glycosaminoglycans (GAGs), which are covalently attached to a protein core. By using a sitting drop crystallization assay under controlled conditions of time, pH and reactants concentration, we have tested several sulfated and non-sulfated GAGs (i.e.: dermatan and keratan sulfate, hyaluronic acid and heparin), differing in their sulfonate and carboxylate degree and pattern, in their ability to modify calcium carbonate crystal morphology as observed under scanning electron microscopy. Without the addition of GAGs, regular \\{104\\} rhombohedral calcite crystals were obtained. When hyaluronic acid (HA), a non-sulfated but carboxylated GAG, was added, 20 mm long piles of unmodified calcite crystals were observed. When desulfated dermatan, which is an epimeric form of HA but shorter polymer, having their carboxylate groups in an inverted configuration, was added, isolated rhombohedral \\{104\\} calcite crystals showing rounded corners with planes oriented parallel to the c axis were observed. When dermatan sulfated was added, isolated calcite crystals exhibit a columnar morphology as a \\{hk0\\} cylinder with three \\{104\\} faces forming a cap at both ends. Heparin activity depends on the fraction added. Fast-moving heparin fraction (FM), is an undersulfated, low-molecular-weight heterogeneous polymer, while slow-moving heparin fraction (SM) is an high-molecular-weight homogeneous polymer rich in trisulfated-disaccharide units. When FM was added, isolated calcite crystals displayed rhombohedrical \\{104\\} faces but flat corners of \\{111\\} faces. The addition of the hypersulfated heparin SM induce the formation of large rosette-like aggregated calcite crystals, where the majority of the \\{104\\} faces appeared not to be lost, although aggregation is done by different kind of faces. It is concluded that, the variation of the sulfate and carboxylate content and configuration drastically changed the morphology of the calcite crystals. The production of calcite particles with defined morphologies could be interesting for the design of novel materials with desirable shape- and texture-depending properties. Granted by FONDAP 11980002.

David, M.; Passalacqua, K.; Neira, A. C.; Fernandez, M. S.

2003-12-01

88

Characterization of physicochemical and thermal properties and crystallization behavior of krabok (irvingia malayana) and rambutan seed fats.  

PubMed

Fatty acid composition, physicochemical and thermal properties and crystallization behavior of fats extracted from the seeds of krabok (Irvingia Malayana) and rambutan (Nephelium lappaceum L.) trees grown in Thailand were studied and compared with cocoa butter (CB). The krabok seed fat, KSF, consisted of 46.9% lauric and 40.3% myristic acids. It exhibited the highest saponification value and slip melting point but the lowest iodine values. The three fats displayed different crystallization behavior at 25°C. KSF crystallized into a mixture of ?' and pseudo-?' structures with a one-step crystallization curve and high solid fat content (SFC). The fat showed simple DSC crystallization and melting thermograms with one distinct peak. The rambutan seed fat, RSF, consisted of 42.5% arachidic and 33.1% oleic acids. Its crystallization behavior was more similar to CB than KSF, displaying a two-step crystallization curve with SFC lower than that of KSF. RSF solidified into a mixture of ?' and pseudo-?' before transforming to ? after 24 h. The large spherulitic microstructures were observed in both KSF and RSF. According to these results, the Thai KSF and RSF exhibited physicochemical, thermal characteristics and crystallization behavior that could be suitable for specific applications in several areas of the food, cosmetic and pharmaceutical industries. PMID:23196867

Sonwai, Sopark; Ponprachanuvut, Punnee

2012-01-01

89

The effects of initial seed size and transients on dendritic crystal growth  

NASA Astrophysics Data System (ADS)

The transient behavior of growing dendritic crystals can be quite complex, as a growing tip interacts with a sidebranch structure set up under an earlier set of conditions. In this work, we report on two observations of transient growth of NH4Cl dendrites in aqueous solution. First, we study growth from initial nearly-spherical seeds. We have developed a technique to initiate growth from a well-characterized initial seed. We find that the approach to steady state is similar for both large and small seeds, in contrast to the simulation findings of Steinbach, Diepers, and Beckermann[1]. Second, we study the growth of a dendrite subject to rapid changes in temperature. We vary the dimensionless supersaturation ? and monitor the tip speed v and curvature ?. During the transient, the tip shape is noticeably distorted from the steady-state shape, and there is considerable uncertainty in the determination of the curvature of that distorted shape. Nevertheless, it appears that the ``selection parameter'' *circ;= 2 d0D / v 2circ remains approximately constant throughout the transient. [1] I. Steinbach, H.-J. Diepers, and C. Beckermann, J. Cryst. Growth, 275, 624-638 (2005).

Dougherty, Andrew; Nunnally, Thomas

2006-03-01

90

Improving the diffraction of full-length human selenomethionyl metavinculin crystals by streak-seeding  

SciTech Connect

Metavinculin is an alternatively spliced isoform of vinculin that has a 68-residue insert in its tail domain (1134 total residues) and is exclusively expressed in cardiac and smooth muscle tissue, where it plays important roles in myocyte adhesion complexes. Mutations in the metavinculin-specific insert are associated with dilated cardiomyopathy (DCM) in man. Crystals of a DCM-associated mutant of full-length selenomethionine-labeled metavinculin grown by hanging-drop vapor diffusion diffracted poorly and were highly sensitive to radiation, preventing the collection of a complete X-ray diffraction data set at the highest possible resolution. Streak-seeding markedly improved the stability, crystal-growth rate and diffraction quality of DCM-associated mutant metavinculin crystals, allowing complete data collection to 3.9 {angstrom} resolution. These crystals belonged to space group P4{sub 3}2{sub 1}2, with two molecules in the asymmetric unit and unit-cell parameters a = b = 170, c = 211 {angstrom}, {alpha} = {beta} = {gamma} = 90{sup o}.

Rangarajan, Erumbi S.; Izard, Tina (Scripps)

2012-06-28

91

Capillary-Seeding Crystallization and Preliminary Crystallographic Analysis of a Solvent-Tolerant Elastase from Pseudomonas aeruginosa Strain K  

PubMed Central

Seeding is a versatile method for optimizing crystal growth. Coupling this technique with capillary counter diffusion crystallization enhances the size and diffraction quality of the crystals. In this article, crystals for organic solvent-tolerant recombinant elastase strain K were successfully produced through microseeding with capillary counter-diffusion crystallization. This technique improved the nucleation success rate with a low protein concentration (3.00 mg/mL). The crystal was grown in 1 M ammonium phosphate monobasic and 0.1 M sodium citrate tribasic dihydrate pH 5.6. The optimized crystal size was 1 × 0.1 × 0.05 mm3. Elastase strain K successfully diffracted up to 1.39 Å at SPring-8, Japan, using synchrotron radiation for preliminary data diffraction analysis. The space group was determined to be monoclinic space group P1211 with unit cell parameters of a = 38.99 ?, b = 90.173 Å and c = 40.60 Å.

Ali, Mohd Shukuri Mohamad; Said, Zatty Syamimi @ Adura Mat; Abd Rahman, Raja Noor Zaliha Raja; Chor, Adam Leow Thean; Basri, Mahiran; Salleh, Abu Bakar

2013-01-01

92

Synthesis and growth of nonlinear infrared crystal material CdSe via seeded oriented temperature gradient solution zoning method  

Microsoft Academic Search

Single crystals of CdSe were grown by using seeded oriented temperature gradient solution zoning (S-TGSZ) method with the\\u000a sizes of 20 mm in diameter and 80 mm in length. The crystals were characterized with X-ray diffraction, transmission spectrophotometer\\u000a and infrared microscope. The transmission spectra showed that the infrared transmission is above 65% and the mean absorption\\u000a was 0.01–0.04 cm?1 in

Youbao Ni; Haixin Wu; Mingsheng Mao; Chen Lin; Ganchao Cheng; Zhenyou Wang

2011-01-01

93

Calcite precipitation rates in the field: Measurement and prediction for a travertine-depositing stream  

SciTech Connect

Rates of calcite precipitation from a travertine-depositing stream were determined from changes in stream composition between consecutive sampling points and were compared with rates predicted from a laboratory-derived rate law. The agreement in rates was generally within an order of magnitude and routinely within a factor of 3. Least agreement between measured and predicted rates was obtained for sections of flowpath where relatively little change in bulk chemical composition occurred, which were the sections with the greatest mass transfer calculation error, and for the stream segment including a waterfall, which was the section with the greatest error in estimated surface area. Reaction rate obtained from the mass of calcite precipitated onto seed crystals placed in the stream significantly underestimated the mass transfer rate. For the travertine-depositing stream of Warm River Cave and Falling Spring Creek, Virginia, the coupling of equilibrium speciation models with mass balance calculations and simple field measurements allowed successful field-based quantification of reaction rates.

Herman, J.S.; Lorah, M.M. (Univ. of Virginia, Charlottesville (USA))

1988-10-01

94

Crystallization and preliminary X-ray diffraction analysis of a new chitin-binding protein from Parkia platycephala seeds  

PubMed Central

A chitin-binding protein named PPL-2 was purified from Parkia platycephala seeds and crystallized. Crystals belong to the orthorhombic space group P212121, with unit-cell parameters a = 55.19, b = 59.95, c = 76.60?Å, and grew over several days at 293?K using the hanging-drop method. Using synchrotron radiation, a complete structural data set was collected to 1.73?Å resolution. The preliminary crystal structure of PPL-2, determined by molecular replacement, presents a correlation coefficient of 0.558 and an R factor of 0.439. Crystallographic refinement is in progress.

Cavada, Benildo S.; Castellon, Rolando E. R.; Vasconcelos, Georg G.; Rocha, Bruno A. M.; Bezerra, Gustavo A.; Debray, Henri; Delatorre, Plinio; Nagano, Celso S.; Toyama, Marcos; Pinto, Vicente P. T.; Moreno, Frederico B. M. B.; Canduri, Fernanda; de Azevedo, Walter F.

2005-01-01

95

Sea urchin tooth mineralization: calcite present early in the aboral plumula.  

PubMed

In both vertebrate bone, containing carbonated hydroxyapatite as the mineral phase, and in invertebrate hard tissue comprised of calcium carbonate, a popular view is that the mineral phase develops from a long-lived amorphous precursor which later transforms into crystal form. Important questions linked to this popular view are: when and where is the crystallized material formed, and is amorphous solid added subsequently to the crystalline substrate? Sea urchin teeth, in which the earliest mineral forms within isolated compartments, in a time and position dependent manner, allow direct investigation of the timing of crystallization of the calcite primary plates. Living teeth of the sea urchin Lytechinus variegatus, in their native coelomic fluid, were examined by high-energy synchrotron X-ray diffraction. The diffraction data show that calcite is present in the most aboral portions of the plumula, representing the very earliest stages of mineralization, and that this calcite has the same crystal orientation as in the more mature adoral portions of the same tooth. Raman spectroscopy of the aboral plumula confirms the initial primary plate mineral material is calcite and does not detect amorphous calcium carbonate; in the more mature adoral incisal flange, it does detect a broader calcite peak, consistent with two or more magnesium compositions. We hypothesize that some portion of each syncytial membrane in the plumula provides the information for nucleation of identically oriented calcite crystals that subsequently develop to form the complex geometry of the single crystal sea urchin tooth. PMID:22940703

Stock, Stuart R; Veis, Arthur; Xiao, Xianghui; Almer, Jonathan D; Dorvee, Jason R

2012-08-24

96

The calcite ??? aragonite transformation in low-Mg marble: Equilibrium relations, transformations mechanisms, and rates  

USGS Publications Warehouse

Experimental transformation of a rather pure natural calcite marble to aragonite marble did not proceed via the expected straightforward polymorphic replacement. Instead, the small amount of Mg in the starting material (0.36 wt %) was excluded from the growing aragonite and diffused preferentially into the remaining calcite grains, producing Mg-rich calcite rods that persisted as relicts. Nucleation of aragonite occurred exclusively on grain boundaries, with aragonite [001] oriented subparallel to calcite [0001]. The aragonite crystals preferentially consumed the calcite crystal on which they nucleated, and the reaction fronts developed preferentially along the {010} and {110} planes of aragonite. Each aragonite neoblast that grew was nearly free of Mg (typically <0.1 wt %). The excess Mg was taken up by the calcite grains in between, stabilizing them and causing a few volume percent rodlike relicts of Mg-enriched calcite (up to 10 wt % MgO) to be left behind by the advancing reaction front. The aragonite growth rates are approximately linear and range from ???3 ?? 10-11 m s-1 at 600??C to ???9 ?? 10-9 m s-1 at 850??C, with an apparent activation enthalpy of 166 ?? 91 kJ mol-1. This reaction mechanism and the resultant texture are akin to cellular precipitation reactions in metals. Similar transformation textures have been reported from high-Mg marbles in Japan and China that disproportionated to low-Mg calcite and dolomite. Copyright 2005 by the American Geophysical Union.

Hacker, B. R.; Rubie, D. C.; Kirby, S. H.; Bohlen, S. R.

2005-01-01

97

Dynamic crystallization of cocoa butter. I. characterization of simple lipids in rapid- and slow-nucleating cocoa butters and their seed crystals  

Microsoft Academic Search

Six cocoa butters with different crystallization induction times and their seed crystals were analyzed for simple lipid composition.\\u000a The rapid-nucleating cocoa butter samples had higher concentrations of 1-palmitoyl-2-oleoyl-3-stearoylglycerol and 1,3-stearoyl-2-oleoylglycerol\\u000a (SOS), and lower concentrations of the diunsaturated triacylglycerols, 1-palmitoyl-2,3-oleoylglycerol and 1-stearoyl-2,3-oleoylglycerol,\\u000a as well as higher stearic acid concentrations within their diacylglycerol fractions when compared to the slow-nucleating samples.\\u000a At the

Siree Chaiseri; Paul S. Dimick

1995-01-01

98

Zweiachsig erscheinende Calcite und Dolomite  

Microsoft Academic Search

The biaxiality of calcites and dolomites is explained by twin lamellae. The amount of the axial angle depends from the part of volume built up by the lamellae of equal orientation, and from the twin angle. The birefringence is of minor influence.

R. Rath; D. Pohl

1971-01-01

99

Unusual calcite cementing of quartz grains on Chandeleur Island Beach, offshore Louisiana  

SciTech Connect

A very unusual calcite cement was found in some beachchips from an unconsolidated beach surface of Chandeleur Island offshore approximately 35 nmi (65 km) south of Mississippi in the Gulf of Mexico. The beachchips are irregularly shaped and are well cemented by this unusual calcite. This calcite crystal structure has not been reported previously as existing in a marine environment. A similar cement has been found in freshwater lake beachrock and in some travertine samples. The calcite crystals are elongate parallel to the c-optic axis, and are composed of bunches of crystallite blades. The crystallite blades of each crystal bunch are pointed and are more bladed than freshwater cement crystals. The intercrystallite pore space contains no fine calcite silt as was observed in the lake samples. Fresh water provided by rainfall may be held in the pore spaces and bounded to the quartz-grain surfaces by ionic attraction. Marine spray above and saline water concentrated underneath from a sandwich effect at the micropore level, allowing rapid growth and precipitation of these very unusual calcite crystals in a single-phase low-salinity fluid.

Mitchell-Tapping, H.J.

1983-09-01

100

Oxygen Isotopic Disequilibrium in Bacteriogenic Soil Calcite Precipitated Near Leaking Oil and Gas Wells in Western Canada  

NASA Astrophysics Data System (ADS)

About one third of all oil and gas wells drilled in Western Canada leak natural gas to surface. Leaking gas is oxidized, sometimes completely, by metanotrophic bacteria in soil near the wells. Depending on soil permeability and rate of gas leakage, zones of bacterial oxidation extend from 20 to >500 cm away from the well bores of leaking wells. Thin coatings and aggregates <300 micrometers across of authigenic calcite of both abiotic and bacteriogenic origin are found in the oxidation zones. Abiotic calcite forms crusts comprised of submicron to micron size sparry, euhedral crystals typical of inorganic soil calcite precipitates. Bacteriogenic calcite is closely associated with bacteria and microbial film and forms massive to porous aggregates of subhedral rounded crystals coating or cementing mineral grains. Growth of nanometer size calcite crystals on top of microbial surfaces indicates active involvement of bacteria with calcite precipitation. To constrain the conditions of calcite precipitation \\delta13C and \\delta18O of soil CO2 and soil temperature at two leaking well sites in Saskatchewan were monitored for ca. two years. The \\delta18O of local soil moisture and groundwater were also measured. Results show that \\delta18O (PDB) of bacteriogenic calcite is from 3 to 6 permil lower than this that would precipitate in isotopic equilibrium with local soil moisture at any time during the year. Although \\delta13C of bacteriogenic calcite may be in isotopic equilibrium with local soil CO2 at temperatures close to freezing, decrease of bacterial metabolic rates at such low temperatures would not favor bacteriogenic calcite precipitation. Therefore, \\delta13C may also reflect disequilibrium. In contrast, both \\delta18O and \\delta13C of abiotic calcite are in isotopic equilibrium with soil moisture and soil CO2 at temperatures close to or higher than the average soil temperatures in the area. The small sizes of bacteriogenic calcite aggregates indicate that these precipitated in confined pools of water where local chemistry and pH are easily modified by the microorganisms. Therefore, oxygen and/or carbon isotopic disequilibrium of bacteriogenic calcite is likely the result of bacterial metabolism. The \\delta18O of soil calcite in paleosols has been widely used as a paleothermometer. Bacteriogenic calcite precipitation however is common in soils and not confined to zones of leaking gas oxidation. Likewise oxygen isotopic disequilibrium of bacteriogenic soil calcite may also be a common phenomenon. Therefore, paleosols should be examined for evidence of bacterial involvement in calcite precipitation prior to the use of \\delta18O of soil calcite as a paleothermometer.

Arkadakskiy, S. V.; Muehlenbachs, K.; Mendoza, C.; Szatkowski, B.

2004-12-01

101

Calcite formation in soft coral sclerites is determined by a single reactive extracellular protein.  

PubMed

Calcium carbonate exists in two main forms, calcite and aragonite, in the skeletons of marine organisms. The primary mineralogy of marine carbonates has changed over the history of the earth depending on the magnesium/calcium ratio in seawater during the periods of the so-called "calcite and aragonite seas." Organisms that prefer certain mineralogy appear to flourish when their preferred mineralogy is favored by seawater chemistry. However, this rule is not without exceptions. For example, some octocorals produce calcite despite living in an aragonite sea. Here, we address the unresolved question of how organisms such as soft corals are able to form calcitic skeletal elements in an aragonite sea. We show that an extracellular protein called ECMP-67 isolated from soft coral sclerites induces calcite formation in vitro even when the composition of the calcifying solution favors aragonite precipitation. Structural details of both the surface and the interior of single crystals generated upon interaction with ECMP-67 were analyzed with an apertureless-type near-field IR microscope with high spatial resolution. The results show that this protein is the main determining factor for driving the production of calcite instead of aragonite in the biocalcification process and that -OH, secondary structures (e.g. ?-helices and amides), and other necessary chemical groups are distributed over the center of the calcite crystals. Using an atomic force microscope, we also explored how this extracellular protein significantly affects the molecular-scale kinetics of crystal formation. We anticipate that a more thorough investigation of the proteinaceous skeleton content of different calcite-producing marine organisms will reveal similar components that determine the mineralogy of the organisms. These findings have significant implications for future models of the crystal structure of calcite in nature. PMID:21768106

Rahman, M Azizur; Oomori, Tamotsu; Wörheide, Gert

2011-07-15

102

An approach for phosphate removal with quartz sand, ceramsite, blast furnace slag and steel slag as seed crystal.  

PubMed

The phosphate removal abilities and crystallization performance of quartz sand, ceramsite, blast furnace slag and steel slag were investigated. The residual phosphate concentrations in the reaction solutions were not changed by addition of the ceramsite, quartz sand and blast furnace slag. The steel slag could provide alkalinity and Ca(2+) to the reaction solution due to its hydration activity, and performed a better phosphate removal performance than the other three. Under the conditions of Ca/P 2.0, pH 8.5 and 10 mg P/L, the phosphate crystallization occurred during 12 h. The quartz sand and ceramsite did not improve the phosphate crystallization, but steel slag was an effective seed crystal. The phosphate concentration decreased drastically after 12 h after addition of steel slag, and near complete removal was achieved after 48 h. The XRD analysis showed that the main crystallization products were hydroxyapatite (HAP) and the crystallinity increased with the reaction time. Phosphate was successfully recovered from low phosphate concentration wastewater using steel slag as seed material. PMID:22378001

Qiu, Liping; Wang, Guangwei; Zhang, Shoubin; Yang, Zhongxi; Li, Yanbo

2012-01-01

103

Crystal structure of a ribonuclease from the seeds of bitter gourd ( Momordica charantia) at 1.75 Å resolution  

Microsoft Academic Search

The ribonuclease MC1 (RNase MC1) from seeds of bitter gourd (Momordica charantia) consists of 190 amino acid residues with four disulfide bridges and belongs to the RNase T2 family, including fungal RNases typified by RNase Rh from Rhizopus niveus and RNase T2 from Aspergillus oryzae. The crystal structure of RNase MC1 has been determined at 1.75 Å resolution with an

Atsushi Nakagawa; Isao Tanaka; Ritsu Sakai; Takashi Nakashima; Gunki Funatsu; Makoto Kimura

1999-01-01

104

Native crystal structure of a nitric oxide-releasing lectin from the seeds of Canavalia maritima.  

PubMed

Here, we report the crystallographic study of a lectin from Canavalia maritima seeds (ConM) and its relaxant activity on vascular smooth muscle, to provide new insights into the understanding of structure/function relationships of this class of proteins. ConM was crystallized and its structure determined by standard molecular replacement techniques. The amino acid residues, previously suggested incorrectly by manual sequencing, have now been determined as I17, I53, S129, S134, G144, S164, P165, S187, V190, S169, T196, and S202. Analysis of the structure indicated a dimer in the asymmetric unit, two metal binding sites per monomer, and loops involved in the molecular oligomerization. These confer 98% similarity between ConM and other previously described lectins, derived from Canavalia ensiformis and Canavalia brasiliensis. Our functional data indicate that ConM exerts a concentration-dependent relaxant action on isolated aortic rings that probably occurs via an interaction with a specific lectin-binding site on the endothelium, resulting in a release of nitric oxide. PMID:16337811

Gadelha, Carlos Alberto de Almeida; Moreno, Frederico Bruno Mendes Batista; Santi-Gadelha, Tatiane; Cajazeiras, João Batista; Rocha, Bruno Anderson Matias da; Assreuy, Ana Maria Sampaio; Lima Mota, Mário Rogério; Pinto, Nilson Vieira; Passos Meireles, Ana Vaneska; Borges, Júlio César; Freitas, Beatriz Tupinamba; Canduri, Fernanda; Souza, Emmanuel Prata; Delatorre, Plínio; Criddle, David Neil; de Azevedo, Walter Filgueira; Cavada, Benildo Sousa

2005-11-14

105

Characterization of Al2O3 coatings oxidized from Al with different proportion of seed crystals at a lower temperature  

NASA Astrophysics Data System (ADS)

Al layer with ?-Al2O3 seed crystals was prepared on the surface of 316L stainless steel (SS) by a double cathodes discharge technique, in which the mixed targets of pure Al doped with different proportions of ?-Al2O3 were used. Then, Al2O3 coatings were obtained after plasma oxidization at 580 °C. The phase composition, microstructure and morphology of the coatings were studied respectively by means of glancing-angle (1°) X-ray diffractometry (GAXRD) and scanning electron microscopy (SEM). Furthermore, the bonding force and corrosion resistance of the coatings were measured. The results indicated that ?-Al2O3 nucleated and grew surrounding the seed crystals as the Volmer-Weber Mode. The Al2O3 coating was compact, performing a good corrosion resistance and metallurgical bonding. The inducing effects of ?-Al2O3 with different fractions were discussed. ?-Al2O3 (5.5 wt.%) was distributed in the Al layer when the target possessing 10% ?-Al2O3 was used. After plasma oxidation, 65.54 wt.% ?-Al2O3 was obtained which was 10.34% more than that obtained by the oxidation of pure Al at the same condition. However, the inducing effects became weak with the further increment of content of ?-Al2O3 seed crystals.

Wang, Chen; Lin, Yuebin; He, Fei; Luo, Xinyi; Tao, Jie

2013-10-01

106

SIMS analyses of minor and trace element distributions in fracture calcite from Yucca Mountain, Nevada, USA  

SciTech Connect

Fracture-lining calcite samples from Yucca Mountain. Nevada. obtained as part of the extensive vertical sampling in studies of this site as a potential high-level waste repository, have been characterized according to microbeam-scale (25-30 {mu}m) trace and minor element chemistry and cathodoluminescent zonation patterns, As bulk chemical analyses are limited in spatial resolution and are subject to contamination by intergrown phases, a technique for analysis by secondary ion mass spectrometry (SIMS) of minor (Mn, Fe, Sr) and trace (REE) elements in calcite was developed and applied to eighteen calcite samples from four boreholes and one trench. SIMS analyses of REE in calcite and dolomite have been shown to be quantitative to abundances < 1 x chondrite. Although the low secondary ion yields associated with carbonates forced higher counting times than is necessary in most silicates. Mn, Fe, Sr, and REE analyses were obtained with sub-ppm detection limits and 2-15% analytical precision. Bulk chemical signatures noted by Vaniman (1994) allowed correlation of minor and trace element signatures in Yucca Mountain calcite with location of calcite precipitation (saturated vs. unsaturated zone). For example, upper unsaturated zone calcite exhibits pronounced negative Ce and Eu anomalies not observed in calcite collected below in the deep unsaturated zone. These chemical distinctions served as fingerprints which were applied to growth zones in order to examine temporal chances in calcite crystallization histories: analyses of such fine-scale zonal variations are unattainable using bulk analytical techniques. In addition, LREE (particularly Ce) scavenging of calcite-precipitating solutions by manganese oxide phases is discussed as the mechanism for Ce-depletion in unsaturated zone calcite. 54 refs., 10 figs., 3 tabs.

Denniston, R.F.; Shearer, C.K.; Layne, G.D. [Univ. of New Mexico, Albuquerque, NM (United States); Vaniman, D.T. [Los Alamos National Lab., NM (United States)

1997-05-01

107

Noncrystallographic calcite dendrites from hot-spring deposits at Lake Bogoria, Kenya  

SciTech Connect

Complex calcite crystals are an integral component of precipitates that form around the orifices of the Loburu and Mawe Moto hot springs on the shores of Lake bogoria, Kenya. Two types of large (up to 4 cm long) noncrystallographic dendrites are important components of these deposits. Feather dendrites are characterized by multiple levels of branching with individual branches developed through crystal splitting and spherulitic growth. Scandulitic (from Latin meaning shingle) dendrites are formed of stacked calcite crystals and are generally more compact than feather dendrites. These developed through the incremental stacking of rectangular-shaped calcite crystals that initially grew as skeletal crystals. Feather and scandulitic dendrites precipitated from the same waters in the same springs. The difference in morphology is therefore related to microenvironments in which they grew. Feather dendrites grew in any direction in pools of free-standing water provided that they were in constant contact with the solute. Conversely, scandulitic dendrites grew on rims of dams where water flowed over the surface in concert with the pulses of spring water. Thus, each calcite crystal in these dendrites represents one episode of crystal growth. The orientation of the component crystals in scandulitic dendrites is controlled by the topography of the dam or surface, not crystallographic criteria. The noncrystallographic dendrites formed from spring waters with initial temperatures of 90--99 C. Surficial water cooling, loss of CO{sub 2}, and presence of other elements that can interfere with crystal growth contributed to the formation of these unusual crystals.

Jones, B. [Univ. of Alberta, Edmonton, Alberta (Canada). Dept. of Geology; Renaut, R.W. [Univ. of Saskatchewan, Saskatoon (Canada). Dept. of Geological Sciences

1995-01-02

108

Capillary-seeding crystallization and preliminary crystallographic analysis of a solvent-tolerant elastase from Pseudomonas aeruginosa strain K.  

PubMed

Seeding is a versatile method for optimizing crystal growth. Coupling this technique with capillary counter diffusion crystallization enhances the size and diffraction quality of the crystals. In this article, crystals for organic solvent-tolerant recombinant elastase strain K were successfully produced through microseeding with capillary counter-diffusion crystallization. This technique improved the nucleation success rate with a low protein concentration (3.00 mg/mL). The crystal was grown in 1 M ammonium phosphate monobasic and 0.1 M sodium citrate tribasic dihydrate pH 5.6. The optimized crystal size was 1 × 0.1 × 0.05 mm³. Elastase strain K successfully diffracted up to 1.39 Å at SPring-8, Japan, using synchrotron radiation for preliminary data diffraction analysis. The space group was determined to be monoclinic space group P12(1)1 with unit cell parameters of a = 38.99 ?, b = 90.173 Å and c = 40.60 Å. PMID:23989606

Ali, Mohd Shukuri Mohamad; Said, Zatty Syamimi Adura Mat; Rahman, Raja Noor Zaliha Raja Abd; Chor, Adam Leow Thean; Basri, Mahiran; Salleh, Abu Bakar

2013-08-28

109

Dislocation Creep in Magnesium Calcite  

NASA Astrophysics Data System (ADS)

To investigate the effect of dissolved Mg on plastic deformation of calcite, we performed triaxial deformation experiments on synthetic calcite with varying amount of Mg content. Mixtures of powders of calcite and dolomite were isostatically hot pressed (HIP) at 850° C and 300 MPa confining pressure for different intervals (2 to 20hrs) resulting in homogeneous aggregates of high-magnesium calcite; Mg content varied from 0.07 to 0.17 mol%. Creep tests were performed at differential stresses from 20 to 160 MPa at 700 to 800° C. Grain sizes before and after deformation were determined from the images obtained from scanning electron microscope (SEM) and optical microscope. Grain sizes are in the range of 5 to 20 microns depending on the HIP time, and decrease with increasing magnesium content. Both BSE images and chemical analysis suggest that all dolomite are dissolved and the Mg distribution is homogeneous through the sample, after 2 hrs HIP. At stresses below 40 MPa, the samples deformed in diffusion region (Coble creep), as described previously by Herwegh. The strength decreases with increasing magnesium content, owing to the difference of grain size. At stresses above 80 MPa, the stress exponent is greater than 3, indicating an increased contribution of dislocation creep. The transition between diffusion to dislocation creep occurs at higher stresses for the samples with higher magnesium content and smaller grain size. Preliminary data suggests a slight increase in strength with increasing magnesium content, but more tests are needed to verify this effect. In a few samples, some strain weakening may have been evident. The activation energy in the transition region (at 80 MPa) is ˜200 KJ/mol with no dependence on magnesium content, agreeing with previous measurements of diffusion creep in natural and synthetic marbles.

Xu, L.; Xiao, X.; Evans, B. J.

2003-12-01

110

Pseudo dissolution of marine calcite  

NASA Astrophysics Data System (ADS)

An enigma exists regarding observations aimed at determining the extent of calcite dissolution on the Ceara Rise in the western equatorial Atlantic. Based on pore-water gradients for Ca, alkalinity, pH, 13C and 14C, a strong case can be constructed that at a depth of 3.3 km about 30% of the calcite rain dissolves [Martin and Sayles, Geochim. Cosmochim. Acta 60 (1996) 243-263; Hales and Emerson, Geochim. Cosmochim. Acta 61 (1997) 501-514; Martin et al., Geochim. Cosmochim. Acta 61 (2000) 1391-1404]. However, benthic chambers deployed at this site record no Ca or alkalinity flux [Jahnke and Jahnke, Geochim. Cosmochim. Acta (2002) in press]. Further, there is no evidence for shell weight loss or shell breakup between 2.8 and 4.1 km water depth on the Ceara Rise [Broecker and Clark, Global Geochem. Cycles (2002) in press]. A possible scenario which accounts for all but the 14C observations [Martin et al., Geochim. Cosmochim. Acta 61 (2000) 1391-1404] is that coatings formed on the calcite are stirred downward into the respiration-CO 2-rich core-top mixed layer and there redissolve. We term this phenomenon pseudo dissolution.

Broecker, W. S.; Clark, Elizabeth

2003-03-01

111

Crystallization and preliminary X-ray diffraction analysis of an anti-H(O) lectin from Lotus tetragonolobus seeds  

PubMed Central

The seed lectin from Lotus tetragonolobus (LTA) has been crystallized. The best crystals grew over several days and were obtained using the vapour-diffusion method at a constant temperature of 293?K. A complete structural data set was collected at 2.00?Å resolution using a synchrotron-radiation source. LTA crystals were found to be monoclinic, belonging to space group P21, with unit-cell parameters a = 68.89, b = 65.83, c = 102.53?Å, ? = ? = 90, ? = 92°. Molecular replacement yielded a solution with a correlation coefficient and R factor of 34.4 and 51.6%, respectively. Preliminary analysis of the molecular-replacement solution indicates a new quaternary association in the LTA structure. Crystallographic refinement is under way.

Moreno, Frederico Bruno Mendes Batista; Martil, Daiana Evelin; Cavada, Benildo Sousa; de Azevedo, Walter Filgueira

2006-01-01

112

Columnar calcites as testimony of diagenetic overprinting at the boundary between Upper Tournaisian dolomites and limestones (Belgium): multiple origins for apparently similar features  

Microsoft Academic Search

In topographic flat areas, sedimentary settings may vary from one outcrop to another. In these settings, calcite precipitates may yield macroscopically similar columnar features, although they are products of different sedimentary or diagenetic processes. Three columnar calcite crystal fabrics, i.e. rosettes, palisade crusts and macro-columnar crystal fans, have been differentiated near and at the contact between Upper Tournaisian dolomites and

PETER N IELSEN; PHILIPPE MUCHEZ; WOUTER H EIJLEN; A. E. T ONY

2005-01-01

113

Microstructural control of calcite via incorporation of intracrystalline organic molecules in shells  

NASA Astrophysics Data System (ADS)

It is widely accepted that organic substances regulate or influence the structure of biominerals, but its direct evidences are not plenty. Here we show that the crystallographic microstructures in biotic calcites arise from incorporated intracrystalline organic molecules (IOMs), through a comparison between biotic calcites in shells and synthetic ones with the IOMs extracted from the shells. Although the prismatic layers of a pearl oyster (Pinctada fucata) and a pen shell (Atrina pectinata) morphologically resemble each other, the crystallographic features of constituent calcites are considerably different; in Pinctada, the IOMs are distributed inhomogeneously to form small-angle grain boundaries and associated crystal defects, whereas in Atrina, the IOMs are distributed almost homogeneously and defects are rare in the calcite crystals. We conducted in vitro calcite syntheses in the presence of the IOMs in EDTA-soluble extracts from the prisms. The IOMs in the extracts from Pinctada and Atrina were incorporated into synthetic calcites in a different manner, exhibiting defect-rich/free features as observed in the natural shells. With regard to amino acid compositions of the IOMs, the extract from Atrina has a higher proportion of acidic amino acids than that from Pinctada, implying that acidic proteins do not correlate directly to their affinity for calcium carbonate crystals.

Okumura, Taiga; Suzuki, Michio; Nagasawa, Hiromichi; Kogure, Toshihiro

2013-10-01

114

Growth of calcium carbonate crystal imitating stalagmite growth in nature  

Microsoft Academic Search

Calcite crystals were prepared by dropping a saturated calcium carbonate aqueous solution on a substrate. The calcite crystals were grown with a growth rate of approximately 0.7?m\\/day under the condition of aqueous temperature of 2°C, aqueous concentration of 0.006mol\\/l, and substrate temperature of 40°C. When the calcite substrate was used, calcite crystals were grown epitaxially. Na2CO3 and CaCl2 aqueous solutions

H. Miyazaki; M. Mizutani; T. Yamashita; H. Aoyama; H. Seue; T. Ota

2006-01-01

115

Role and influence of impurities on GaN crystal grown from liquid solution under high nitrogen pressure in multi-feed-seed configuration  

NASA Astrophysics Data System (ADS)

Role and influence of impurities like: oxygen, indium and magnesium, on GaN crystals grown from liquid solution under high nitrogen pressure in multi-feed-seed configuration is shown. The properties of differently doped GaN crystals are presented. The crystallization method and the technology based on it (for obtaining high quality GaN substrates) are described in details. Some electronic and optoelectronic devices built on those GaN substrates are demonstrated.

Bo?kowski, Michal; Lucznik, Boleslaw; Sochacki, Tomasz; Amilusik, Mikolaj; Litwin-Staszewska, Elzbieta; Piotrzkowski, Ryszard; Grzegory, Izabella

2013-03-01

116

The seeded growth of calcium sulfate dihydrate crystals in NaCl solutions up to 6 m and 90 C  

SciTech Connect

The kinetics of calcium sulfate crystal growth is of importance in various fields, such as geochemistry, desalination technology, petroleum industry, and water and wastewater treatment. The seeded crystal growth rate of calcium sulfate dihydrate was measured as a function of supersaturation in NaCl electrolyte solutions from 0 to 6 m at temperatures of 25, 50, 70, and 90 C. The growth followed a second-order parabolic rate law with activation energies greater than 53 kJ/mol which suggested the surface reaction as the rate-limiting step. It was observed that the rate constant and the activation energy are solution composition dependent. The rate constant increases with NaCl concentration up to 3 molal and then begins to fall slightly. The activation energy dropped from 61 kJ/mol in the pure Ca-SO[sub 4]H[sub 2]O system to 53 kJ/mol in 3.0 m NaCl solutions. The electrolyte effect was similar to the crystal solubility behavior in aqueous electrolyte solutions resulting from the variation of the mean activity coefficient of the crystal with ionic strength. The rate constant is proportional to the crystal solubility and inversely proportional to the edge work or the interfacial tension as expected in the classic BCF model.

He, S.; Oddo, J.E.; Tomson, M.B. (Rice Univ., Houston, TX (United States). Dept. of Environmental Science and Engineering)

1994-03-15

117

Alternative origins for nannobacteria-like objects in calcite  

NASA Astrophysics Data System (ADS)

More than 40 calcite-precipitation experiments were performed under sterile conditions in order to investigate the origins of 25 300 nm spherical-, rod-, and ovoid-shaped objects that have been widely interpreted as evidence of nanometer-scale life (i.e., nannobacteria). Individual experiments included the addition of soluble organic compounds, common species of eubacteria, or phage-induced eubacterial lysates. These experiments indicate that many of the nanometer-scale objects have inorganic or nonnannobacterial origins. In the precipitation experiments, calcite formed euhedral crystals 50 800 nm in diameter and smaller (<50 nm) anhedral or rounded particles or protocrystals. The small anhedral or rounded solids resembled nannobacteria. The relative amount of anhedral or rounded calcite was greatest in experiments with a dissolved organic component. These controlled experiments are in accord with observations that rounded nanometer-scale objects are more common in minerals formed in organic-rich environments. Bacterial fragments occur as rounded to irregularly shaped particles that included cell-wall fragments, expulsed cytoplasm, and relict capsules that also closely resembled nannobacteria. Acid etching of the large euhedral crystals produced in the precipitation experiments also resulted in the formation of nanometer-scale features that resembled nannobacteria in natural carbonates. The shapes of the etching artifacts vary as a function of the strength of the acid and the duration of etching. Much caution is advisable in interpreting the origin of rounded features <50 nm.

Kirkland, Brenda L.; Lynch, F. Leo; Rahnis, Michael A.; Folk, Robert L.; Molineux, Ian J.; McLean, Robert J. C.

1999-04-01

118

Chiral solutes can seed the formation of enantiomorphic domains in a twist-bend nematic liquid crystal  

NASA Astrophysics Data System (ADS)

The twist-bend nematic, an enantiomorphic liquid-crystalline phase, exhibited by the structurally symmetric liquid-crystal dimer CB7CB is induced to form a single domain of uniform handedness, in the bulk, by the addition of the dopant chiral solute (S)-1-phenylethanol. Addition of a nonracemic (or scalemic) mixture of both R and S enantiomers of this solute produced equal volumes of P and M chiral domains for the twist-bend nematic phase. This seeding of the domains in an enantiomorphic nematic conglomerate is revealed using deuterium NMR spectroscopy.

Emsley, James W.; Lesot, Philippe; Luckhurst, Geoffrey R.; Meddour, Abdelkrim; Merlet, Denis

2013-04-01

119

Controls of carbonate mineralogy and solid-solution of Mg in calcite: evidence from spelean systems  

SciTech Connect

Precipitation of carbonate minerals in spelean systems occurs under a wide range of fluid chemistry, Mg-Ca ratios, alkalinities, pH and temperatures; thus, spelean systems provide ideal settings to determine factors controlling the mineralogy of precipitated carbonates and solid-solution of Mg in calcite. Cave waters and actively-precipitating carbonate speleothems were collected from Carlsbad Caverns National Park, New Mexico and the Mammoth-Flint Cave System, Kentucky. Carbonate mineralogy of precipitated phases was determined by x-ray diffraction, and major and minor element composition of waters and accompanying minerals were determined by Atomic Absorption Spectrophotometry. Results demonstrate that at a constant CO3 concentration the precipitation threshold for calcite to aragonite is controlled dominantly by the Mg/Ca ratio of the ambient fluid. Aragonite precipitation is favored by high Mg/Ca ratios. Conversely, with increasing CO3 concentration at constant fluid Mg/Ca ratios, calcite is preferentially precipitated. Solid-solution of Mg in calcite is positively correlated with both increased Mg/Ca ratios and CO3 concentrations. These data suggest that Mg contents of calcite can not be defined solely in terms of a homogeneous distribution coefficient. Rather, Mg concentrations can be also be affected by the CO3 concentration and degree of calcite saturation, suggesting that the rate of crystal growth also plays and important role in Mg solid-solution in calcites.

Gonzalez, L.A.; Lohmann, K.C.

1985-01-01

120

Calcite twin analysis in syntectonic calcite, Cape Fold Belt, South Africa: Implications for fold and cleavage formation within a shallow thrust front  

NASA Astrophysics Data System (ADS)

Lithologies of two formations in the frontal ranges of the Cape Fold Belt, South Africa, host synorogenic calcite precipitated at low metamorphic grade during the end-Permian deformation of this Gondwana-wide orogen. The calcite crystals are mechanically twinned and this deformation is linked to both folding and the formation of an axial planar cleavage with associated extensional veins. Twinning strain analysis reveals a complex, rotational synfolding history in the upper, thin-layered Prince Albert Formation. Twinned calcite within the fold preserves two unique strain events. The primary shortening strain fabric is layer-parallel and transport-parallel (˜north south). The shortening strain overprint (-5.82%) is layer-normal and plunges steeply to the northeast. These results are inconsistent with a flexural-slip folding mechanism. By contrast, a homogeneous syn-cleavage stress strain field is recorded in the underlying, massive thick-bedded Dwyka Tillite. Analyses of calcite twins in clast-hosted, syn-cleavage fibrous calcite and rare limestone clasts define a sub-horizontal N S shortening, and sub-horizontal, E W extension. The intermediate axis (?2) is vertical and preserves shortening (-5.3%). The extension axis (?3) is horizontal and parallel to the clast ‘tension gash’ (fracture) plane. These deformed syn-cleavage calcite materials provide an independent constraint to the debate about contemporaneous stress strain fields associated with folding and formation of an axial planar cleavage.

Craddock, John P.; McKiernan, Alex W.; de Wit, Maarten J.

2007-07-01

121

Seed-mediated growth of noble metal nanocrystals: crystal growth and shape control  

NASA Astrophysics Data System (ADS)

Controlled synthesis of noble metal nanocrystals has received enormous attention due to the ability of tailoring the properties of nanocrystals by tuning their shape, size, and composition. The seed-mediated growth method is one of the most reliable and versatile methods to control the shapes of noble metal nanocrystals. This feature article highlights recent strategies regarding shape-controlled synthesis of noble metal nanocrystals by the seed-mediated growth method, with the aim of introducing new strategies and offering new mechanistic insights into nanocrystal shape evolution. Critical parameters affecting the nucleation and growth of noble metal NCs are systemically introduced and analyzed. New developments of extended seed-mediated growth methods were also introduced. Finally, the perspectives of future research on the seed-mediated growth method are also discussed.

Niu, Wenxin; Zhang, Ling; Xu, Guobao

2013-03-01

122

Sr / Mg ratios of modern marine calcite: Empirical indicators of ocean chemistry and precipitation rate  

NASA Astrophysics Data System (ADS)

Holocene biotic and abiotic marine calcite have a similar range of Mg contents (0 to 22 and 4 to 21 mol% MgCO 3 , respectively), yet biotic calcite has Sr 2+ concentrations that are consistently 1250 ppm higher than those of abiotic calcite. As in laboratory experiments, a positive linear relation is observed between D Sr and calcite Mg content. This produces two distinct linear trends on a plot of Sr 2+ vs. Mg 2+ concentrations. Principal axes of variation for both trends have similar slopes, yet distinctly different Sr 2+ concentration intercepts. (Biotic: y = 0.024 x + 1298, r 2 = 0.70; Abiotic: y = 0.027 x + 47, r 2 = 0.77). The similar slopes of these trends reflect the constancy of Mg / Ca and Sr / Ca ratios of modern seawater. Equations describing the dependence of D Sr on calcite Mg content are derived from both trends (Biotic: D Sr = 3.16 x 10 t -6 (ppm Mg) + 0.169; Abiotic: D Sr = 3.52 x 10 -6 (ppm Mg) + 0.0062). Characterization of Sr-Mg trends for Holocene materials allows comparison with analogous trends from ancient samples to estimate relative changes in seawater Mg / Ca and Sr / Ca ratios. The relatively high Sr contents of biotic calcite result from rapid precipitation rates associated with shell accretion in marine organisms. Calcites precipitated from seawater in laboratory experiments have D sr values that are similar to those of biotic marine calcite, suggesting that both precipitate at approximately the same rate. Our estimates of surface area-normalized precipitation rates in planktonic and benthonic foraminifera are comparable to those of seeded, pH-stat experiments. We conclude that the D Sr values for biotic and experimental marine calcite are kinetically controlled, whereas the lower precipitation rates of abiotic marine calcite yield D Sr values that approximate equilibrium conditions. Experimentally derived equations describing the relation between D Sr and calcite precipitation rate indicate that the offset in Sr content between biotic and abiotic calcite is the result of abiotic precipitation rates that are two to five orders of magnitude lower than those of biotic precipitates. However, observations of naturally occurring marine cements suggest that the five-order-of-magnitude offset best represents natural system processes.

Carpenter, Scott J.; Lohmann, Kyger C.

1992-05-01

123

Influence of temperature on the composition of magnesian calcite overgrowths precipitated from sea water  

SciTech Connect

A constant disequilibrium technique was used to determine the composition of magnesian calcite overgrowths precipitated on pure calcite seeds from artificial sea water at 5, 25 and 40/sup 0/C. The amount of magnesium incorporated in the overgrowths at a given temperature is independent of the precipitation rate over a wide range of saturation states and is believed to correspond to a composition in true equilibrium with sea water. The distribution coefficient of magnesium, D/sub Mg/sup 2 +///sup c/, in the magnesian calcite overgrowths increases almost linearly with temperature, being 0.0121 +- 0.0013 at 5/sup 0/C, 0.0172 +- 0.0022 at 25/sup 0/C, and 0.0271 +- 0.0013 at 40/sup 0/C. These values apply only to magnesian calcites precipitated from standard composition sea water, since a previous study has shown D/sub Mg/sup 2 +///sup c/ to be a function of the (Mg/sup 2 +/)(Ca/sup 2 +/) ratio in the parent solution. Results of this study are compared with values reported previously by other workers, and with the compositional distribution of naturally occurring magnesian calcite cements and ooids found in sea water. It appears that variations in temperature are not sufficient to account for the compositional variability of naturally occurring inorganic marine magnesian calcite cements.

Mucci, A.

1987-07-01

124

Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater  

NASA Astrophysics Data System (ADS)

A constant disequilibrium technique was used to determine the composition of magnesian calcite overgrowths precipitated on pure calcite seeds from artificial seawater at 5, 25 and 40°C. The amount of magnesium incorporated in the overgrowths at a given temperature is independent of the precipitation rate over a wide range of saturation states and is believed to correspond to a composition in true equilibrium with seawater. The distribution coefficient of magnesium, D Mg 2+ c , in the magnesian calcite overgrowths increases almost linearly with temperature, being 0.0121 ± 0.0013 at 5°C, 0.0172 ± 0.0022 at 25°C, and 0.0271 ± 0.0013 at 40°C. These values apply only to magnesian calcites precipitated from standard composition seawater, since a previous study has shown D Mg 2+ c to be a function of the [ Mg 2+ ]/[ Ca 2+ ] ratio in the parent solution. Results of this study are compared with values reported previously by other workers, and with the compositional distribution of naturally occurring magnesian calcite cements and ooids found in seawater. It appears that variations in temperature are not sufficient to account for the compositional variability of naturally occurring "inorganic" marine magnesian calcite cements.

Mucci, Alfonso

1987-07-01

125

Flexible minerals: self-assembled calcite spicules with extreme bending strength.  

PubMed

Silicatein-? is responsible for the biomineralization of silicates in sponges. We used silicatein-? to guide the self-assembly of calcite "spicules" similar to the spicules of the calcareous sponge Sycon sp. The self-assembled spicules, 10 to 300 micrometers (?m) in length and 5 to 10 ?m in diameter, are composed of aligned calcite nanocrystals. The spicules are initially amorphous but transform into calcite within months, exhibiting unusual growth along [100]. They scatter x-rays like twinned calcite crystals. Whereas natural spicules evidence brittle failure, the synthetic spicules show an elastic response, which greatly enhances bending strength. This remarkable feature is linked to a high protein content. With nano-thermogravimetric analysis, we measured the organic content of a single spicule to be 10 to 16%. In addition, the spicules exhibit waveguiding properties even when they are bent. PMID:23493708

Natalio, Filipe; Corrales, Tomas P; Panthöfer, Martin; Schollmeyer, Dieter; Lieberwirth, Ingo; Müller, Werner E G; Kappl, Michael; Butt, Hans-Jürgen; Tremel, Wolfgang

2013-03-15

126

Experimental investigation of surface energy and subcritical crack growth in calcite  

NASA Astrophysics Data System (ADS)

Subcritical cracking behavior and surface energies are important factors in geological processes, as they control time-dependent brittle processes and the long-term stability of rocks. In this paper, we present experimental data on subcritical cracking in single calcite crystals exposed to glycol-water mixtures with varying water content. We find upper bounds for the surface energy of calcite that decrease with increasing water concentration and that are systematically lower than values obtained from molecular dynamics simulations. The relation of surface energy to water concentration can explain water weakening in chalks. The rate of subcritical crack growth in calcite is well described by a reaction rate model. The effect of increasing water on crack velocity is to lower the threshold energy release rate required for crack propagation. The slope of the crack velocity curve remains unaffected, something which strongly suggests that the mechanism for subcritical cracking in calcite does not depend on the water concentration.

RøYne, Anja; Bisschop, Jan; Dysthe, Dag Kristian

2011-04-01

127

Microstructure of calcite in the CM2 carbonaceous chondrite LON 94101: Implications for deformation history during and/or after aqueous alteration  

NASA Astrophysics Data System (ADS)

The microstructure of calcite in the CM2 carbonaceous chondrite LON 94101 has been characterized using electron backscatter diffraction (EBSD) analysis, to reconstruct the parent body deformation history during and/or after aqueous alteration. The results suggest that at least two events of calcite crystallization have taken place during aqueous alteration, and at least three episodes of deformation are recorded by the calcite. The first event of calcite crystallization produced calcite grains scattered throughout the matrix, and the second event formed a calcite vein via localized fluid flow. The first episode of deformation is recorded in the crystallographic preferred orientations of the calcite grains and occurred via a directed stress probably induced by compaction in shallow crustal levels of the parent body. The second episode of deformation is recorded in an e-twin microstructure and it suggests a deformation induced via directed stress by impact processing, also in shallow crustal levels. The third episode of deformation generated subgrains in the calcite vein and in some calcite grains, and fragmented and disrupted the calcite vein. This could have been a result of a relatively forceful deformation event, perhaps when the meteorite was released from its parent body. This study shows that carbonate microstructures in carbonaceous chondrites is a powerful and versatile tool for reconstructing the history of deformation during and/or after aqueous alteration.

Lindgren, Paula; Lee, Martin R.; Sofe, Mahmood; Burchell, Mark J.

2011-06-01

128

Crystallization of GaN by HVPE on pressure grown seeds  

Microsoft Academic Search

Growth of GaN under pressure from solution in gallium results in almost dislocation free plate-like crystals but with size limited to approx. 1 cm (lateral) and 100 mum (thickness). On the other hand, deposition of GaN by HVPE on the pressure grown substrates allows stable crystallization with rates of a few hundreds mum\\/h. The crystals with a thickness exceeding 2

I. Grzegory; B. Lucznik; M. Bockowski; B. Pastuszka; G. Kamler; G. Nowak; M. Krysko; S. Krukowski; S. Porowski

2006-01-01

129

Fabrication of a good-quality single grain YBCO sample through the control of seed crystals  

SciTech Connect

The authors investigated the growth mode of YBa{sub 2}Cu{sub 3}O{sub 7{minus}y} (Y123) grains and its effect on the levitation forces and trapped magnetic field of top-seeded melt processed YBCO samples. When a slab-like Sm-seed was used, undesirable subsidiary Y123 grains were formed, while the formation of the subsidiary Y123 grains was suppressed and a nearly single Y123 grain grew when a thick and wide seed was used. The formation of the subsidiary Y123 grains decreased the levitation forces and trapped magnetic field, due to the presence of grain boundaries with weak link characteristics.

Kim, C.J.; Hong, G.W.; Jee, Y.A.; Han, Y.H.; Han, S.C.; Sung, T.H.

1999-09-01

130

Optimizing seeded casting of mono-like silicon crystals through numerical simulation  

NASA Astrophysics Data System (ADS)

Recently, silicon ingots produced by typical multicrystalline casting systems but having monocrystalline features are entering the photovoltaic market. In order to look into the particular properties of this novel method, the normal silicon casting process is numerically simulated, and compared to an optimized mono-like casting process, based on the use of oriented monocrystalline silicon seeds. The seeding process was optimized by reducing the time spent in the melt in order to reduce the back diffusion of harmful non-feedstock metal impurities such as iron, reducing the thermomechanical stress to avoid dislocation multiplication, and decreasing the overall interface curvature. Additionally, the growth conditions in the zone just above the seeds were fine-tuned to increase the production of vacancy point defects, in the hope of achieving “vacancy passivation” of harmful interstitial iron by moving it to substitutional position.

Black, Andrés; Medina, Juan; Piñeiro, Axa; Dieguez, Ernesto

2012-08-01

131

Micromorphology of calcite-silica deposits, Yucca Mountain, Nevada  

SciTech Connect

High-level nuclear waste placed at the proposed Yucca Mountain repository must remain isolated for many thousands of years. Concern remains that calcite-silica vein deposits along fault fractures near the proposed repository are not solely pedogenic, but rather record upwelling of groundwater and thus possible routes for transporting radionuclides to the land surface. The purpose of this study was to provide an additional test for whether the calcite-silica deposits have a groundwater (phreatic) origin or a pedogenic origin. This test involved comparing the micromorphic features of the controversial vein deposits with the micromorphic features of known pedogenic and phreatic deposits. Using a combination of thin sections, scanning electron microscopy, x-ray diffraction, and cathodoluminescence, the following micromorphic features were compared : argillans, calcified microorganisms, detrital grains, aquatic microfossils, calcite crystal size and sorting, and opal content. Based on these comparisons, the controversial vein deposits are more similar to pedogenic deposits than phreatic deposits, and thus appear to be of pedogenic origin. 71 refs., 4 figs., 3 tabs.

Monger, H.C.; Adams, H.P. [New Mexico State Univ., Las Cruces, NM (United States)

1996-03-01

132

Modeling steps and kinks on the surface of calcite.  

PubMed

This work presents modeling results on the cleavage face of calcite as well as on steps and isolated kinks on this face. We used static lattice energy minimization and interatomic potentials fitted to bulk properties. The energy needed to cleave a bulk calcite crystal along the [1 0 (-)1 4] plane was calculated to be 0.59 J m(-2) in agreement with previous studies using the same potentials. The perfect surface reconstructs in the top few atomic layers, but its symmetry corresponds to the bulk termination. By contrast, the (1 0 (-)1 4) surface with cleavage steps present reconstructs to form a (2 x 1) super cell. This may help explain experimental observations of (2 x 1) symmetry on calcite surfaces. The energy required to form a monatomic obtuse step is calculated to be 1.3 x 10(-10) J m(-1) and for the acute step, 2.4 x 10(-10) J m(-1), suggesting that obtuse steps dominate on cleaved surfaces. Along the two types of steps, a total of 16 kink geometries exist. We calculated kink defect energy with two different approaches: one where kink pairs were added onto infinitely long steps and one where kinks were placed inside pits on a cleavage surface. Calculations on infinitely long steps show that for vacuum conditions, kink pairs possess roughly identical formation energy, about 1.2-2.2 eV, so based on energetics one cannot expect significant differences in kink site frequency PMID:15511175

Kristensen, Rune; Stipp, S L S; Refson, Keith

2004-11-01

133

Modeling steps and kinks on the surface of calcite  

NASA Astrophysics Data System (ADS)

This work presents modeling results on the cleavage face of calcite as well as on steps and isolated kinks on this face. We used static lattice energy minimization and interatomic potentials fitted to bulk properties. The energy needed to cleave a bulk calcite crystal along the {1 0 1 4} plane was calculated to be 0.59 J m-2 in agreement with previous studies using the same potentials. The perfect surface reconstructs in the top few atomic layers, but its symmetry corresponds to the bulk termination. By contrast, the (1 0 1 4) surface with cleavage steps present reconstructs to form a (2×1) super cell. This may help explain experimental observations of (2×1) symmetry on calcite surfaces. The energy required to form a monatomic obtuse step is calculated to be 1.3×10-10 J m-1 and for the acute step, 2.4×10-10 J m-1, suggesting that obtuse steps dominate on cleaved surfaces. Along the two types of steps, a total of 16 kink geometries exist. We calculated kink defect energy with two different approaches: one where kink pairs were added onto infinitely long steps and one where kinks were placed inside pits on a cleavage surface. Calculations on infintely long steps show that for vacuum conditions, kink pairs possess roughly identical formation energy, about 1.2-2.2 eV, so based on energetics one cannot expect significant differences in kink site frequency.

Kristensen, Rune; Stipp, S. L. S.; Refson, Keith

2004-11-01

134

Surface analysis of Ln(2-x)Ce(x)CuO4 (Ln = Pr and Nd) single crystals grown by the top seeded solution method  

Microsoft Academic Search

Large single crystals of Ln(2-x)Ce(x)CU04 (Ln = Pr and Nd), the average size is approximately 5 mm x 5 mm x 1 mm, were grown by the Top Seeded Solution Growth (TSSG) technique. The optical constants of the crystals were determined by Variable Angle Spectroscopic Ellipsometry (VASE) and the results seem to indicate that a 1.5 eV absorption peak appears

Louise C. Sengupta; Somnath Sengupta; Wendy E. Kosik; J. D. Demaree

1992-01-01

135

Direct Observation of Microbial Inhibition of Calcite Dissolution  

PubMed Central

Vertical scanning interferometry (VSI) provides a method for quantification of surface topography at the angstrom to nanometer level. Time-dependent VSI measurements can be used to study the surface-normal retreat across crystal and other solid surfaces during dissolution or corrosion processes. Therefore, VSI can be used to directly and nondestructively measure mineral dissolution rates with high precision. We have used this method to compare the abiotic dissolution behavior of a representative calcite (CaCO3) cleavage face with that observed upon addition of an environmental microbe, Shewanella oneidensis MR-1, to the crystal surface. From our direct observations, we have concluded that the presence of the microbes results in a significant inhibition of the rate of calcite dissolution. This inhibition appears to be a 2nd-order effect that is related to the formation of etch pits. The opening of etch pits was greatly inhibited in the presence of added bacteria, suggesting that the bacterial cells exert their effect by inhibiting the formation of etch pits at high-energy sites at the crystal surface caused by lattice defects, e.g., screw or point dislocations. The experimental methodology thus provides a nondestructive, directly quantifiable, and easily visualized view of the interactions of microbes and minerals during weathering (or corrosion) processes or during mineral precipitation.

Luttge, Andreas; Conrad, Pamela G.

2004-01-01

136

Simulating Succinate-Promoted Dissolution at Calcite {104} Steps  

NASA Astrophysics Data System (ADS)

Organic molecules of a wide range of molecular weights from small organic acids, amino-acids, acidic peptides and acidic proteins to humic and fulvic acids play a key role in modulating nucleation, crystal growth and dissolution of calcium carbonate polymorphs. In general, these acidic molecules inhibit calcite growth and, promote dissolution preferentially along specific crystallographic directions, in the process, regulating crystal shape and size, and even whether a metastable polymorph (e.g., vaterite or aragonite) is nucleated first. For example, chiral faces of calcite are selected by chiral amino-acids and the unusual {hk0} faces are expressed in the presence of amino-acids [Orme et al., 2001], and unusual heptagonal dissolution etch-pit are seen in the presence of succinate compared to the normal rhombohedral pits in water alone [Teng et al., 2006]. Thus, the presence of unusual crystal morphologies may indicate organic-mediated growth, thus serving as a biosignature. We have conducted the Molecular Dynamics (MD) simulations using the Consistent Valence Force Field (CVFF) as implemented in the FORCITE© module of the Materials Studio © software package (Accelrys, Inc. TM) to model the adsorption of succinate, a dicarboxylic acid, and charge- balancing Na+ ions on dry and hydrated steps in different directions on the {104} cleavage face of calcite [Mkhonto and Sahai, in prep.]. At the site of succinate adsorption, we find elongation of the interatomic distances (Ca-OCO3,i) between surface Ca2+ cation and the oxygen of the underlying inorganic CO32- anion the first surface layer of calcite, compared to the corresponding distances in the presence of water alone, suggesting greater ease of surface Ca2+ detachment. This result is consistent with the empirically observed increase in overall dissolution rate with succinate [Teng et al., 2006]. Furthermore, succinate adsorption lowers the step energies, which explains the appearance of steps in the unsusual [42-1] and [010] directions in the presence of succinate as observed by Atomic Force Microscopy [Teng et al., 2006]. Finally, the calculated succinate adsorption energies at hydrated steps decreases in the sequence [-441] > [42-1] > [010], which also corresponds to the sequence of steps opening at dissolution etch-pits in the presence of succinate [Teng et al., 2006]. Our study provides a strong example of the utility of MD simulations in exploring organic-mediated mineral crystal dissolution (and growth) mechanisms, resulting in unusual morphologies, that may serve as biosignatures on Earth, and potentially on Mars and other worlds.

Mkhonto, D.; Sahai, N.

2008-12-01

137

pH-dependence of calcite growth kinetics at constant solution calcium to carbonate activity ratio and supersaturation: an in situ Atomic Force Microscopy study  

Microsoft Academic Search

Calcite-solution reactions (growth, dissolution and replacement) are critical in a range of both engineering and natural processes. Classical crystal growth theory relates calcite growth rates to the degree of supersaturation. The solution composition may also affect the growth rate of carbonate minerals, via the Ca2+ to CO32- concentration ratio (Nehrke et al., 2007; Perdikouri et al., 2009), ionic strength (Zuddas

Encarnación Ruiz-Agudo; Christine V. Putnis; Carlos Manuel Rodriguez-Navarro; Andrew Putnis

2010-01-01

138

Crystallization of Fe(NH4)26H2O on a Seed in Magnetic Fields Up to 140 Koe.  

National Technical Information Service (NTIS)

The present communication reports isothermal crystal growth experiments in high magnetic fields. We find that both the isothermal linear velocity of growth or dissolution R of a seed of Fe(NH4)2 (SO4)2 . 6H2O is increased when a magnetic field H is applie...

M. M. Schieber

1966-01-01

139

Arsenate uptake by calcite: Macroscopic and spectroscopic characterization of adsorption and incorporation mechanisms  

NASA Astrophysics Data System (ADS)

Batch uptake experiments and X-ray element mapping and spectroscopic techniques were used to investigate As(V) (arsenate) uptake mechanisms by calcite, including adsorption and coprecipitation. Batch sorption experiments in calcite-equilibrated suspensions (pH 8.3; PCO 2 = 10 -3.5 atm) reveal rapid initial sorption to calcite, with sorption rate gradually decreasing with time as available sorption sites decrease. An As(V)-calcite sorption isotherm determined after 24 h equilibration exhibits Langmuir-like behavior up to As concentrations of 300 ?M. Maximum distribution coefficient values ( Kd), derived from a best fit to a Langmuir model, are ˜190 L kg -1. Calcite single crystals grown in the presence of As(V) show well-developed rhombohedral morphology with characteristic growth hillocks on (101¯4) surfaces at low As(V) concentrations (?5 ?M), but habit modification is evident at As(V) concentrations ?30 ?M in the form of macrostep development preferentially on the - vicinal surfaces of growth hillocks. Micro-X-ray fluorescence element mapping of (101¯4) surfaces shows preferential incorporation of As in the - vicinal faces relative to + vicinals. EXAFS fit results for both adsorption and coprecipitation samples confirm that As occurs in the 5+ oxidation state in tetrahedral coordination with oxygen, i.e., as arsenate. For adsorption samples, As(V) forms inner-sphere surface complexes via corner-sharing with Ca octahedra. As(V) coprecipitated with calcite substitutes in carbonate sites but with As off-centered, as indicated by two Ca shells, and with likely disruption of local structure. The results indicate that As(V) interacts strongly with the calcite surface, similar to often-cited analog phosphate, and uptake can occur via both adsorption and coprecipitation reactions. Therefore, calcite may be effective for partial removal of dissolved arsenate from aquatic and soil systems.

Alexandratos, Vasso G.; Elzinga, Evert J.; Reeder, Richard J.

2007-09-01

140

The potential for phosphorus pollution remediation by calcite precipitation in UK freshwaters  

NASA Astrophysics Data System (ADS)

This paper examines the potential for calcium carbonate to reduce phosphate pollution in freshwaters by co-precipitation, a process known as a "self cleansing mechanism". Calcium carbonate saturation levels and phosphate concentrations (SRP - soluble reactive phosphate) across the major eastern UK river basins are examined to test for solubility controls. The study shows that calcite saturation varies for each catchment as a function of flow and biological activity rather than by direct regulation by SRP. Indeed, there is no evidence, for any of the rivers studied, that calcite solubility controls hold. However, for groundwater and groundwater-fed springs in the Chalk of the Thames basin, calcite saturation is observed with associated low SRP levels. A self-cleansing mechanism may well be operative within the Chalk due to two factors. Firstly, there is a high potential for nucleation on the calcite micro-crystals in the aquifer. Secondly, there are within aquifer reactions that remove the calcite nucleating inhibitors (SRP and dissolved organic carbon, DOC) to levels lower than those occurring within the rivers do. These inhibitors enter the catchment at very high concentrations in association with agricultural pollution (fertilizer application and animal slurry) and household contamination (e.g. sewage sources from septic tanks). Under low flow conditions, when the saturation index for calcite is at its highest, so too is the concentration of the nucleation inhibitor SRP. Companion work shows that calcite precipitation can occur at the water-sediment interface of the river and this may involve SRP removal. The data, as a whole, define an apparent bound for calcite solubility control where in the presence of nucleating centres, SRP must be less than 4 mM-P l-1 and DOC must be less than 150 mM-C l-1: a condition that does not seem to pertain within most UK rivers.

Neal, Colin

141

Calcite Dissolution in Saline Waters  

NASA Astrophysics Data System (ADS)

The specific effect of ionic strength on the reaction kinetics of calcite dissolution in intermediate to high ionic strength (0.5 < I < 6.0) solutions applicable to natural waters has been investigated using classical free-drift methods where all other parameters (mCa2+, PCO2, and T) have been held constant. Both phosphate-free solutions of potassium chloride (KCl) and sodium chloride (NaCl) as the dominant ionic strength determining salt were investigated where calcium concentrations were held constant in all solutions at approximately 0.010 molal. Reaction rates were found to vary significantly as a function of ionic strength of the reacting solution, which we suggest is due to the lowered activity of water with an increase in ionic strength which decreases the rate of cation hydration. When modeled with the general rate equation, R = k(1-Ømega)n, first-order kinetics (n=1) are sufficient to fit the experimental data. Furthermore, the rate constant (k) appears to be a function of the square root of the ionic strength of the reacting solution. These results may have potential applications to the response of the ocean to acidification by fossil fuel CO2 as well as CO2 sequestration in subsurface saline waters in calcium carbonate-hosted reservoirs.

Finneran, D. W.; Morse, J. W.

2007-12-01

142

Biaxially-Textured Photovoltaic Film Crystal Silicon on Ion Beam Assisted Deposition CaF2 Seed Layers on Glass  

SciTech Connect

We grow biaxially textured heteroepitaxial crystal silicon (c-Si) films on display glass as a low-cost photovoltaic material. We first fabricate textured CaF{sub 2} seed layers using ion-beam assisted deposition, then coat the CaF{sub 2} with a thin, evaporated epitaxial Ge buffer and finally deposit heteroepitaxial silicon on the Ge. The silicon is grown by hot-wire chemical vapor deposition, a high-rate, scalable epitaxy technology. Electron and X-ray diffraction confirm the biaxial texture of the CaF{sub 2} and epitaxial growth of the subsequent layers. Transmission electron microscopy reveals columnar silicon grains about 500 nm across. We fabricate a proof-of-concept epitaxial film c-Si solar cell with an open circuit voltage of 375 mV that is limited by minority carrier lifetime.

Groves, J. R.; Li, J. B.; Clemens, B. M.; LaSalvia, V.; Hasoon, F.; Branz, H. M.; Teplin, C. W.

2012-05-01

143

Timing and mechanism of late-Pleistocene calcite vein formation across the Dead Sea Fault Zone, northern Israel  

NASA Astrophysics Data System (ADS)

The emplacement of calcite-filled veins perpendicular to the Dead Sea Fault Zone in northern Israel reflects strain partitioning during transpression. We present structural, geochemical, and U-Th geochronological data that constrain the mechanism, conditions and timing of vein formation. Vein walls are strongly brecciated and commonly cemented with coarsely crystalline calcite, whereas calcite-filled veins are composed of wall-parallel bands of calcite crystals. Elongated blocky and fibrous calcite crystals grew perpendicular to the vein walls and are characterised by a truncate sealing-hiatus morphology, indicating episodes of partial or complete sealing of the fractures during calcite precipitation. Stable isotope and rare-earth element and yttrium (REY) analyses indicate that calcite-filled veins precipitated by karst processes, involving meteoric water and limited fluid-rock interactions. U-Th dating results show a prolonged history of vein growth. While some veins initiated prior to 500 ka, the majority of the veins were active between 358 and 17 ka. Age constraints on vein activity correspond to an ˜E-W regional shortening phase in this sector of the Dead Sea Fault Zone, associated with an increased component of convergence during the late-Pleistocene.

Nuriel, Perach; Weinberger, Ram; Rosenbaum, Gideon; Golding, Suzanne D.; Zhao, Jian-Xin; Tonguc Uysal, I.; Bar-Matthews, Miryam; Gross, Michael R.

2012-03-01

144

The seeding effect of floating zone growth on Nd1.85Ce0.15CuO4 and Bi2Sr2CaCu2O8- ? single crystals  

NASA Astrophysics Data System (ADS)

Single crystals with the [100] orientation were selected and used as seeds to investigate the effect of travelling solvent floating zone growth on superconducting oxides of Nd1.85Ce0.15CuO4 and Bi2Sr2CaCu2O8-?. The number of nuclei was remarkably reduced and random nuclei could be eased when the seeding was applied during the growth of Nd1.85Ce0.15CuO4 single crystals, compared to the crystals grown without seed. The crystal could preferentially grow on the seed although some additional nuclei occurred at the solid-liquid interface during the initial growth process. In consequence, the crystal ingot obtained is a large single grain having dimensions of 5 mm in diameter and 40 mm in length. The orientation of the seeded growth crystal was found to be 5°off the [100] seed identified by an x-ray Laue pattern. For the growth of Bi2Sr2CaCu2O8-?, it was found that the seed crystal was well melted to decompose and mix with the molten zone flux composition. This led to crystal growth in accordance with the spontaneous nucleation procedure. The chemical composition of the interface between the seed and the grown crystal was determined by EDX. The growth behaviour and crystal habit show no significant change from those when seed samples are not used. This indicates that seeding does not influence the growth of Bi2Sr2CaCu2O8-? for this incongruent melt, i.e., neither seed-orientated nor size-improved crystal is obtained. The crystal size is seen to increase significantly and the number of grains is remarkably reduced with the length of the crystal ingot grown. The crystallographic directions of [100] and [010] are well aligned along the growth direction of the crystal ingot. The composition of the compounds and secondary phases were determined in the boundary region between the seed and newly grown crystals. The superconducting transition temperatures were also determined for the as-grown and post-annealed samples.

Lin, C. T.; Maljuk, A.; Liang, B.

2002-12-01

145

Thermodynamic and kinetic description of dolomitization of calcite and calcitization of dolomite (dedolomitization)  

Microsoft Academic Search

Replacement dolomitization of calcite is a chemical process where dolomite comes to occupy the spatial position and volume\\u000a of calcite through simultaneous dissolution of calcite and precipitation of dolomite without effecting a volume change in\\u000a the crystalline portion of the rock. Dissolution and precipitation occur adjacent to one another in conjunction with a narrow\\u000a aqueous boundary film width which separates

James A. Dockal

1988-01-01

146

Coprecipitation of Chromate with Calcite: Batch Experiments and X-ray Absorption Spectroscopy  

SciTech Connect

Batch experiments, combined with in situ spectroscopic methods, are used to examine the coprecipitation of Cr(VI) with calcite, including partitioning behavior, site-specific distribution of Cr on the surface of calcite single crystals, and local coordination of Cr(VI) in the calcite structure. It is found that the concentration of Cr incorporated in calcite increases with increasing Cr concentration in solution. The calculated apparent partition coefficient, K{sub d}*, is highest at low Cr solution concentration, and decreases to a constant value with increasing Cr solution concentration. DIC images of the (1 0 {bar 1} 4) surface of calcite single crystals grown in the presence of CrO{sub 4}{sup 2-}exhibit well-defined growth hillocks composed of two pairs of symmetrically nonequivalent vicinal faces, denoted as + and -, which reflect the orientation of structurally nonequivalent growth steps. Micro-XRF mapping of the Cr distribution over a growth hillock shows preferential incorporation of Cr into the -- steps, which is considered to result from differences in surface structure geometry. XANES spectra confirm that incorporated Cr is hexavalent, and no reduction of Cr(VI) in the X-ray beam was observed up to 2 days at room temperature. EXAFS fit results show the incorporated Cr(VI) has the expected first shell of 4 O at {approx}1.64 {+-} 0.01 {angstrom}, consistent with CrO{sub 4}{sup 2-}. Best fit results show that the second shell is split with {approx}2.5 Ca at {approx}3.33 {+-} 0.05 and {approx}2.2 Ca at 3.55 {+-} 0.05 {angstrom}, which confirms the incorporation of chromate into calcite. Consideration of possible local coordination indicates that significant distortion or disruption is required to accommodate CrO{sub 4}{sup 2-}in the calcite structure.

Tang,Y.; Elzinga, E.; Lee, Y.; Reeder, R.

2007-01-01

147

Coprecipitation of chromate with calcite: Batch experiments and X-ray absorption spectroscopy  

NASA Astrophysics Data System (ADS)

Batch experiments, combined with in situ spectroscopic methods, are used to examine the coprecipitation of Cr(VI) with calcite, including partitioning behavior, site-specific distribution of Cr on the surface of calcite single crystals, and local coordination of Cr(VI) in the calcite structure. It is found that the concentration of Cr incorporated in calcite increases with increasing Cr concentration in solution. The calculated apparent partition coefficient, Kd?, is highest at low Cr solution concentration, and decreases to a constant value with increasing Cr solution concentration. DIC images of the (101¯4) surface of calcite single crystals grown in the presence of CrO42- exhibit well-defined growth hillocks composed of two pairs of symmetrically nonequivalent vicinal faces, denoted as + and -, which reflect the orientation of structurally nonequivalent growth steps. Micro-XRF mapping of the Cr distribution over a growth hillock shows preferential incorporation of Cr into the—steps, which is considered to result from differences in surface structure geometry. XANES spectra confirm that incorporated Cr is hexavalent, and no reduction of Cr(VI) in the X-ray beam was observed up to 2 days at room temperature. EXAFS fit results show the incorporated Cr(VI) has the expected first shell of 4 O at ˜1.64 ± 0.01 Å, consistent with CrO42-. Best fit results show that the second shell is split with ˜2.5 Ca at ˜3.33 ± 0.05 and ˜2.2 Ca at ˜3.55 ± 0.05 Å, which confirms the incorporation of chromate into calcite. Consideration of possible local coordination indicates that significant distortion or disruption is required to accommodate CrO42- in the calcite structure.

Tang, Yuanzhi; Elzinga, Evert J.; Jae Lee, Young; Reeder, Richard J.

2007-03-01

148

Do organic ligands affect calcite dissolution rates?  

NASA Astrophysics Data System (ADS)

Steady state Iceland-spar calcite dissolution rates were measured at 25 °C in aqueous solutions containing 0.1 M NaCl and up to 0.05 M dissolved bicarbonate at pH from 7.9 to 9.1 in the presence of 13 distinct dissolved organic ligands in mixed-flow reactors. The organic ligands considered in this study include those most likely to be present in either (1) aquifers at the conditions pertinent to CO 2 sequestration or (2) soil/early diagenetic environments: acetate, phthalate, citrate, EDTA 4-, succinate, D-glucosaminate, L-glutamate, D-gluconate, 2,4-dihydroxybenzoate, 3,4-dihydroxybenzoate, fumarate, malonate, and gallate. Results show that the presence of <0.05 mol/kg of these organic anions changes calcite dissolution rates by less than a factor of 2.5 with the exception of citrate and EDTA 4-. The presence of 0.05 mol/kg citrate and EDTA 4- increases calcite dissolution rates by as much as a factor of 35 and 500, respectively, compared to rates in organic anion-free solutions. Further calcite dissolution experiments were performed in the presence of organic polymers similar to bacterial exudates, cell exopolysaccharides, and analogs of microbial cell envelopes: alginate, lichen extract, humic acid, pectin, and gum xanthan. In no case did the presence of <100 ppm of these organics change calcite dissolution rates by more than a factor of 2.5. Results obtained in this study suggest that the presence of aqueous organic anions negligibly affects calcite forward dissolution rates in most natural environments. Some effect on calcite reactivity may be observed, however, by the presence of organic anions if they change substantially the chemical affinity of the fluid with respect to calcite.

Oelkers, Eric H.; Golubev, Sergey V.; Pokrovsky, Oleg S.; Bénézeth, Pascale

2011-04-01

149

Magnesian calcite sorbent for carbon dioxide capture  

Microsoft Academic Search

Magnesian calcite with controlled properties was synthesized for the removal of carbon dioxide. The results from characterization, reactivity and CO2 capture capacity for different synthesis conditions are reported. The magnesian calcite samples (CaCO3:MgCO3) were synthesized by the coprecipitation of specific amounts of commercially available CaO and MgO by carbon dioxide. Characterization was done with BET, SEM\\/EDS, particle size analysis and

James C. Mabry; Kanchan Mondal

2011-01-01

150

Crystallization and preliminary X-ray studies of a galactose-specific lectin from the seeds of bitter gourd (Momordica charantia).  

PubMed

A galactose-specific lectin from the seeds of bitter gourd (Momordica charantia) is a four-chain type II ribosome-inactivating protein (RIP) resulting from covalent association through a disulfide bridge between two identical copies of a two-chain unit. The available structural information on such four-chain RIPs is meagre. The bitter gourd lectin was therefore crystallized for structural investigation and the crystals have been characterized. It is anticipated that the structure of the orthorhombic crystals will be analysed using molecular replacement by taking advantage of its sequence, and presumably structural, homology to normal two-chain type II RIPs. PMID:20823520

Chandran, Thyageshwar; Sharma, Alok; Vijayan, M

2010-08-26

151

CALCITE PRECIPITATION IN LOW TEMPERATURE GEOTHERMAL SYSTEMS: AN EXPERIMENTAL APPROACH  

Microsoft Academic Search

One of the most common production problems in geothermal fields is calcite (calcium carbonate) scale deposition. Calcite formed in the wellbore and in near wellbore region significantly decreases the output of a production well. Calcite scaling is experienced in almost all the geothermal fields around the world. Calcite may form from hydrolysis, boiling and heating of cooler peripheral fluids. Although

Omer Izgec; Birol Demiral; Henri Bertin; Serhat Akin

152

Are Picoplankton Responsible for Calcite Precipitation in Lakes?  

Microsoft Academic Search

Deposits of lacustrine calcite are important records of en- vironmental changes. In order to interpret these archives, knowledge about the origin of the calcite is essential. It has been accepted that calcite precipitation can be induced by bacteria and algae. However, the detailed mechanisms are still unclear. This review summarizes what is known about the interactions between calcite precipitation and

Maria Dittrich; Martin Obst

2004-01-01

153

A model for the distribution of trace elements between calcite and dolomite  

Microsoft Academic Search

A crystal-growth model is proposed, which allows ions of a trace element to enter the Ca and Mg sites of dolomite in proportion to the size of the ions relative to that of Ca and Mg ions, and which assigns equal portions of the trace element to the Ca site of dolomite and the Ca site of associated calcite. The

R. Kretz

1982-01-01

154

Sea Urchin Spine Calcite Forms via a Transient Amorphous Calcium Carbonate Phase  

Microsoft Academic Search

The skeletons of adult echinoderms comprise large single crystals of calcite with smooth convoluted fenestrated morphologies, raising many questions about how they form. By using water etching, infrared spectroscopy, electron diffraction, and environmental scanning electron microscopy, we show that sea urchin spine regeneration proceeds via the initial deposition of amorphous calcium carbonate. Because most echinoderms produce the same type of

Yael Politi; Talmon Arad; Eugenia Klein; Steve Weiner; Lia Addadi

2004-01-01

155

Inhibition of calcite growth by alginate  

NASA Astrophysics Data System (ADS)

The kinetics of calcite precipitation in the presence of alginate was investigated using the constant composition technique. In the concentration range investigated (0.0002-0.005 g L -1), alginate inhibits calcite precipitation. The extent of inhibition increased with increased alginate concentration and decreased solution supersaturation. Alginate adsorption, derived from normalized calcite precipitation rates, is described satisfactorily by the Langmuir adsorption model. At lowest supersaturation, alginate adsorption onto calcite probably reaches its maximal uptake of 7.5E-4 g m -2, corresponding to surface coverage of one molecule for each 200-300 nm 2, depending on the molecular mass of alginate. This means that one alginate molecule can be bound over 100-150 Ca surface sites. Initially, on the surface of the inhibited calcite, XPS identified alginate but after further time in solution, when the system had recovered, XPS demonstrated that it disappeared from the surface, presumably buried under the newly formed calcite. The alginate affinity constant decreases with increasing supersaturation, evidence for incomplete adsorption. A simple model based on competition between growth and desorption effectively describes the observed change in the adsorption constant.

Lakshtanov, L. Z.; Bovet, N.; Stipp, S. L. S.

2011-07-01

156

Effects of temperature and transport conditions on calcite growth in the presence of Mg 2+: Implications for paleothermometry  

Microsoft Academic Search

This study links direct measurement of Mg-calcite growth kinetics with high-spatial-resolution analysis of Mg contents in experimental crystals, with particular attention to the effects of temperature on growth rate and reactant transport conditions on Mg distribution. In contrast to previous experiments on Mg partitioning into calcite, here the layer-growth mechanism was observed in situ and step speeds precisely measured with

Laura E. Wasylenki; Patricia M. Dove; James J. De Yoreo

2005-01-01

157

Freshwater-phreatic calcite cementation, Schooner Cays, Bahamas  

SciTech Connect

Freshwater-phreatic calcite cementation is an active process on 700 and 2700 yr-old ooid-sand islands in the Schooner Cays, Bahamas. Cement fabrics and textures indicate a general, four-stage model of pore infilling. (1) The precipitation of isolated, decimicron-sized, rhombohedrons of calcite on grain surfaces forms an incipient circumgranular cement. (2) Continued precipitation enlarges crystal sizes and forms new rhombohedral crystals, resulting in a continuous circumgranular rim of cement. (3) Additional cementation quickly masks the circumgranular fabric, producing a partial pore-filling mosaic. (4) The remaining pore space is occluded with a mosaic of calcite cement. Petrographic evidence for the earlier circumgranular rim of cement is not necessarily apparent after the last stage of cementation. Empty pores and all four stages of phreatic-zone cementation were observed in the diagenetically immature 700 yr-old rocks, but only stages 2 through 4 were observed in the diagenetically more mature 2700 yr-old phreatic zone samples. Cements are distributed homogeneously within each pore at every stage, yet because each pore may proceed through the four stages at different rates, each pore can be at a different stage of infilling. This results in an inhomogeneous distribution of cement between pores during the initial stages of cementation. Recognition of a cement stratigraphy similar to that described here should aid in the identification of freshwater-phreatic diagenesis in ancient carbonate rock sequences. Variability in the amount of freshwater-phreatic cement between pores should be expected and not interpreted as the product of different paragenetic sequences.

Budd, D.A.

1985-02-01

158

Crystallization and preliminary X-ray diffraction analysis of the lectin from Canavalia boliviana Piper seeds  

PubMed Central

Plant lectins are the most studied group of carbohydrate-binding proteins. Despite the high similarity between the members of the Diocleinae subtribe (Leguminosae) group, they present differing biological activities. Canavalia boliviana lectin (Cbol) was purified using a Sephadex G-50 column and crystallized in the presence of X-Man by hanging-drop vapour diffusion at 293?K. After optimization, crystals suitable for diffraction were obtained under the condition 0.1?M HEPES pH 7.5 and 3.0?M sodium formate. The crystal belonged to the monoclinic space group C2, with unit-cell parameters a = 126.70, b = 66.64, c = 64.99?Å, ? = 90.0, ? = 120.8, ? = 90.0°. Assuming the presence of a dimer in the asymmetric unit, the solvent content was estimated to be about 46%. A complete data set was collected at 1.5?Å resolution.

Moura, Tales Rocha; Bezerra, Gustavo Arruda; Bezerra, Maria Julia Barbosa; Teixera, Cicero Silvano; Bezerra, Eduardo Henrique Salviano; Benevides, Raquel Guimaraes; da Rocha, Bruno Anderson Matias; de Souza, Luiz Augusto Gomes; Delatorre, Plinio; Nagano, Celso Shiniti; Cavada, Benildo Sousa

2009-01-01

159

Crystallization and preliminary X-ray diffraction analysis of the lectin from Canavalia boliviana Piper seeds.  

PubMed

Plant lectins are the most studied group of carbohydrate-binding proteins. Despite the high similarity between the members of the Diocleinae subtribe (Leguminosae) group, they present differing biological activities. Canavalia boliviana lectin (Cbol) was purified using a Sephadex G-50 column and crystallized in the presence of X-Man by hanging-drop vapour diffusion at 293 K. After optimization, crystals suitable for diffraction were obtained under the condition 0.1 M HEPES pH 7.5 and 3.0 M sodium formate. The crystal belonged to the monoclinic space group C2, with unit-cell parameters a = 126.70, b = 66.64, c = 64.99 A, alpha = 90.0, beta = 120.8, gamma = 90.0 degrees . Assuming the presence of a dimer in the asymmetric unit, the solvent content was estimated to be about 46%. A complete data set was collected at 1.5 A resolution. PMID:19255467

Moura, Tales Rocha; Bezerra, Gustavo Arruda; Bezerra, Maria Julia Barbosa; Teixera, Cícero Silvano; Bezerra, Eduardo Henrique Salviano; Benevides, Raquel Guimarães; da Rocha, Bruno Anderson Matias; de Souza, Luiz Augusto Gomes; Delatorre, Plínio; Nagano, Celso Shiniti; Cavada, Benildo Sousa

2009-02-12

160

Precipitation of calcite induced by Synechocystis sp. PCC6803.  

PubMed

Calcite with laminate structure was successfully prepared by culturing Synechocystis sp. PCC6803 with different concentrations of calcium chloride (CaCl2) in BG11 media. S. PCC6803 was examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser confocal scanning microscope (LCSM) and energy dispersive spectroscopy (EDS). The effects of Ca(2+) concentrations and pH values on calcification were investigated and the micro morphs of the CaCO3 crystals were observed by means of SEM. These results showed that CaCO3 crystals could be more easily formed with increasing the concentration of CaCl2 in S. PCC6803 culture solution. S. PCC6803 could largely bind calcium ions, most of which were present in extracellular polymeric substances and on the cell wall. Inside the cells there were a lot of circular areas rich in calcium ions without the crystallization of calcium. Some cells produced a thicker gelatinous sheath outside of the translucent organic thin layer. And the cells inside also produced major changes that the original chloroplasts were almost transformed into starch grains whose sizes were from 0.5 to 1 ?m with relatively uniform in sizes. At the same time the cell sizes significantly reduced to only about 8-9 ?m almost changing to half of its original diameters. The calcite crystals with a highly preferred orientation induced by S. PCC6803 were observed with X-ray diffraction (XRD). A critical implication was that S. PCC6803 could induce bio-calcification and then mediate the further growth of CaCO3 crystals in the biological system. PMID:23543209

Han, Zuozhen; Yan, Huaxiao; Zhou, Shixue; Zhao, Hui; Zhang, Yan; Zhang, Ningning; Yao, Chuankai; Zhao, Lin; Han, Chunyan

2013-03-31

161

Eu3+ uptake by calcite: preliminary results from coprecipitation experiments and observations with surface-sensitive techniques.  

PubMed

A lack of information in databases for contamination risk assessment about the transport behaviour of the trivalent f-orbital elements in groundwater systems where calcite is at equilibrium motivated this study of Eu(3+) uptake. The free drift technique was used to examine the effects of Eu(3+) concentration, presence of Na(+) or K(+) and temperature, as well as calcite nucleation and precipitation kinetics, on the partitioning of calcite. Changes in surface composition and morphology resulting from exposure of single crystals of Iceland spar to Eu(3+)-bearing solutions were observed with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). First results confirm that calcite has high affinity for Eu(3+). Rates of nucleation and precipitation strongly affect the extent of uptake but the presence of Na(+) and K(+) has no effect, suggesting formation of solid solution as CaCO(3)-EuOHCO(3). Surface-sensitive techniques prove that Eu(3+) is adsorbed to calcite even when the surface is dissolving and adsorption is not accompanied by precipitation of a separate Eu(3+)-solid phase. Adsorbed Eu modifies calcite's dissolution behaviour, roughening terraces and rounding step edges, and producing surface morphology where some surface sites appear blocked. Results imply that Eu(3+) concentrations in natural calcites are limited by Eu(3+) availability rather than by a lack of ability to fit into calcite's atomic structure. This behaviour can probably be expected for other trivalent rare Earth elements (REE), actinides and fission products whose behaviour is similar to that of Eu(3+). These elements are likely to be incorporated within the calcite bulk in systems where it is precipitating and the demonstrated strong partitioning ensures some uptake even where calcite is at or under saturation. PMID:12598092

Stipp, S L S; Lakshtanov, L Z; Jensen, J T; Baker, J A

2003-03-01

162

Upscaling calcite growth rates from the mesoscale to the macroscale.  

PubMed

Quantitative prediction of mineral reaction rates in the subsurface remains a daunting task partly because a key parameter for macroscopic models, the reactive site density, is poorly constrained. Here we report atomic force microscopy (AFM) measurements on the {1014} calcite surface of monomolecular step densities, treated as equivalent to the reactive site density, as a function of aqueous calcium-to-carbonate ratio and saturation index. Data for the obtuse step orientation are combined with existing step velocity measurements to generate a model that predicts overall macroscopic calcite growth rates. The model is quantitatively consistent with several published macroscopic rates under a range of alkaline solution conditions, particularly for two of the most comprehensive data sets, without the need for additional fit parameters. The model reproduces peak growth rates, and its functional form is simple enough to be incorporated into reactive transport or other macroscopic models designed for predictions in porous media. However, it currently cannot model equilibrium or pH effects and it may overestimate rates at high aqueous calcium-to-carbonate ratios. The discrepancies in rates at high calcium-to-carbonate ratios may be due to differences in pretreatment, such as exposing the seed material to SI ? 1.0 to generate/develop growth hillocks, or other factors. PMID:23713769

Bracco, Jacquelyn N; Stack, Andrew G; Steefel, Carl I

2013-06-20

163

Upscaling Calcite Growth Rates From the Mesoscale to the Macroscale  

SciTech Connect

Quantitative prediction of mineral reaction rates in the subsurface remains a daunting task partly because a key parameter for macroscopic models, the reactive site density, is poorly constrained. Here we report atomic force microscopy (AFM) measurements on the calcite surface of monomolecular step densities, treated as equivalent to the reactive site density, as a function of aqueous calcium-to-carbonate ratio and saturation index. Data for the obtuse step orientation are combined with existing step velocity measurements to generate a model that predicts overall macroscopic calcite growth rates. The model is quantitatively consistent with several published macroscopic rates under a range of alkaline solution conditions, particularly for two of the most comprehensive data sets without the need for additional fit parameters. The model reproduces peak growth rates and its functional form is simple enough to be incorporated into reactive transport or other macroscopic models designed for predictions in porous media. However, it currently cannot model equilibrium, pH effects, and may overestimate rates at high aqueous calcium-to-carbonate ratios. The discrepancies in rates at high calcium-to-carbonate ratios may be due to differences in pre-treatment, such as exposing the seed material to SI 1.0 to generate/develop growth hillocks, or other factors.

Bracco, Jacquelyn N [ORNL; Stack, Andrew G [ORNL; Steefel, Carl I [Lawrence Berkeley National Laboratory (LBNL)

2013-01-01

164

Understanding the Molecular-Scale Processes of Biomineralization: The Role of Mg2+ and Sr2+ in Calcite Growth  

NASA Astrophysics Data System (ADS)

Determining the precise role that common inorganic species assume during the complex processes of biomineralization is central to understanding biomineralization strategies and for the unambiguous use of biogenic minerals as paleoclimate indicators. Inorganic impurities influence the solubility, reactivity, and polymorphic expression of biogenic carbonates leading to a dynamic relationship between the evolution of biomineralizing organisms and the long-term chemistry of the world's oceans. Biological control over the minor and trace element contents of skeletal materials is therefore a fundamental aspect of the strategies employed by organisms to govern mineral formation. These vital effects, however, impact the use of biogenic minerals as tools for understanding past climates. Resolving the chemical and physical factors that control trace element incorporation during calcite mineralization is critical to providing the inorganic baseline needed for assessing both paleoclimate signals and biological processes. It is now possible to determine the fundamental thermodynamic and kinetic parameters of calcite growth by making direct quantitative measurements of the microscopic processes occurring at the mineral-water interface. These molecular-scale observations are often able to provide crucial information concerning the actual mechanisms of growth that is otherwise unobtainable through macroscopic techniques. Here we use in situ fluid-cell atomic force microscopy (AFM) to achieve a physical understanding of the role of magnesium and strontium in governing calcite formation. Despite the biogeochemical importance of this system, macroscopic studies have been unable to discern the actual mechanism by which Mg2+ mediates calcite growth and morphology. AFM was used to resolve the controversial mechanism of calcite inhibition by magnesium through molecular-scale determination of the thermodynamic and kinetic controls of Mg2+ on calcite morphology and growth. Comparison of directly observed monomolecular step velocities to standard crystal growth impurity models demonstrated that calcite growth inhibition was due to enhanced mineral solubility through Mg2+ incorporation. Terrace width measurements on calcite growth spirals confirmed that magnesium thermodynamically inhibits calcite growth by independently demonstrating decreased effective supersaturations in the presence of magnesium. In contrast, the addition of strontium as an impurity in the growth system caused measured step velocities to decrease by a mechanism that corresponds to the step-pinning model of impurity inhibition. Therefore strontium provides a kinetic barrier to calcite growth by physically blocking the migration of monomolecular steps. The results of this study indicate that impurity size and ability to form solid solutions that are isomorphous with the host crystal play a major role in determining the specific mechanism of inhibition of biogenic calcite crystals. Further, these fundamental empirical observations may explain why Mg/Ca ratios have proved to be a more reliable paleothermometer than Sr/Ca ratios in biogenic calcite.

Davis, K. J.; Dove, P. M.; De Yoreo, J. J.

2001-12-01

165

Nano-cluster composite structure of calcitic sponge spicules--a case study of basic characteristics of biominerals.  

PubMed

Spicules of calcareous sponges are elaborately shaped skeletal elements that nonetheless show characteristics of calcite single-crystals. Our atomic force microscopic and transmission electron microscopic investigation of the triradiate spicules of the sponge Pericharax heteroraphis reveals a nano-cluster structure with mostly well-aligned small crystal domains and pockets with accumulated domain misalignments. Combined high-resolution and energy-filtering transmission electron microscopy revealed carbon enrichments located in between crystal domain boundaries, which strongly suggests an intercalated network-like proteinaceous organic matrix. This matrix is proposed to be involved in the nano-clustered calcite precipitation via a transient phase that may enable a 'brick-by-brick' formation of composite and yet single-crystalline spicules with elaborate morphologies. This composite cluster structure reduces the brittleness of the material by dissipating strain energy and deflecting crack propagation from the calcite cleavage planes, but the lattice symmetry and anisotropic growth properties of calcite still play a major role in the morphogenesis of these unusual calcite single-crystals. Our structural, crystallographic, textural, and chemical analysis of sponge spicules corroborates the view that nano-clustered crystal growth, induced by organic matrices, is a basic characteristic of biomineralisation that enables the production of composite materials with elaborate morphologies. PMID:16321444

Sethmann, Ingo; Hinrichs, Ruth; Wörheide, Gert; Putnis, Andrew

2006-01-01

166

Soliton self-frequency shift controlled by a weak seed laser in tellurite photonic crystal fibers.  

PubMed

We report the first demonstration of soliton self-frequency shift (SSFS) controlled by a weak continuous-wave (CW) laser, from a tellurite photonic crystal fiber pumped by a 1560 nm femtosecond fiber laser. The control of SSFS is performed by the cross-gain modulation of the 1560 nm femtosecond laser. By varying the input power of the weak CW laser (1560 nm) from 0 to 1.17 mW, the soliton generated in the tellurite photonic crystal fiber blue shifts from 1935 to 1591 nm. The dependence of the soliton wavelength on the operation wavelength of the weak CW laser is also measured. The results show the CW laser with a wavelength tunable range of 1530-1592 nm can be used to control the SSFS generation. PMID:23903161

Liu, Lai; Meng, Xiangwei; Yin, Feixiang; Liao, Meisong; Zhao, Dan; Qin, Guanshi; Ohishi, Yasutake; Qin, Weiping

2013-08-01

167

Ages and Origins of Calcite and Opal in the Exploratory Studies Facility Tunnel, Yucca Mountain, Nevada  

USGS Publications Warehouse

Deposits of calcite and opal are present as coatings on open fractures and lithophysal cavities in unsaturated-zone tuffs at Yucca Mountain, Nevada, site of a potential high-level radioactive waste repository. Outermost layers of calcite and opal have radiocarbon ages of 16,000 to 44,000 years before present and thorium-230/uranium ages of 28,000 to more than 500,000 years before present. These ages are young relative to the 13-million-year age of the host rocks. Multiple subsamples from the same outer layer typically show a range of ages with youngest ages from the thinnest subsamples. Initial uranium-234/uranium-238 activity ratios between 1 and 9.5 show a distinct negative correlation with thorium-230/uranium age and are greater than 4 for all but one sample younger than 100,000 years before present. These data, along with micrometer-scale layering and distinctive crystal morphologies, are interpreted to indicate that deposits formed very slowly from water films migrating through open cavities. Exchanges of carbon dioxide and water vapor probably took place between downward-migrating liquids and upward-migrating gases at low rates, resulting in oversaturation of mineral constituents at crystal extremities and more or less continuous deposition of very thin layers. Therefore, subsamples represent mixtures of older and younger layers on a scale finer than sampling techniques can resolve. Slow, long-term rates of deposition (less than about 5 millimeters of mineral per million years) are inferred from subsamples of outermost calcite and opal. These growth rates are similar to those calculated assuming that total coating thicknesses of 10 to 40 millimeters accumulated over 12 million years. Calcite has a wide range of delta carbon-13 values from about -8.2 to 8.5 per mil and delta oxygen-18 values from about 10 to 21 per mil. Systematic microsampling across individual mineral coatings indicates basal (older) calcite tends to have the largest delta carbon-13 values and smallest delta oxygen-18 values compared to calcite from intermediate and outer positions. Basal calcite has relatively small strontium-87/strontium-86 ratios, between 0.7105 and 0.7120, that are similar to the initial isotopic compositions of the strontium-rich tuff units, whereas outer calcite has more radiogenic strontium-87/strontium-86 ratios between 0.7115 and 0.7127. Isotopic compositions of strontium, oxygen, and carbon in the outer (youngest) unsaturated-zone calcite are coincident with those measured in Yucca Mountain calcrete, which formed by pedogenic processes. The physical and isotopic data from calcite and opal indicate that they formed from solutions of meteoric origin percolating through a limited network of connected fracture pathways in the unsaturated zone rather than by inundation from ascending ground water originating in the saturated zone. Mineral assemblages, textures, and distributions within the unsaturated zone are distinctly different from those deposited below the water table at Yucca Mountain. The calcite and opal typically are present only on footwall surfaces of a small fraction of fractures and only on floors of a small fraction of lithophysal cavities. The similarities in the carbon, oxygen, and strontium isotopic compositions between fracture calcite and soil-zone calcite, as well as the gradation of textures from detritus-rich micrite in the soil to detritus-free spar 10 to 30 meters below the surface, also support a genetic link between the two depositional environments. Older deposits contain oxygen isotope compositions that indicate elevated temperatures of mineral formation during the early stages of deposition; however, in the youngest deposits these values are consistent with deposition under geothermal gradients similar to modern conditions. Correlations between mineral ages and varying Pleistocene climate conditions are not apparent from the current data. Cumulative evidence from calcite and opal deposits indicate

Paces, James B.; Neymark, Leonid A.; Marshall, Brian D.; Whelan, Joseph F.; Peterman, Zell E.

2001-01-01

168

The role of citrate and phthalate during Co(II) coprecipitation with calcite  

NASA Astrophysics Data System (ADS)

The influence of citrate and phthalate on Co coprecipitation with calcite was investigated using a combination of batch experiments, Fourier-transform infra-red (FT-IR) spectroscopy, and thermogravimetric analysis (TGA) over a wide range of precipitation rates. Steady-state growth conditions (at constant [Ca], [Co], DIC, and pH) were generally achieved within 3-5 h, after which Co(II) partitioning into calcite was evaluated. Only minor differences are observed in the partition coefficient ( Kd) trends with and without citrate and phthalate as a function of calcite precipitation rate except at very low rates. Slight inhibition of calcite growth is observed in the presence of citrate or phthalate, which can be attributed to adsorption at surface sites. TGA curves for samples coprecipitated with citrate show a significant mass loss between 375 and 550 °C, whereas the weight-loss curves for the Co-phthalate coprecipitates are indistinguishable from those of the organic-free Co coprecipitates. This indicates that citrate is incorporated into calcite during calcite crystallization, whereas phthalate is excluded. FT-IR spectra for the sample with citrate show a broad absorption in the range 3700-3100 cm -1, which is attributable to water molecules coordinated to citrate coprecipitated with calcite. The preferential incorporation of citrate over phthalate likely reflects differences in both aqueous speciation and conformation of the carboxylate groups. This new finding may provide new insight to the factors that control the behavior of macromolecules and their incorporation into the structure of calcium carbonate during biomineralization.

Lee, Young J.; Reeder, Richard J.

2006-05-01

169

Oxygen isotopes in calcite grown under cave-analogue conditions  

NASA Astrophysics Data System (ADS)

Speleothem oxygen isotopes and growth rates are valuable proxies for reconstructing climate history. There is debate, however, about the conditions that allow speleothems to grow in oxygen isotope equilibrium, and about the correct equilibrium fractionation factors. We report results from a series of carbonate growth experiments in karst-analogue conditions in the laboratory. The setup closely mimics natural processes (e.g. precipitation driven by CO 2-degassing, low ionic strength solution, thin solution film) but with a tight control on growth conditions (temperature, pCO 2, drip rate, calcite saturation index and the composition of the initial solution). Calcite is dissolved in water in a 20,000 ppmV pCO 2 environment. This solution is dripped onto glass plates (coated with seed-carbonate) in a lower pCO 2 environment (<2500 ppmV), where degassing leads to calcite growth. Experiments were performed at 7, 15, 25 and 35 °C. At each temperature, calcite was grown at three drip rates (2, 6 and 10 drips per minute) on separate plates. The mass of calcite grown in these experiments varies with temperature ( T in K) and drip rate ( d_ r in drips min -1) according to the relationship daily growth mass = 1.254 + 1.478 ? 10 -9 ? e0.0679 T + ( e0.00248 T - 2) ? (-0.779 d_ r2 + 10.05 d_ r + 11.69). This relationship indicates a substantial increase of growth mass with temperature, a smaller influence of drip rate on growth mass at low temperature and a non-linear relationship between drip rate and growth mass at higher temperatures. Low temperature, fast dripping conditions are found to be the most favourable for reducing effects associated with evaporation and rapid depletion of the dissolved inorganic carbon reservoir (rapid DIC-depletion). The impact of evaporation can be large so caves with high relative humidity are also preferable for palaeoclimate reconstruction. Even allowing for the maximum offsets that may have been induced by evaporation and rapid DIC-depletion, ? 18O measured in some of our experiments remain higher than those predicted by Kim and O'Neil (1997). Our new results are well explained by equilibrium at a significantly higher ?calcite-water, with a kinetic-isotope effect that favours 16O incorporation as growth rate increases. This scenario agrees with recent studies by Coplen (2007) and Dietzel et al. (2009). Overall, our results suggest that three separate processes cause ? 18O to deviate from true isotope equilibrium in the cave environment. Two of these drive ? 18O to higher values (evaporation and rapid DIC-depletion) while one drives ? 18O to lower values (preferential incorporation of 16O in the solid carbonate at faster growth rates). While evaporation and DIC-depletion can be avoided in some settings, the third may be inescapable in the cave environment and means that any temperature to ? 18O relationship is an approximation. The controlled conditions of the present experiments also display limitations in the use of the Hendy test to identifying equilibrium growth.

Day, C. C.; Henderson, G. M.

2011-07-01

170

Oxygen self-diffusion in calcite: Dependence on temperature and water fugacity  

NASA Astrophysics Data System (ADS)

Oxygen self-diffusion in natural calcite single crystals was studied hydrothermally at 400-800°C and 10-350 MPa confining pressure. Diffusion coefficients ( D) were determined from 18O concentration profiles measured with an ion microprobe. At 100 MPa confining pressure, the Arrhenius parameters yield an activation energy ( Q) = 173 ± 6 kJ/mole and pre-exponential factor ( D0) = 7 × 10 -9 m 2/s over the temperature range 400-800°C, and there is no measurable anisotropy. Constant temperature experiments at 700°C indicate that there is a strong linear correlation (slope = 0.9) of D with water fugacity over the range 4-240 MPa, consistent with the interpretation that, in the presence of water, the dominant oxygen-bearing transport species in calcite is molecular water. A sample containing 1180 ppm Mn shows a marked increase in observed D values at temperatures below ˜ 600°C. However, when pre-annealed at ? 700°C the D values obtained at 550°C are within a factor of four of the value obtained for the low (100 ppm) Mn sample. The results of this study confirm that in contrast to carbon, oxygen diffusion rates in calcite are greatly enhanced when water is present. The difference in the effect of water on Doxygen and Dcarbon in calcite may provide valuable information for evaluating the role of fluids in the thermal histories of calcite-bearing rocks.

Farver, John R.

1994-02-01

171

Low-magnesium calcite produced by coralline algae in seawater of Late Cretaceous composition.  

PubMed

Shifts in the MgCa ratio of seawater driven by changes in midocean ridge spreading rates have produced oscillations in the mineralogy of nonskeletal carbonate precipitates from seawater on time scales of 10(8) years. Since Cambrian time, skeletal mineralogies of anatomically simple organisms functioning as major reef builders or producers of shallow marine limestones have generally corresponded in mineral composition to nonskeletal precipitates. Here we report on experiments showing that the ambient MgCa ratio actually governs the skeletal mineralogy of some simple organisms. In modern seas, coralline algae produce skeletons of high-Mg calcite (>4 mol % MgCO(3)). We grew three species of these algae in artificial seawaters having three different MgCa ratios. All of the species incorporated amounts of Mg into their skeletons in proportion to the ambient MgCa ratio, mimicking the pattern for nonskeletal precipitation. Thus, the algae calcified as if they were simply inducing precipitation from seawater through their consumption of CO(2) for photosynthesis; presumably organic templates specify the calcite crystal structure of their skeletons. In artificial seawater with the low MgCa ratio of Late Cretaceous seas, the algae in our experiments produced low-Mg calcite (<4 mol % MgCO(3)), the carbonate mineral formed by nonskeletal precipitation in those ancient seas. Our results suggest that many taxa that produce high-Mg calcite today produced low-Mg calcite in Late Cretaceous seas. PMID:12399549

Stanley, Steven M; Ries, Justin B; Hardie, Lawrence A

2002-10-24

172

Mathematical model of biofilm induced calcite precipitation.  

PubMed

Microbially modulated carbonate precipitation is a fundamentally important phenomenon of both engineered and natural environments. In this paper, we propose a mixture model for biofilm induced calcite precipitation. The model consists of three phases - calcite, biofilm and solvent - which satisfy conservation of mass and momentum laws with addition of a free energy of mixing. The model also accounts for chemistry, mechanics, thermodynamics, fluid and electrodiffusion transport effects. Numerical simulations qualitatively capturing the dynamics of this process and revealing effects of kinetic parameters and external flow conditions are presented. PMID:20489270

Zhang, T; Klapper, I

2010-01-01

173

Transformation mechanism of amorphous calcium carbonate into calcite in the sea urchin larval spicule  

PubMed Central

Sea urchin larval spicules transform amorphous calcium carbonate (ACC) into calcite single crystals. The mechanism of transformation is enigmatic: the transforming spicule displays both amorphous and crystalline properties, with no defined crystallization front. Here, we use X-ray photoelectron emission spectromicroscopy with probing size of 40–200 nm. We resolve 3 distinct mineral phases: An initial short-lived, presumably hydrated ACC phase, followed by an intermediate transient form of ACC, and finally the biogenic crystalline calcite phase. The amorphous and crystalline phases are juxtaposed, often appearing in adjacent sites at a scale of tens of nanometers. We propose that the amorphous-crystal transformation propagates in a tortuous path through preexisting 40- to 100-nm amorphous units, via a secondary nucleation mechanism.

Politi, Yael; Metzler, Rebecca A.; Abrecht, Mike; Gilbert, Benjamin; Wilt, Fred H.; Sagi, Irit; Addadi, Lia; Weiner, Steve; Gilbert, P. U. P. A.

2008-01-01

174

Calcitic microlenses as part of the photoreceptor system in brittlestars  

NASA Astrophysics Data System (ADS)

Photosensitivity in most echinoderms has been attributed to `diffuse' dermal receptors. Here we report that certain single calcite crystals used by brittlestars for skeletal construction are also a component of specialized photosensory organs, conceivably with the function of a compound eye. The analysis of arm ossicles in Ophiocoma showed that in light-sensitive species, the periphery of the labyrinthic calcitic skeleton extends into a regular array of spherical microstructures that have a characteristic double-lens design. These structures are absent in light-indifferent species. Photolithographic experiments in which a photoresist film was illuminated through the lens array showed selective exposure of the photoresist under the lens centres. These results provide experimental evidence that the microlenses are optical elements that guide and focus the light inside the tissue. The estimated focal distance (4-7µm below the lenses) coincides with the location of nerve bundles-the presumed primary photoreceptors. The lens array is designed to minimize spherical aberration and birefringence and to detect light from a particular direction. The optical performance is further optimized by phototropic chromatophores that regulate the dose of illumination reaching the receptors. These structures represent an example of a multifunctional biomaterial that fulfills both mechanical and optical functions.

Aizenberg, Joanna; Tkachenko, Alexei; Weiner, Steve; Addadi, Lia; Hendler, Gordon

2001-08-01

175

Cd sup 2+ uptake by calcite, solid-state diffusion, and the formation of solid-solution: Interface processes observed with near-surface sensitive techniques (XPS, LEED, and AES)  

Microsoft Academic Search

X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES) allowed direct observations of structure and bonding environments at the calcite surface. Pure calcite crystals were cleaved from precleaned Iceland spar and were briefly exposed to aqueous solutions containing various concentrations of Cd{sup 2+}, COâ²⁻, ClOâ⁻, and\\/or Cl⁻. LEED results demonstrate that the calcite surface is atomically

S. L. Stipp; M. F. Jr. Hochella; G. A. Parks; J. O. Leckie

1992-01-01

176

The inhibitory effects of carboxymethyl inulin on the seeded growth of calcium carbonate.  

PubMed

Kinetics of precipitation of calcite (CaCO(3)) from aqueous solution in the presence of carboxymethyl inulin (CMI) was investigated under strictly controlled temperature, pH, supersaturation ratio (S=4.8) and ionic strength (I=0.1M). The highly reproducible constant composition technique was used to study the influence of biopolymers of crystal growth of CaCO(3), on CaCO(3) seed crystals at pH 8.5 and 25°C. The crystal growth of calcium carbonate (CaCO(3)) was inhibited in the presence of CMI at low concentration (2.5×10(-9) to 25×10(-9)mol/L). The larger number of negatively charged functional groups exhibited a 95% growth rate inhibition at a concentration of 15×10(-9)mol/L. The higher inhibition efficiency is related to the maximum surface charge density due to adsorbed polymer. PMID:22079106

K?rboga, Semra; Oner, Mualla

2011-10-25

177

Cyclic Cratonic Carbonates and Phanerozoic Calcite Seas.  

ERIC Educational Resources Information Center

|Discusses causes of cyclicity in cratonic carbonate sequences and evidence for and potential significance of postulated primary calcite sediment components in past Paleozoic seas, outlining problems, focusing on models explaining existing data, and identifying background. Future sedimentary geologists will need to address these and related areas…

Wilkinson, Bruce H.

1982-01-01

178

EXAFS Characterization of Uranyl Interaction at the Calcite-Water Interface  

NASA Astrophysics Data System (ADS)

Recent studies using X-ray absorption and luminescence spectroscopies have demonstrated uptake and incorporation of uranyl by calcite. X-ray microprobe studies on uranyl-reacted calcite single crystals reveal that uranyl exhibits preferences for specific surface sites. These preferences are likely expressed upon adsorption of uranyl species at the calcite-water interface. We have used X-ray absorption fine structure (EXAFS), combined with luminescence spectroscopy, to characterize the interactions of uranyl carbonate species with the calcite surface in the pH range 7.4-8.3. To minimize the potential for dissolution or precipitation, solutions were pre-equilibrated with reagent calcite powder for 3 weeks, with pH stabilizing at 8.3. For other experiments, the pH was adjusted initially to achieve a final pH of 7.4 after aging for 3 weeks. Uranyl carbonate solution was added to the pre-equilibrated suspensions to achieve initial U(VI) concentrations ranging from 5 uM to 5 mM. Suspensions were filtered to recover the calcite, but were kept moist to retain the sorbed complexes at the calcite-water interface. Multiple EXAFS spectra were collected using a multi-element Ge detector at beamline 12-BM at the APS and analyzed using standard methods. Fourier transforms (FT) of the spectra contain features corresponding to two axial oxygens and a shell of equatorial oxygens, as well as weaker features at higher R. For suspensions having initial U(VI) concentrations from 5-100 uM, a shoulder is observed in the FT on the high-R side of the equatorial oxygen, and fitting suggests minor splitting of this shell. For higher initial U(VI) concentrations, including the highest U(VI) concentration, 5 mM, the splitting becomes more pronounced. Weak features in the FT at higher-R can be fitted with backscattering from carbon, oxygen, and calcium shells, although fits without calcium provide nearly the same agreement. Luminescence spectroscopy reveals the presence of multiple uranyl species sorbed on the calcite from suspensions with 100 uM U(VI) and lower concentrations. The presence of multiple species may account for the splitting of the equatorial coordination indicated by EXAFS. For higher U(VI) concentrations, the luminescence results are consistent with formation of a schoepite-like precipitate. Understanding the complex interactions of uranyl at the calcite-water interface requires complementary data from multiple techniques.

Reeder, R. J.; Elzinga, E. J.; Tait, C. D.

2002-12-01

179

Constraining the Pressure Threshold of Impact Induced Calcite Twinning  

NASA Astrophysics Data System (ADS)

To better constrain the pressure threshold of impact-induced calcite twinning, calcite targets have been experimentally impacted. This study has implications for the deformation history of carbonaceous chondrite parent bodies.

Lindgren, P.; Price, M. C.; Lee, M. R.; Burchell, M.

2012-03-01

180

Inhibition of calcite precipitation by natural organic material: Kinetics, mechanism, and thermodynamics  

USGS Publications Warehouse

The inhibition of calcite precipitation by natural organic material (NOM) in solutions seeded with calcite was investigated using a pH-stat system. Experiments were carried out using three NOMs with different physical/chemical properties. For each of the materials, inhibition was found to be more effective at lower carbonate/calcium ratios and lower pH values. The reduction in the precipitation rate could be explained by a Langmuir adsorption model using a conditional equilibrium constant. By identification of the type of site on the NOM molecules that is involved in the adsorption reaction, the "conditional" equilibrium constants obtained at different solution compositions converged to a single "nonconditional" value. The thermodynamic data determined at 25??C and 1 atm suggest that the interaction between NOM molecules and the calcite surface is chemisorptive in nature and that adsorption is an endothermic reaction driven by the entropy change. The greatest degree of inhibition was observed for the NOM with the highest molecular weight and aromatic carbon content. For a given type of NOM, the degree of inhibition of calcite precipitation was dictated by the balance between the enthalpy change and the entropy change of the adsorption reaction. ?? 2005 American Chemical Society.

Lin, Y. -P.; Singer, P. C.; Aiken, G. R.

2005-01-01

181

Controls on trace-element partitioning in cave-analogue calcite  

NASA Astrophysics Data System (ADS)

We report trace-element data from a series of carbonate growth experiments in cave-analogue conditions in the laboratory with the goal of better understanding environmental controls on trace-element incorporation in stalagmites. The experimental setup closely mimics natural processes (e.g. precipitation driven by CO2-degassing, low ionic strength solution, thin solution-film) but with a tight control on growth conditions (temperature, pCO2, drip rate, calcite saturation index and the composition of the initial solution). Calcite is dissolved in deionized water in a 20,000 ppmV pCO2 environment, with trace-elements (Li, Na, Mg, Co, Sr, Cd, Ba, U) at appropriate concentrations to mimic natural cave drip-waters. This solution is dripped onto glass plates (coated with seed-calcite) in a lower pCO2 environment at 7, 15, 25 and 35 °C and drip rates of 2, 6 and 10 drips per minute. D(Sr) was shown to be statistically invariant over the full range of temperature and growth rate studied. No relationship between Sr/Ca and growth rate is therefore expected in stalagmite samples over comparable growth rates. D(Mg) has a relationship with temperature defined by D(Mg) = 0.01e0.02[±0.006]T, but temperature is not expected to be the dominant control on Mg/Ca in cave calcite due to the larger impact of calcite precipitation on Mg/Ca. Over short timescales, in conditions where temperature is well buffered, the fraction of calcium remaining in solution (f) is likely to be the dominant control on Mg/Ca and other trace-element ratios. But differences in the response of trace-elements to f and T may allow their combined use to assess past cave conditions. High Cd/Castalagmite is particularly indicative of low amounts of prior calcite precipitation and Cd/Ca would be a useful addition to trace-element studies of natural stalagmites. Significant scatter is observed in trace-element ratios during the laboratory experiments, which cannot be explained by simple Rayleigh distillation. This scatter is well explained by solution mixing and by the mixing of calcite with different fractions of calcite growth. Accounting for the effects of mixing on trace-element concentrations or ratios may help to achieve more robust interpretations of stalagmite chemistry as part of a multi-proxy approach to assessment of past environments.

Day, Christopher C.; Henderson, Gideon M.

2013-11-01

182

Quantitative Forward Modeling on the Effect of Growth, Diffusion and Exsolution on Calcite- Dolomite Thermometry Data  

NASA Astrophysics Data System (ADS)

Calcite-dolomite solvus thermometry is commonly used to constrain metamorphic temperatures in carbonates. The temperature dependent Mg-exchange between calcite and dolomite is experimentally well calibrated. Its application to natural contact metamorphic samples appears to be complex, because large variations in Mg- composition are often observed within individual grains, which do not exhibit simple (concentric) zoning patterns. We developed quantitative forward models to evaluate the effect of growth zoning, volume diffusion and formation of submicroscopic exsolution lamellae (<1?m) on the measured Mg-distribution in individual calcite crystals and compare the modeling results to field data. We present detailed calcite-dolomite thermometry data on multiple scales from the Ubehebe Peak contact aureole (USA). At the aureole scale, measured X(Mg) in calcite increases as a function of increasing proximity to the intrusive contact. On the other hand, several hundred analyses obtained on single thin sections commonly result in Gaussian-shaped distributions of measured Mg-content. These variations are approximately ten times higher than the analytical uncertainty, covering a span of more than 100°C in the calculated temperatures. Nevertheless, domains of lower and higher X(Mg)-values that correlate with adjacent silicate minerals are observed at the thin section scale. Lower values correspond to the presence of tremolite and higher values to that of forsterite. This correlation is in agreement with phase petrology, where tremolite is typically formed at lower temperatures than forsterite. Hence, the apparent lower temperatures of calcite in the vicinity of tremolite suggest that the original Mg-content during crystallization is at least partially preserved. Modeling results, based on our best temperature-time path estimates for the Ubehebe Peak aureole and published experimentally derived Mg-diffusion data in calcite, reveal that the original prograde Mg-zoning of calcite will be reset, if the calcite is assumed to grow slowly along a prograde path. In contrast, models of calcite growth in which the entire grain forms over a small temperature interval, as expected for infiltration driven mineral reactions, show that parts of the original lower Mg-compositions can be preserved. Modeled Mg-distributions were transformed into histograms by taking into account intersection probabilities of grains in thin sections and random microprobe analyses. Resulting histograms are similar to the measured ones, but in contrast to our observations the predicted Mg-distribution exhibit clear concentric zoning. For further insight we modeled the effect of retrograde equilibration within a calcite grain through submicroscopic exsolution lamellae. Modeling results for selected values of spot and lamellae sizes yield randomly distributed higher and lower X(Mg)-values, hiding a possible zoning that potentially survived the T-t-path. We thus demonstrate that the Mg-composition of calcite does reflect a memory of the temperature-reaction path, which is partially obscured by diffusion and exsolution.

Mueller, T.; Baumgartner, L.; Foster, C. T.; Roselle, G. T.

2006-12-01

183

Cyanobacterial Calcite Precipitation - Laboratory Study on Different Spatial Scales  

Microsoft Academic Search

Lacustrine calcite precipitation with sedimentation rates up to 1 mm per year can result in large carbonate deposits. Varved lake sediments high in calcite content are now intensively studied as high-resolution continental archives for environmental change. Especially in oligotrophic hardwater lakes, eukaryotic and prokaryotic picoplankton was found to be important in the overall process of calcite precipitation. Rates and mechanisms

M. Obst; D. Mavrocordatos; P. Gasser; M. Dittrich

2003-01-01

184

Structural evolution of calcite at high temperatures: Phase V unveiled.  

PubMed

The calcite form of calcium carbonate CaCO3 undergoes a reversible phase transition between Rc and Rm at ~1240?K under a CO2 atmosphere of ~0.4?MPa. The joint probability density function obtained from the single-crystal X-ray diffraction data revealed that the oxygen triangles of the CO3 group in the high temperature form (Phase V) do not sit still at specified positions in the space group Rm, but migrate along the undulated circular orbital about carbon. The present study also shows how the room temperature form (Phase I) develops into Phase V through an intermediate form (Phase IV) in the temperature range between ~985?K and ~1240?K. PMID:24084871

Ishizawa, Nobuo; Setoguchi, Hayato; Yanagisawa, Kazumichi

2013-10-02

185

Structural evolution of calcite at high temperatures: Phase V unveiled  

PubMed Central

The calcite form of calcium carbonate CaCO3 undergoes a reversible phase transition between Rc and Rm at ~1240?K under a CO2 atmosphere of ~0.4?MPa. The joint probability density function obtained from the single-crystal X-ray diffraction data revealed that the oxygen triangles of the CO3 group in the high temperature form (Phase V) do not sit still at specified positions in the space group Rm, but migrate along the undulated circular orbital about carbon. The present study also shows how the room temperature form (Phase I) develops into Phase V through an intermediate form (Phase IV) in the temperature range between ~985?K and ~1240?K.

Ishizawa, Nobuo; Setoguchi, Hayato; Yanagisawa, Kazumichi

2013-01-01

186

Fluid inclusion studies of calcite veins from Yucca Mountain, Nevada, Tuffs: Environment of formation  

SciTech Connect

Calcite vein and vug fillings at four depths (130-314m), all above the present water table in USW G-1 bore hole at Yucca Mountain, Nevada, contain primary fluid inclusions with variable vapor/liquid raitos: Most of these inclusions are either full of liquid or full of vapor. The liquid-filled inclusions show that most of the host calcite crystallized from fluids at <100{degrees}C. The vapor-filled inclusions provide evidence that a separate vapor phase was present in the fluid during crystallization. Studies of these vapor-filled inclusions on the microscope crushing stage were interpreted in an earlier paper as indicating trapping of an air-water-CO{sub 2} vapor phase at ``<100{degrees}C``. Our new studies reveal the additional presence of major methane in the vapor-filled inclusion, indicating even lower temperatures of trapping, perhaps at near-surface temperatures. They also show that the host calcite crystals grew from a flowing film of water on the walls of fractures open to the atmosphere, the vapor-filled inclusions representing bubbles that exsolved from this film onto the crystal surface.

Roedder, E. [Harvard Univ., Cambridge, MA (United States); Whelan, J.F. [Geological Survey, Denver, CO (United States); Vaniman, D.T. [Los Alamos National Lab., NM (United States)

1994-12-31

187

Purification, crystallization and X-ray characterization of a Kunitz-type trypsin inhibitor protein from the seeds of chickpea (Cicer arietinum)  

PubMed Central

A Kunitz-type trypsin inhibitor protein (CPTI) purified from chickpea seeds was estimated to have a molecular mass of 18?kDa on SDS–PAGE. The IC50 value of CPTI was determined to be 2.5?µg against trypsin. The inhibitory activity of CPTI is 114 TIU (trypsin inhibitory units) per milligram of protein, which is high compared with those of other known Kunitz-type trypsin inhibitors from legumes. CPTI crystallized in three different orthorhombic crystal forms: P21212 form A, P21212 form B and P212121. The crystals of P21212 form A, with unit-cell parameters a = 37.2, b = 41.2, c = 104.6?Å, diffracted to 2.0?Å resolution at the home source and to 1.4?Å on beamline BM14 at the ESRF. Data were also collected from crystals grown in the presence of iodine. The Matthews coefficient for these crystals was calculated to be 2.37?Å3?Da?1, corresponding to a solvent content of 42%. The other two crystal forms (P21212 form B and P212121) diffracted comparatively poorly.

Sharma, Urvashi; Suresh, C. G.

2011-01-01

188

Crystals  

NSDL National Science Digital Library

In this earth science/math/art activity, learners use simple ingredients to grow crystals and examine the repeating geometric shapes and patterns. Learners compare the growth of crystals from four types of crystal-starters (table salt, Borax, sand, and Epsom salt) to see which starter grows the most crystals in 14 days. Learners report their results online and find out what other learners discovered. Afterward, learners can use the crystals they grew to create works of art.

Science, Lawrence H.

2009-01-01

189

Biosorption of divalent Pb, Cd and Zn on aragonite and calcite mollusk shells.  

PubMed

The potential of using mollusk shell powder in aragonite (razor clam shells, RCS) and calcite phase (oyster shells, OS) to remove Pb(2+), Cd(2+) and Zn(2+) from contaminated water was investigated. Both biogenic sorbents displayed very high sorption capacities for the three metals except for Cd on OS. XRD, SEM and XPS results demonstrated that surface precipitation leading to crystal growth took place during sorption. Calcite OS displayed a remarkably higher sorption capacity to Pb than aragonite RCS, while the opposite was observed for Cd. However, both sorbents displayed similar sorption capacities to Zn. These could be due to the different extent of matching in crystal lattice between the metal bearing precipitate and the substrates. The initial pH of the solution, sorbent's dosage and grain size affected the removal efficiency of the heavy meals significantly, while the organic matter in mollusk shells affected the removal efficiency to a lesser extent. PMID:21550150

Du, Yang; Lian, Fei; Zhu, Lingyan

2011-05-06

190

Atomic force microscopy investigation of the mechanism of calcite microcrystal growth under Kitano conditions.  

PubMed

A combined atomic force microscopy (AFM)-inverted optical microscopy technique has been used to image the surface of calcite single microcrystals, with dimensions of 10-20 microm, at high resolution. The microcrystals were grown on a glass substrate using the Kitano method, a process that involves the outgassing of carbon dioxide from a saturated solution of calcium carbonate. The resulting increase in the supersaturation of the solution, with respect to calcium carbonate, induces crystallization. It is demonstrated, for the first time, that calcite microcrystals formed in this way exhibit a single spiral growth hillock on the (104) surface, as evidenced by a spiral step pattern, indicating that growth occurs at steps arising from an individual screw dislocation. The subsequent reactivity of these crystals under Kitano conditions has been followed in situ using AFM imaging. PMID:15697268

Dobson, Phillip S; Bindley, Lucy A; Macpherson, Julie V; Unwin, Patrick R

2005-02-15

191

The seeding effect of floating zone growth on Nd1.85Ce0.15CuO4 and Bi2Sr2CaCu2O8- delta single crystals  

Microsoft Academic Search

Single crystals with the [100] orientation were selected and used as seeds to investigate the effect of travelling solvent floating zone growth on superconducting oxides of Nd1.85Ce0.15CuO4 and Bi2Sr2CaCu2O8-delta. The number of nuclei was remarkably reduced and random nuclei could be eased when the seeding was applied during the growth of Nd1.85Ce0.15CuO4 single crystals, compared to the crystals grown without

C. T. Lin; A. Maljuk; B. Liang

2002-01-01

192

The seeding effect of floating zone growth on Nd1.85Ce0.15CuO4 and Bi2Sr2CaCu2O8?? single crystals  

Microsoft Academic Search

Single crystals with the [100] orientation were selected and used as seeds to investigate the effect of travelling solvent floating zone growth on superconducting oxides of Nd1.85Ce0.15CuO4 and Bi2Sr2CaCu2O8??. The number of nuclei was remarkably reduced and random nuclei could be eased when the seeding was applied during the growth of Nd1.85Ce0.15CuO4 single crystals, compared to the crystals grown without

C T Lin; A Maljuk; B Liang

2002-01-01

193

Carbon and oxygen diffusion in calcite: Effects of Mn content and P H 2 O  

Microsoft Academic Search

The diffusion rates of carbon and oxygen in two calcite crystals of different Mn contents have been studied between 500° and 800° C in a CO2-H2O atmosphere (PCO2=1-5 bars, PH2O=0.02-24 bars) labeled with 13C and 18O. Isotope concentration gradients within annealed specimens were measured using a secondary ion microprobe by depth profiling parallel and perpendicular to the c axis. Despite

Andreas K. Kronenberg; Richard A. Yund; Bruno J. Giletti

1984-01-01

194

Model study of initial adsorption of SO 2 on calcite and dolomite  

Microsoft Academic Search

The rate of calcareous stone degradation is to a significant extent controlled by their surface chemistry with SO2. Initial surface sulphite is converted to a harmful gypsum upon, e.g. NO2 catalysed oxidation. However, it has been observed by scanning electron microscopy that the lateral distributions of gypsum crystals differ between calcitic and dolomitic marbles. The first-principles density functional theory is

Katarina Malaga-Starzec; Itai Panas; Oliver Lindqvist

2004-01-01

195

Site-specific incorporation of uranyl carbonate species at the calcite surface  

Microsoft Academic Search

Spatially resolved luminescence spectra from U(VI) co-precipitated at the (101?4) growth surface of synthetic calcite single crystals confirm heterogeneous incorporation corresponding to the distribution of structurally non-equivalent steps composing the vicinal surfaces of spiral growth hillocks. Spectral structure from U(VI) luminescence at the “–” vicinal regions and featureless, weak luminescence at the “+” vicinal regions are consistent with previously reported

Richard J. Reeder; Evert J. Elzinga; C. Drew Tait; K. D. Rector; Robert J. Donohoe; David E. Morris

2004-01-01

196

Site-specific incorporation of uranyl carbonate species at the calcite surface  

Microsoft Academic Search

Spatially resolved luminescence spectra from U(VI) co-precipitated at the (101¯4) growth surface of synthetic calcite single crystals confirm heterogeneous incorporation corresponding to the distribution of structurally non-equivalent steps composing the vicinal surfaces of spiral growth hillocks. Spectral structure from U(VI) luminescence at the `` '' vicinal regions and featureless, weak luminescence at the ``+'' vicinal regions are consistent with previously

Richard J. Reeder; Evert J. Elzinga; C. Drew Tait; K. D. Rector; Robert J. Donohoe; David E. Morris

2004-01-01

197

Textural properties of synthetic nano-calcite produced by hydrothermal carbonation of calcium hydroxide  

NASA Astrophysics Data System (ADS)

The hydrothermal carbonation of calcium hydroxide (Ca(OH)2) at high pressure of CO2 (initial PCO2 1/4 55 bar) and moderate to high temperature (30 and 90 1C) was used to synthesize fine particles of calcite. This method allows a high carbonation efficiency (about 95% of Ca(OH)2-CaCO3 conversion), a significant production rate (48 kg/m3 h) and high purity of product (about 96%). However, the various initial physicochemical conditions have a strong influence on the crystal size and surface area of the synthesized calcite crystals. The present study is focused on the estimation of the textural properties of synthesized calcite (morphology, specific surface area, average particle size, particle size distribution and particle size evolution with reaction time), using Rietveld refinements of X-ray diffraction (XRD) spectra, Brunauer-Emmett-Teller (BET) measurements, and scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. This study demonstrate that the pressure, the temperature and the dissolved quantity of CO2 have a significant effect on the average particle size, specific surface area, initial rate of precipitation, and on the morphology of calcium carbonate crystals. In contrast, these PTx conditions used herein have an insignificant effect on the carbonation efficiency of Ca(OH)2. Finally, the results presented here demonstrate that nano-calcite crystals with high specific surface area (SBET 1/4 6-10m2/g) can be produced, with a high potential for industrial applications such as adsorbents and/or filler in papermaking industry.

Montes-Hernandez, G.; Fernández-Martínez, A.; Charlet, L.; Tisserand, D.; Renard, F.

2008-05-01

198

Rare earth elements in natural calcite  

NASA Astrophysics Data System (ADS)

Rare earth elements (REE) have been used for years as a tool for interpreting rock genesis but recently they have come to be used as good analogues for the tri--valent actinides. Although there are abundant data in the literature describing their relative proportions in rocks, data for individual minerals and factors controlling their uptake are lacking. Better understanding of the extent of REE incorporation in all minerals would provide valuable information about the extent of immobilisation one could expect in the event of escape of radioactive elements. When concrete waste repositories break down, calcite forms in the high pH, Ca-rich solutions. Calcium's electronic structure permits significant distortion in its octahedral coordination, so divalent ions substitute extensively when size is compatible. The trivalent REE's, whose radii are similar to calcium, substitute to some extent but the range of possible uptake is unknown. This study was designed to fill that gap. With help from museums and mineral collectors internationally, we have collected a suite of calcite samples from well--characterised genetic environments. These include limestone, dolomite, marble, pegmatite, hydrothermal vein fillings, regionally metamorphosed settings and skarn, as well as several types of calcitic fossils. We used inductively coupled plasma-mass spectroscopy (ICP--MS) with isotope dilution to analyse the REE content. Data show expected trends. The most interesting result so far is that the highest natural REE concentrations are significantly lower than expected from the distribution coefficients we determined experimentally, suggesting calcite takes up essentially all REE's available from solution. This is good news for natural attenuation in the vicinity of decomposing concrete repositories because one could expect immobilisation of the actinides whose behaviour is similar to that of the lanthanides. The negative side is that recrystallisation or dissolution could release them again.

Jensen, J. T.; Harstad, A. O.; Waight, T. E.; Stipp, S. L. S.

2003-04-01

199

Arsenate Uptake by Calcite: Macroscopic and Spectroscopic Characterization of Adsorption and Incorporation Mechanisms  

SciTech Connect

Batch uptake experiments and X-ray element mapping and spectroscopic techniques were used to investigate As(V) (arsenate) uptake mechanisms by calcite, including adsorption and coprecipitation. Batch sorption experiments in calcite-equilibrated suspensions (pH 8.3; PCO{sub 2} = 10{sup -3.5} atm) reveal rapid initial sorption to calcite, with sorption rate gradually decreasing with time as available sorption sites decrease. An As(V)-calcite sorption isotherm determined after 24 h equilibration exhibits Langmuir-like behavior up to As concentrations of 300 {mu}M. Maximum distribution coefficient values (K{sub d}), derived from a best fit to a Langmuir model, are {approx}190 L kg{sup -1}. Calcite single crystals grown in the presence of As(V) show well-developed rhombohedral morphology with characteristic growth hillocks on (10{bar 1}4) surfaces at low As(V) concentrations ({<=}5 {mu}M), but habit modification is evident at As(V) concentrations {>=}30 {mu}M in the form of macrostep development preferentially on the - vicinal surfaces of growth hillocks. Micro-X-ray fluorescence element mapping of (10{bar 1}4) surfaces shows preferential incorporation of As in the - vicinal faces relative to + vicinals. EXAFS fit results for both adsorption and coprecipitation samples confirm that As occurs in the 5+ oxidation state in tetrahedral coordination with oxygen, i.e., as arsenate. For adsorption samples, As(V) forms inner-sphere surface complexes via corner-sharing with Ca octahedra. As(V) coprecipitated with calcite substitutes in carbonate sites but with As off-centered, as indicated by two Ca shells, and with likely disruption of local structure. The results indicate that As(V) interacts strongly with the calcite surface, similar to often-cited analog phosphate, and uptake can occur via both adsorption and coprecipitation reactions. Therefore, calcite may be effective for partial removal of dissolved arsenate from aquatic and soil systems.

Alexandratos,V.; Elzinga, E.; Reeder, R.

2007-01-01

200

The influence of mineralization pathways on the Mg content and fractionation patterns in calcite  

NASA Astrophysics Data System (ADS)

Chemical proxy models are based upon the long-standing assumption that the uptake of minor and trace elements into a growing mineral reflects equilibrium fractionation processes. This picture is rooted in fundamental assumptions from BCF crystal growth theory- the thermodynamic-based model that was derived for step growth at very near equilibrium conditions. However, the original assumptions of this theory have been lost. Moreover, the applicability of step growth processes to biomineralization is being called into question with the realization that many carbonate biominerals form by non-classical processes. Here, mineralization begins with the accumulation of an amorphous calcium carbonate (ACC) in a localized environment that subsequently transforms to the complex mesocrystal/organic composites we know as skeletal structures. This study investigates the Mg content of calcites that form from ACC over a range of Mg/Ca ratios. At low Mg/Ca levels where Mg content is insufficient to inhibit step growth, the ACC transforms into crystallites of Mg calcites with 0-20 mol % MgCO3. These calcites exhibit the expected linear fractionation with Mg/Ca of the initial solution. In contrast, when initial Mg levels are above the threshold where step growth is prohibited, ACC transforms to nanoparticle aggregates of very high Mg calcite (30-50 mol% MgCO3). The Mg content of calcites formed by this process is approximately independent of solution composition- without evidence of fractionation. Mineralization is biased to this alternative pathway when the Mg content of the local environment is too high for calcite growth and saturation increases rapidly to reach ACC solubility before aragonite precipitation can occur. The pathway is allowed because high levels of supersaturation render thermodynamic barriers to nucleation less significant than kinetic barriers, which are apparently larger for the crystalline phases. Thus, the alternative pathway is a consequence of interplays between kinetic and thermodynamic factors. In the absence of high initial Mg levels or rapid rates of saturation, low Mg calcite or aragonite, respectively, form by the classical crystal growth process. Thus, carbonates with diverse compositions are produced by a systematic and predictable relationship to the Mg/Ca of the initial solution and saturation state. Insights from this study suggest a geochemical framework for interpreting Mg contents within the context of mineralization process and may reconcile apparent discrepancies in reported fractionation patterns.

Dove, P. M.; Wang, D.; DeYoreo, J.

2011-12-01

201

Binding of ethanol on calcite: the role of the OH bond and its relevance to biomineralization.  

PubMed

The interaction of OH-containing compounds with calcite, CaCO(3), such as is required for the processes that control biomineralization, has been investigated in a low-water solution. We used ethanol (EtOH) as a simple, model, OH-containing organic compound, and observed the strength of its adsorption on calcite relative to OH from water and the consequences of the differences in interaction on crystal growth and dissolution. A combination of atomic force microscopy (AFM) and molecular dynamics (MD) simulations showed that EtOH attachment on calcite is stronger than HOH binding and that the first adsorbed layer of ethanol is highly ordered. The strong ordering of the ethanol molecules has important implications for mineral growth and dissolution because it produces a hydrophobic layer. Ethanol ordering is disturbed along steps and at defect sites, providing a bridge from the bulk solution to the surface. The strong influence of calcite in structuring ethanol extends further into the liquid than expected from electrical double-layer theory. This suggests that in fluids where water activity is low, such as in biological systems optimized for biomineralization, organic molecules can control ion transport to and from the mineral surface, confining it to specific locations, thus providing the organism with control for biomineral morphology. PMID:20812690

Sand, K K; Yang, M; Makovicky, E; Cooke, D J; Hassenkam, T; Bechgaard, K; Stipp, S L S

2010-10-01

202

Origin of zoning within dedolomite and calcitized gypsum of the Mississippian Arroyo Penasco Group  

SciTech Connect

The Mississippian Arroyo Penasco Group carbonates are the oldest Paleozoic rocks present in north-central New Mexico. These supratidal to shallow,subtidal sediments exhibit complex diagenetic fabrics produced by periods of pre-Pennsylvanian subaerial exposure. Both extensive recrystallization of the Espiritu Santo carbonates and brecciation of the overlying Macho Member of the Tererro Formation resulted from an extended period of Mississippian subaerial exposure of broad, low-relief tidal flats. Cathodoluminescent petrography indicates that the recrystallized limestones consist of calcite pseudomorphs of dolomite and gypsum. Dedolomite and calcitized gypsum crystals, with /sup 13/C//sup 12/C ratios of -2 to +1.5% PDB, range from highly zoned to uniformly luminescent. Electron microprobe analyses reveals variable Mn and Fe contents across the pseudomorphs which are responsible for differences in observed luminosity. These features are interpreted to reflect a period of subaerial exposure after deposition of Macho Member sediments, which caused dissolution of gypsum and dolomite by sulfate and Mg depleted meteoric fluids and produced the collapse breccia. Preservation of zoning within some pseudomorphs required simultaneous dissolution of gypsum and dolomite and precipitation of calcite. C-isotope data indicates a meteoric to mixed phreatic origin for pore fluids which precipitated calcite; repetitive zoning within dolomite and gypsum pseudomorphs is indicative of interactions between marine and meteoric phreatic fluids in the intertidal environment.

Ulmer, D.S.

1985-01-01

203

An experimental study of calcite and limestone dissolution rates as a function of pH from ?1 to 3 and temperature from 25 to 80°C  

Microsoft Academic Search

Dissolution rates of single calcite crystals, limestones, and compressed calcite powders were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed in aqueous HCl solutions over the bulk solution pH range ?1 to 3, and at temperatures of 25°, 50°, and 80°C. Corresponding rates of the three different sample types are identical within experimental uncertainty. Interpretation

Marwan Alkattan; Eric H Oelkers; Jean-Louis Dandurand; Jacques Schott

1998-01-01

204

Fast diffusion along mobile grain boundaries in calcite  

NASA Astrophysics Data System (ADS)

Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44Ca from a layer of 44Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 ?m wide adjacent to the powder layer is heterogeneously enriched in 44Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in 44Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature, Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width ( D GB ?) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value of D GB ? in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than regions of anomalous mineralogy, water- or melt-content as is generally suggested.

McCaig, Andrew; Covey-Crump, Stephen J.; Ben Ismaïl, Walid; Lloyd, Geoffrey E.

2007-02-01

205

Quartz Growth on X-Seeds.  

National Technical Information Service (NTIS)

This invention concerns itself with a method for producing single crystal, seed material for use in growing synthetic quartz crystals. This seed material comprises a Z-faced seed crystal cut or sliced from the X-plus growing region of a synthetic quartz, ...

A. F. Armington

1983-01-01

206

Reaction mechanism for the replacement of calcite by dolomite and siderite: implications for geochemistry, microstructure and porosity evolution during hydrothermal mineralisation  

NASA Astrophysics Data System (ADS)

Carbonate reactions are common in mineral deposits due to CO2-rich mineralising fluids. This study presents the first in-depth, integrated analysis of microstructure and microchemistry of fluid-mediated carbonate reaction textures at hydrothermal conditions. In doing so, we describe the mechanisms by which carbonate phases replace one another, and the implications for the evolution of geochemistry, rock microstructures and porosity. The sample from the 1.95 Moz Junction gold deposit, Western Australia, contains calcite derived from carbonation of a metamorphic amphibole—plagioclase assemblage that has further altered to siderite and dolomite. The calcite is porous and contains iron-rich calcite blebs interpreted to have resulted from fluid-mediated replacement of compositionally heterogeneous amphiboles. The siderite is polycrystalline but nucleates topotactically on the calcite. As a result, the boundaries between adjacent grains are low-angle boundaries (<10°), which are geometrically similar to those formed by crystal-plastic deformation and recovery. Growth zoning within individual siderite grains shows that the low-angle boundaries are growth features and not due to deformation. Low-angle boundaries develop due to the propagation of defects at grain faces and zone boundaries and by impingement of grains that nucleated with small misorientations relative to each other during grain growth. The cores of siderite grains are aligned with the twin planes in the parent calcite crystal showing that the reactant Fe entered the crystal along the twin boundaries. Dolomite grains, many of which appear to in-fill space generated by the siderite replacement, also show alignment of cores along the calcite twin planes, suggesting that they did not grow into space but replaced the calcite. Where dolomite is seen directly replacing calcite, it nucleates on the Fe-rich calcite due to the increased compatibility of the Fe-bearing calcite lattice relative to the pure calcite. Both reactions are interpreted as fluid-mediated replacement reactions which use the crystallography and elemental chemistry of the calcite. Experiments of fluid-mediated replacement reactions show that they proceed much faster than diffusion-based reactions. This is important when considering the rates of reactions relative to fluid flow in mineralising systems.

Pearce, Mark A.; Timms, Nicholas E.; Hough, Robert M.; Cleverley, James S.

2013-10-01

207

Reaction mechanism for the replacement of calcite by dolomite and siderite: implications for geochemistry, microstructure and porosity evolution during hydrothermal mineralisation  

NASA Astrophysics Data System (ADS)

Carbonate reactions are common in mineral deposits due to CO2-rich mineralising fluids. This study presents the first in-depth, integrated analysis of microstructure and microchemistry of fluid-mediated carbonate reaction textures at hydrothermal conditions. In doing so, we describe the mechanisms by which carbonate phases replace one another, and the implications for the evolution of geochemistry, rock microstructures and porosity. The sample from the 1.95 Moz Junction gold deposit, Western Australia, contains calcite derived from carbonation of a metamorphic amphibole—plagioclase assemblage that has further altered to siderite and dolomite. The calcite is porous and contains iron-rich calcite blebs interpreted to have resulted from fluid-mediated replacement of compositionally heterogeneous amphiboles. The siderite is polycrystalline but nucleates topotactically on the calcite. As a result, the boundaries between adjacent grains are low-angle boundaries (<10°), which are geometrically similar to those formed by crystal-plastic deformation and recovery. Growth zoning within individual siderite grains shows that the low-angle boundaries are growth features and not due to deformation. Low-angle boundaries develop due to the propagation of defects at grain faces and zone boundaries and by impingement of grains that nucleated with small misorientations relative to each other during grain growth. The cores of siderite grains are aligned with the twin planes in the parent calcite crystal showing that the reactant Fe entered the crystal along the twin boundaries. Dolomite grains, many of which appear to in-fill space generated by the siderite replacement, also show alignment of cores along the calcite twin planes, suggesting that they did not grow into space but replaced the calcite. Where dolomite is seen directly replacing calcite, it nucleates on the Fe-rich calcite due to the increased compatibility of the Fe-bearing calcite lattice relative to the pure calcite. Both reactions are interpreted as fluid-mediated replacement reactions which use the crystallography and elemental chemistry of the calcite. Experiments of fluid-mediated replacement reactions show that they proceed much faster than diffusion-based reactions. This is important when considering the rates of reactions relative to fluid flow in mineralising systems.

Pearce, Mark A.; Timms, Nicholas E.; Hough, Robert M.; Cleverley, James S.

2013-06-01

208

New Shocked Calcite and Fe Grains from Noerdlingen Ries Impact Crater  

NASA Astrophysics Data System (ADS)

Shocked minerals with simple chemical composition of silica, carbon or iron material reveal high-pressure phase during compression process [1]. As calcite mineral shows high-pressure phase of aragonite it should have the "shocked phase" formed from high-pressure phase mixed with quenched materials of gas-melt reaction [2,3,4]. The main purposes of this paper are (1) to discuss new shocked materials of calcite found in the Noerdlingen Ries impact crater. Germany and artificial impact phases. and (2) to show new findings of fine Fe-grains and anomalous plagioclase in suevite. Single grains of anomalous calcite for X-ray analysis were selected from limestone with wormy or bubble-included texture in Buschelberg Ries impact crater (sample R8) [3 4]. Powdered and single grain samples of anomalous calcites show low X-ray intensity and high Bragg angle shift. compared with standard calcite of Akiyoshi limestone as listed in Table 1 [2.3.4]. The unit-cell dimensions of the single grain measured with the four-axes X-ray diffractometer in Yamaguchi University were determined by the least square calculation from the "highest X-ray intensity peak" of each crystal plane which is the same data of the powdered X-ray diffraction sample as an average structure . Both powdered and single grain data reveal high density of 2.76 g/cm^3 (between aragonite of density: rho=2.8g/cm^3, and normal calcite: rho=2.71g/cm^3) of the calcite cell which is the same characters of shocked phases of quartz or graphite [1,2,3,4] (Table 1). Anomalous calcite contains minor amount of Si Fe Ti, though major cation is Ca of 99.4% in cation content. These foreign elements are mixed with during the formation in vapor plume [5]. Table 1, showing the x-ray density and unit-cell parameters of anomalous shocked calcite with high density from Ries and artificial impact craters, appears here in the hard copy. Two types of shocked calcites are found in artificial impact experiments of railgun of the ISAS. Japan. (a) Shocked calcite formed on oxidation condition (near the rim of vapor plume of impact [7]). (b)Shocked calcite formed on reduction condition (interior of vapor plume of impact [7]). The shocked calcites of the Buschelberg (R8) inside the crater are correspondent to shocked calcite with reduction state. which suggests recrystallization inside the crater. The detailed experiments of the R8 samples are checked on yellow (Fe-rich) colored sample of R8Yl and white colored sample (R8W). From the X-ray diffraction data of Bragg angle shift, the R8W1 is more close to reduction state during impact. The shocked calcites under oxidation state are formed as non-stoichiometric compound (and high density) after gas of C02 formation mainly near rim of vapor plume, which is the same data of artificial impact experiments of oxidation condition (i.e. sample No. LSRGl). The shocked calcite under reduction state are formed relatively as normal calcite (with normal or a little high density) and carbon after formation of gaseous materials of CO, 02 and Ca. Thus cabon source from target rock of limestone [2,3,4] can be explained as reduction formation of impact, as listed in Table 2. Table 2, showing, the formation process of shocked calcite materials by shock impact, appears here in the hard copy. References [1] Miura Y.(l991): Shock Waves, 1, 135-141. ?2] Miura Y. (1994): Proc. ISAS Lunar & Planet. Sci. (ISAS, Japan), 27, 204-207. [3] Miura Y. et al. (1995): Shock Waves, Japan, 613-624. [4] Miura Y. and Okanoto M. (19995): Antarctic Meteorites (NIP?), 20, 169-171. ?1989): Tma;l(-.t. r?r;lt.?rin? ?rhrfnr? llniv?r?i?v Pr? )A? 7?;

Miura, Y.

1995-09-01

209

Occurrence of microdiamond in UHP calcite marble from the Kokchetav Massif  

NASA Astrophysics Data System (ADS)

Small amount of microdiamond was discovered in UHP calcite marble from Kumdy-kol, the Kokchetav Massif northern Kazakhstan. UHP calcite marble is characterized by 1) pure calcite (after aragonite) is only one carbonate phase, 2) presence of titanite, K-feldspar and diopside with phengite and K-feldspar lamella, 3) coesite exsolution from supersilicic titanite. This marble contains UHP evidence as coesite of exsolution origin in titanite and lamella texture in diopside. Minimum P condition was constrained as 6 GPa by precursor composition of titanite. So far, we have described this calcite marble as diamond-free UHP carbonate rock (Ogasawara et al., 2002); however, our recent observation found microdiamond in some domains (B-type) of this rock. Microdiamond occurs only in diopside and its amount is very low compared with diamond-bearing dolomite marble. We found 61 microdiamond grains in two thin sections (c.f. in dolomite marble, highest concentration domain contains over 4000 grains in one thin section). Microdiamond grains are heterogeneously distributed in some layers. The domain (A-type) containing relatively large amount of titanite does not contain diamond. The microdiamond in calcite marble shows 3 to 20 micrometers in diameter, rounded or cubic form, pale yellowish color and translucent character. Laser Raman spectroscopy indicated that FWHM of a Raman band at about 1332 cm-1 ranges from 2.99 to 3.64 cm-1 (average: 3.28 cm-1) and this average value is the smallest among core of S-type, rim of S-type, R-type and diamond in gneiss. Based on the morphology and other features except for FWHM of Raman band, the microdiamond in calcite marble is similar to R-type in dolomite marble (Ishida et al., 2003). No S-type diamond (_gStar_h-shaped form with core and rim) occurs in this calcite marble, and this indicates that the second stage growth of microdiamond probably from multicomponent aqueous fluid had not occurred in calcite marble. Low amount of microdiamond occurs only in diopside, no diamond occurs in titanite (stable at extremely low-XCO2 conditions), and lack of the second stage growth of microdiamond in calcite marble; all these features may be related with extremely low-XCO2 condition under UHP metamorphism. One of the possible explanations for this diamond occurrence is that microdiamonds in calcite marble are relic crystal formed before the second stage growth (the rim stage of S-type microdiamond in dolomite marble). Therefore, it is very important to confirm whether these microdiamond in B-type calcite marble had been growing or dissolving under such extremely low-XCO2 conditions. References Ishida et al. (2003): Journal of Metamorphic Geology, Vol. 21, p. 515-522. Ogasawara et al. (2000): The Island Arc, Vol. 9, p. 400-416. Ogasawara et al. (2002): American Mineralogist, Vol. 87, p. 454-461.

Ogasawara, Y.; Adachi, T.; Aoki, K.

2004-12-01

210

First-principles calculations of structural, electronic, optical and elastic properties of magnesite MgCO 3 and calcite CaCO 3  

Microsoft Academic Search

Detailed ab initio calculations of the structural, electronic, optical and elastic properties of two crystals – magnesite (MgCO3) and calcite (CaCO3) – are reported in the present paper. Both compounds are important natural minerals, playing an important role in the carbon dioxide cycling. The optimized crystal structures, band gaps, density of states diagrams, elastic constants, optical absorption spectra and refractive

M. G. Brik

2011-01-01

211

First-principles calculations of structural, electronic, optical and elastic properties of magnesite MgCO3 and calcite CaCO3  

Microsoft Academic Search

Detailed ab initio calculations of the structural, electronic, optical and elastic properties of two crystals - magnesite (MgCO3) and calcite (CaCO3) - are reported in the present paper. Both compounds are important natural minerals, playing an important role in the carbon dioxide cycling. The optimized crystal structures, band gaps, density of states diagrams, elastic constants, optical absorption spectra and refractive

M. G. Brik

2011-01-01

212

Basal slip and texture development in calcite: new results from torsion experiments  

SciTech Connect

The deformation behavior of calcite has been of longstanding interest. Through experiments on single crystals, deformation mechanisms were established such as mechanical twinning on e = {l_brace}{bar 1}018{r_brace} <40{bar 4}1> in the positive sense and slip on r = {l_brace}10{bar 1}4{r_brace} <20{bar 2}{bar 1}> and f = {l_brace}{bar 1}012{r_brace} <0{bar 2}2{bar 1}> both in the negative sense. More recently it was observed that at higher temperatures f{l_brace}{bar 1}012{r_brace} <10{bar 1}1> slip in both senses becomes active and, based on slip line analysis, it was suggested that e(0001) <11{bar 2}0> slip may occur. So far there had been no direct evidence for basal slip, which is the dominant system in dolomite. With new torsion experiments on calcite single crystals at 900 K and transmission electron microscopy, this study identifies (0001) <11{bar 2}0> slip unambiguously by direct imaging of dislocations and diffraction contrast analysis. Including this slip system in polycrystal plasticity simulations, enigmatic texture patterns observed in compression and torsion of calcite rocks at high temperature can now be explained, resolving a long-standing puzzle.

Barber, D. J.; Wenk, H.-R.; Gomez-Barreiro, J.; Rybacki, E.; Dresen, G. (UCB); (Essex); (GFZ)

2008-06-09

213

Carbonates in thrust faults: High temperature investigations into deformation processes in calcite-dolomite systems  

NASA Astrophysics Data System (ADS)

The role of dolomite on the strength and evolution of calcite-dolomite fold and thrust belts and nappes (as observed in the Canadian Rockies, the Swiss Alps, the Italian Apennines, and the Naukluft Nappe Complex) is largely unknown. Field investigations indicate that strain in natural systems is localized in calcite, resulting in a ductile response, while dolomite deforms in a dominantly brittle manner. To date, experimental studies on polymineralic carbonate systems are limited to homogeneous, fine-grained, calcite-dolomite composites of relatively low dolomite content. The effect of dolomite on limestone rheology, the onset of crystal-plastic deformation in dolomite in composites, and the potential for strain localization in composites have not yet been fully quantified. Constant displacement rate (3x10-4 s-1and 10-4 s-1), high confining pressure (300 MPa) and high temperature (750° C and 800° C) torsion experiments were conducted to address the role of dolomite on the strength of calcite-dolomite composites. Experiments were performed on samples produced by hot isostatic pressing (HIP) amalgams of a natural, pure dolomite and a reagent, pure calcite. We performed experiments on the following mixtures (given as dolomite%): 25%, 35%, 50%, and 75%. These synthetic HIP products eliminated concerns of mineralogical impurities and textural anomalies due to porosity, structural fabrics (e.g., foliation) and fossil content. The samples were deformed up to a maximum finite shear strain of 5.0 and the experimental set up was unvented to inhibit sample decarbonation. Mechanical data shows a considerable increase in sample yield strength with increasing dolomite content. Experimental products with low starting dolomite content (dol%: 25% and 35%) display macroscopic strain localization along compositionally defined foliation. Experimental products with high dolomite content (dol%: 50% and 75%) demonstrate no macroscopic foliation. Post-deformation microstructure analysis shows that small dolomite grains (~50 ?m) are characterized by well-defined grain boundaries and cleavage controlled fracture. There is evidence of the interruption of foliation development due to the presence of large-grained dolomite. Calcite grains are characterized by triple junction grain boundaries, providing evidence for recrystallization. Ongoing microstructural analyses (including: thin section analysis, EBSD, SEM, and Microprobe analysis) are being conducted to better constrain the deformation mechanisms and the degree of strain localization in these composites. Our experiments provide insights into the processes controlling rheology within bimodal calcite-dolomite systems, which can be used to improve models of the evolution of fold and thrust belt systems.

Kushnir, A.; Kennedy, L.; Misra, S.; Benson, P.

2012-04-01

214

CO2 emission accompanying the fracture of calcite  

NASA Astrophysics Data System (ADS)

Time resolved mass spectroscopy of the emissions accompanying the fracture of calcite (rhombohedral CaCO3) show that the principle volatile product, CO2, is released in bursts milliseconds after the fracture event. Similar measurements during the abrasion of calcite and during low temperature thermal decomposition of pulverized calcite show similar CO2 bursts. We argue that the observed bursts reflect localized decomposition of the calcite during the relaxation of reversible plastic deformation created by fracture and abrasion. This implies that mechanical, non-thermal processes play an important role in producing the observed decomposition products.

Dickinson, J. T.; Jensen, L. C.; Langford, S. C.; Rosenberg, P. E.; Blanchard, D. L.

1991-12-01

215

Calcite dissolution: Effects of trace cations naturally present in Iceland spar calcites  

NASA Astrophysics Data System (ADS)

In situ dissolution experiments on a set of pure, optical quality Iceland spar calcite samples from four different localities showed etch pit step retreat rates to be inversely proportional to total inherent trace cation composition. Atomic absorption spectroscopy (AAS) revealed Fe 2+, Mg 2+, Mn 2+ and Sr 2+ in amounts varying from a few to hundreds of ppm. We used a very simple experimental set-up, with an Atomic Force Microscope (AFM) fluid cell and a droplet of MilliQ water. As the calcite dissolved and approached equilibrium with the solution, trace cations were released, which were then present for interaction with the dissolving surface. We monitored continuous free-drift dissolution, in situ, on fresh {101¯4} cleavage surfaces for up to 40 min. Dissolution produced one-layer-deep, rhombic etch pits that continually expanded as we collected images. The rhombohedral symmetry of calcite defines two obtuse and two acute edges on the cleavage surface of etch pits and these, as expected from previous work, had different dissolution rates. Despite identical experimental conditions for all samples, we observed lower step retreat rates for both obtuse and acute edges on calcite characterised by relatively high trace cation composition. Increased cation concentration, particularly Mn, was also correlated with rounding of obtuse-obtuse corners, resulting in obtuse step retreat rates similar to those for acute sides. Physcial limitations of the AFM technique were taken into account when measuring step rate retreat and results were collected only from single-layer etch pits, which represent crystalline calcite with minimal defects. Dissolution rates presented here are thus lower than previous reports for studies of deep etch pits and where the physical limitations of imaging may not have been considered. In addition to molecular-level proof that divalent cations inherent at ppm levels in the calcite affect the dissolution process, these results show that pure, optical quality Iceland spar calcite should not be considered pure in the chemical sense. The results imply that dissolution rates determined for ideal systems with pure, synthetic or natural, materials may be considered as the boundary condition for dissolution in real systems in nature, where cations are always present both in the solution and in the initial solid.

Harstad, A. O.; Stipp, S. L. S.

2007-01-01

216

Dissolution dynamics of the calcite-water interface observed in situ by glancing-incidence X-ray scattering  

SciTech Connect

Glancing-incidence X-ray scattering measurements made at the National Synchrotron Light Source were used to investigate dissolution dynamics in situ at the calcite-water interface. The relation between calcite saturation state and roughness of the calcite (1014) cleavage surface as a function of time was examined during pH titrations of an initially calcite-saturated solution. Systematic variations in roughness were observed as a function of saturation state as pH was titrated to values below that of calcite saturation. Different steady-state values of roughness were evident at fixed values of {Delta}G{sub r}, and these were correlated with the extent of undersaturation. A significant increase in roughness begins to occur with increasing undersaturation at a {Delta}G{sub r} value of approximately {minus}2.0 kcal/mol. The dissolution rate corresponding to this increase is about 1.5 x 10{sup 7} mmol/cm {center_dot} sec. This increase in roughness is attributed to a transition in the principal rate-determining dissolution mechanism, and is consistent with both powder-reaction studies of dissolution kinetics and single-crystal dissolution studies by atomic force microscopy. These data indicate some important potential applications of GIXS in the study of mineral-water interface geochemistry.

Sturchio, N.C.; Chiarello, R.P.

1995-06-02

217

Interaction of Copper with the Surface of Calcite.  

National Technical Information Service (NTIS)

The interaction of Cu ions in solution with the surface of calcite has been studied in a range of solutions from pure water to seawater. Observations of the uptake of Cu from solution onto calcite indicates that the process is rapid and strong in both dis...

M. L. Franklin J. W. Morse

1981-01-01

218

Dissolution of coccolithophorid calcite by microzooplankton and copepod grazing  

NASA Astrophysics Data System (ADS)

Independent of the ongoing acidification of surface seawater, the majority of the calcium carbonate produced in the pelagial is dissolved by natural processes above the lysocline. We investigate to what extent grazing and passage of coccolithophorids through the guts of copepods and the food vacuoles of microzooplankton contribute to calcite dissolution. In laboratory experiments where the coccolithophorid Emiliania huxleyi was fed to the rotifer Brachionus plicatilis, the heterotrophic flagellate Oxyrrhis marina and the copepod Acartia tonsa, calcite dissolution rates of 45-55%, 37-53% and 5-22% of ingested calcite were found. We ascribe higher loss rates in microzooplankton food vacuoles as compared to copepod guts to the strongly acidic digestion and the individual packaging of algal cells. In further experiments, specific rates of calcification and calcite dissolution were also measured in natural populations during the PeECE III mesocosm study under differing ambient pCO2 concentrations. Microzooplankton grazing accounted for between 27 and 70% of the dynamic calcite stock being lost per day, with no measurable effect of CO2 treatment. These measured calcite dissolution rates indicate that dissolution of calcite in the guts of microzooplankton and copepods can account for the calcite losses calculated for the global ocean using budget and model estimates.

Antia, A. N.; Suffrian, K.; Holste, L.; Müller, M. N.; Nejstgaard, J. C.; Simonelli, P.; Carotenuto, Y.; Putzeys, S.

2008-01-01

219

Crystallization and preliminary X-ray analysis of luffaculin, a ribosome-inactivating protein from sponge-gourd seeds.  

PubMed

Luffaculin is a ribosome-inactivating protein. Crystals suitable for X--ray diffraction were first obtained using the hanging-drop vapour-diffusion method. X-ray studies show that the crystals belong to space group C2, with unit-cell parameters a = 89.90, b = 59.82, c = 55.18 A, beta = 120.81 degrees, and have one molecule in the crystallographic asymmetric unit. The crystals diffract X-rays to at least 2.0 A resolution. PMID:10666599

Ma, Q; Yao, G; Wu, S; Li, H; Li, J; Dong, Y

2000-02-01

220

Differences in the immobilization of arsenite and arsenate by calcite  

NASA Astrophysics Data System (ADS)

The sorption and coprecipitation experiments of arsenic (As) with calcite coupled with determinations of the chemical state of As both in the reaction fluid and in calcite were conducted to investigate the influence of the As oxidation state on its immobilization into calcite. The oxidation states of As in calcite and water were determined via As K-edge XANES and HPLC-ICP-MS analysis, respectively. The results of the sorption experiments at pH 8.2 show that only As(V) is distributed to calcite regardless of the As oxidation state in the solution. In coprecipitation experiments, As(V) is preferentially incorporated into calcite over a wide range of pH (7-12). On the other hand, the incorporation of As(III) into calcite is not observed at circumneutral pH. This difference between As(III) and As(V) is attributed to the fact that their dissolved species are neutral vs. negatively charged, respectively, at circumneutral pH (arsenite as H3AsO3; arsenate as H2AsO4- or HAsO42-). As the pH increases (>9), up to 33% of As(III)/Astotal ratio is partitioned into calcite or a precursor of calcite (metastable vaterite formed during the early stage of precipitation). The higher interaction of As with calcite at an alkaline pH compared with circumneutral pH is due to the negative charge of As(III) at alkaline pH. However, the As(III)/Astotal ratio decreases as time progresses and only As(V) can be found finally in calcite. The ratio of distribution coefficients of As(III) and As(V) into calcite (KAs(V)/KAs(III)) at pH ˜7 is larger than 2.1 × 103, suggesting that the oxidation state of As is a significant issue in considering the interaction between As and calcite in groundwater. Moreover, low KAs(III) shows that the sequestration of As via coprecipitation with calcite is not an important chemical process under reducing conditions, such as in the groundwaters in Bangladesh and other As-contaminated areas where As(III) is the dominant dissolved species of As. In the system spiked only with As(III), the XANES analysis detected As(V) in calcite, where the partitioning of As into calcite is very limited as stated above. Therefore, HPLC-ICP-MS and cyclic voltammetric measurements were performed to clarify the As(V)-incorporation mechanism in the As(III) system. As a result, the oxidation of As(III) to As(V) and the stabilization of As(V) in a solution in the presence of the Ca2+ ion are observed. These phenomena result from the formation of calcium arsenate complexes in the aqueous phase. This complexation is an important process for the coprecipitation of As with calcite. Hence, As is incorporated into calcite as As(V) even from the As(III) solution at circumneutral pH, wherein As(III) exists as neutral arsenite with little interaction with calcite.

Yokoyama, Yuka; Tanaka, Kazuya; Takahashi, Yoshio

2012-08-01

221

Crystal Structures of Vegetative Soybean Lipoxygenase VLX-B and VLX-D, and Comparisons with Seed Isoforms LOX-1 and LOX-3  

PubMed Central

The lipoxygenase family of lipidperoxidizing, nonheme iron dioxygenases form products that are precursors for diverse physiological processes in both plants and animals. In soybean (Glycine max), five vegetative isoforms, VLX-A, VLX-B, VLX-C, VLX-D, VLX-E, and four seed isoforms LOX-1, LOX-2, LOX-3a, LOX-3b have been identified. In this study, we determined the crystal structures of the substrate-free forms of two major vegetative isoforms, with distinct enzymatic characteristics, VLX-B and VLX-D. Their structures are similar to the two seed isoforms, LOX-1 and LOX-3, having two domains with similar secondary structural elements: a ?-barrel N-terminal domain containing highly flexible loops and an ?-helix-rich C-terminal catalytic domain. Detailed comparison of the structures of these two vegetative isoforms with the structures of LOX-1 and LOX-3 reveals important differences that help explain distinct aspects of the activity and positional specificity of these enzymes. In particular, the shape of the three branches of the internal subcavity, corresponding to substrate-binding and O2 access, differs among the isoforms in a manner that reflects the differences in positional specificities.

Youn, Buhyun; Sellhorn, George E.; Mirchel, Ryan J.; Gaffney, Betty J.; Grimes, Howard D.; Kang, ChulHee

2009-01-01

222

Existence and release of fluid inclusions in bornite and its associated quartz and calcite  

NASA Astrophysics Data System (ADS)

The existence and release of fluid inclusions in bornite and its associated minerals, namely, quartz and calcite were investigated and confirmed. The structures, forms, and phases of these large quantities of fluid inclusions were also studied. A mass of fluid inclusions with various sizes, distributions, shapes, and phases exist in bornite and its associated minerals. Their sizes vary from a few micrometers to tens of micrometers, and the forms appear as negative crystals, or elongated, elliptical, and irregular. At room temperature, fluid inclusions were mainly characterized as gas-liquid twophase. However, small amounts of fluid inclusions with pure gas phase and pure liquid single-phase were also observed in quartz and calcite. These fluid inclusions initially broke during the ore crushing and grinding process and then released into the flotation pulp in the flotation process. The quantitative analysis of fluid inclusions in the solution and the comparisons of mineral dissolution show that the amount of copper and iron released by fluid inclusions in the bornite sample is higher than the amount dissolved by the mineral; fluid inclusions in the associated gangue minerals, quartz, and calcite also make contribution.

Deng, Jiu-shuai; Wen, Shu-ming; Wu, Dan-dan; Liu, Jian; Zhang, Xiao-lin; Shen, Hai-ying

2013-09-01

223

Prediction of calcite morphology from computational and experimental studies of mutations of a de novo-designed peptide.  

PubMed

Many organisms use macromolecules, often proteins or peptides, to control the growth of inorganic crystals into complex materials. The ability to model peptide-mineral interactions accurately could allow for the design of novel peptides to produce materials with desired properties. Here, we tested a computational algorithm developed to predict the structure of peptides on mineral surfaces. Using this algorithm, we analyzed energetic and structural differences between a 16-residue peptide (bap4) designed to interact with a calcite growth plane and single- and double-point mutations of the charged residues. Currently, no experimental method is available to resolve the structures of proteins on solid surfaces, which precludes benchmarking for computational models. Therefore, to test the models, we chemically synthesized each peptide and analyzed its effects on calcite crystal growth. Whereas bap4 affected the crystal growth by producing heavily stepped corners and edges, point mutants had variable influences on morphology. Calculated residue-specific binding energies correlated with experimental observations; point mutations of residues predicted to be crucial to surface interactions produced morphologies most similar to unmodified calcite. These results suggest that peptide conformation plays a role in mineral interactions and that the computational model supplies valid energetic and structural data that can provide information about expected crystal morphology. PMID:21797243

Schrier, Sarah B; Sayeg, Marianna K; Gray, Jeffrey J

2011-08-22

224

Marine low-magnesian calcite cement from periphery of Mururoa atoll  

SciTech Connect

Reef-associated rudstones and grainstones from the periphery of Mururoa atoll (Pacific Ocean 21/degrees/50'N-138/degrees/50'W) are significantly more cemented than their lagoonal counterparts. With increasing depth, the centrifugal distribution of the marine lithification is associated with a change in the cement composition. Peripheral facies above 500 m are cemented by three types of high-magnesian calcite (HMC) with distinct mole % MgCO/sub 3/: 14.4-16.9 for fibrous crusts, 7.6-13.5 for bladed cement, and 4.3-11.2 for sparitic, stubby forms. Large pores favor fibrous HMC; small, isolated pores are preferential sites for sparitic HMC, the bladed HMC occurring in association with the two end members of this morphological and geochemical variation. Below 500 m, marine HMCs are progressively replaced by acicular crystals forming fringes less than 100 ..mu..m thick around original grains. Individual crystals are 10-80 ..mu..m long and 2-10 ..mu..m wide. These acicular crystals are low-magnesian calcite (LMC) with less than 3.9 mole % MgCO/sub 3/ and up to 1470 ppm strontium. The marine origin of LMC from the deep periphery of Mururoa atoll is deduced from (1) continuous occurrence over more than 200 m within a major shallowing-upward sequence; (2) absence of emergence within this sequence; (3) strontium content similar to that of marine HMC; and (4) deep infiltration of cold marine waters that may have favored the precipitation of magnesium-depleted calcite along the periphery of the atoll. Recognition of marine LMC cementation at Mururoa atoll indicates LMC is not an exclusive indicator of meteoric diagenesis. Marine LMC cementation may have been more common than previously reported in ancient carbonates, and all acicular LMC cements are not necessarily secondary.

Aissaoui, D.M.

1989-03-01

225

Crystals  

NSDL National Science Digital Library

This page contains four documents explaining crystals. The topic is covered at an advanced level in relation to nanotechnology and requires background knowledge in eight grade science. A powerpoint with illustrations and instructor guide containing activities are included to aid in teaching this subject. Additionally, links to related videos, a lab handout, and discussion questions are available.

2012-10-16

226

Dynamic and topographic observation of calcite dissolution using enhanced in-situ phase-shift interferometry  

NASA Astrophysics Data System (ADS)

The dissolution rate of calcite in water with and without flow was measured using enhanced phase-shift interferometry (PSI). This technique uses a white light source instead of laser light to shorten the coherent wave-length providing quick and precise measurements of mineral dissolution and growth over relatively short time scales (several tens of minutes). Phase shift interferometry is non-invasive technique that allows surface observation at the molecular level in water as well as direct, simultaneous measurement of step velocity and the behavior of crystal surfaces during dissolution. The theoretical PSI resolution in the vertical direction in water is 0.78 nm; however the practical detection limit of the calcite dissolution experiments was 4.2 nm due to the effects of external and internal inferences such as apparatus drift, air drag, etc.The calcite dissolution rate at zero water flow was estimated from the retreat velocity as 0.0494 nm/s, which corresponds to flux rate of 1.33E-10 mol/cm2/s. This is around the lower limit of previously published data obtained mainly by powder experiments. The dissolution rates for different vicinal slopes of the surface were also measured. These rates were well-correlated with the vicinal slopes in accordance with BCF theory indicating that there is a mutual interaction between neighboring growth/dissolution steps via diffusion, consistent with previously published AFM measurements.The effect of topographical features on the microscopic dissolution behavior was clearly observed during the PSI measurements. The dissolution velocity at the isolated step in pure water was obtained as 2.8 nm/s by fitting the experimental data. Thus, it is possible to examine and determine global calcite dissolution rates using the topographical measurements determined from PSI experiments.

Ueta, Shinzo; Satoh, Hisao; Nishimura, Yoshihiro; Ueda, Akira; Tsukamoto, Katsuo

2013-01-01

227

An oxygen isotope study of illite and calcite in three Appalachian Paleozoic vertic paleosols  

SciTech Connect

The oxygen isotope compositions of pedogenic calcite and illite in three Paleozoic vertic red bed paleosols record a complex history of pedogenic and diagenetic processes. The paleosols are preserved in the upper Mississippian (Chesterian) Mauch Chunk (Pennsylvania), Hinton (West Virginia), and Pennington (Tennessee) Formations in the Appalachian Basin and were buried to 7--8 km (230--260 C), 3--4 km (100--145 C), and 1.5--3 km (70--100 C), respectively. The submicron-size clay fractions of the paleosols are presently composed of R3-ordered illite/smectite (I/S), inferred from K-Ar dates and soil morphology to have been authigenically produced by the reaction of smectite to illite. Oxygen isotope fractionations, calculated pore-fluid compositions, and K-Ar dates were used to constrain the conditions and timing of I/S and pedogenic calcite (re)crystallization in each of the paleosols. The data were interpreted in light of three possible end-member environments for formation and recrystallization: (1) the pedogenic environment; (2) the burial diagenetic environment; and (3) a burial environment dominated by hot, tectonically driven fluid. The {delta}{sup 18}O values of pedogenic calcite and authigenic I/S in vertic red bed paleosols generally reflect recrystallization and exchange during burial diagenesis and are thus not suitable for interpretation of paleoenvironment or paleoclimate. The involvement of orogenic fluids during burial cannot be ruled out, but it is not required by any of the paleosols. In the coastal-margin Pennington Formation paleosol, I/S formation may have been initiated in the pedogenic environment in response to cyclical wetting and drying of the soil clay matrix. Continued smectite transformation and coarsening and ordering of I/S during a long period of shallow burial led to its isotopic reequilibration with pore fluids at slightly elevated temperature, but oxygen isotope compositions of pedogenic calcite in the paleosol are preserved.

Mora, C.I.; Sheldon, B.T.; Driese, S.G. [Univ. of Tennessee, Knoxville, TN (United States). Dept. of Geological Sciences; Elliott, W.C. [Georgia State Univ., Atlanta, GA (United States). Dept. of Geology

1998-05-01

228

Ab initio vibrational spectra and dielectric properties of carbonates: magnesite, calcite and dolomite  

Microsoft Academic Search

The equilibrium geometry, the Raman and IR vibrational spectra at the ? point, TO–LO splitting, IR intensities, Born and dielectric\\u000a tensors of magnesite MgCO3, dolomite MgCa(CO3)2 and calcite CaCO3 have been calculated with the periodic ab initio program CRYSTAL, by using an all-electron gaussian type basis set and the\\u000a B3LYP hamiltonian. LO (longitudinal-optical) modes are computed by correcting the dynamical

L. Valenzano; Y. Noël; R. Orlando; C. M. Zicovich-Wilson; M. Ferrero; R. Dovesi

2007-01-01

229

Peak effect, oxygen deficiency, and scaling behavior of magnetization for YBa2Cu3O7-? crystals prepared by top-seeded solution growth  

NASA Astrophysics Data System (ADS)

We present the magnetic properties of YBa2Cu3O7-? crystals prepared by top-seeded solution growth (TSSG). The crystals showed the peculiar peak effect (fish-tail) for fields parallel to the c-axis, for which the hysteresis width in the magnetic isotherm M(H) showed a single maximum at Hp>0. This M(H) was observed up to 85 K, just below the maximum Tc = 86 K, and was found to be insensitive to various oxygen annealings. On the other hand, oxygen reduction, which resulted in a decrease in c, by up to 4 K, showed a scaling behavior of the normalized (M.H) for both temperatures and oxygen contents, indicating a single pinning mechanism. This implies that the oxygen vacancies or twin planes are unlikely to account for this peak effect. We also discuss the effects of defects such as twisted boundaries, localized stacking faults, and impurities on the peak effect. Together with the fish-tail associated with oxygen vacancies, we suggest that the peak effect is not related to specific ions, but to material processing.

Hyun, O. B.; Yoshida, M.; Kitamura, T.; Hirabayashi, I.

1996-02-01

230

A biophysical elucidation for less toxicity of agglutinin than abrin-a from the seeds of Abrus precatorius in consequence of crystal structure.  

PubMed

X-ray crystal structure determination of agglutinin from Abrus precatorius in Taiwan is presented. The crystal structure of agglutinin, a type II ribosome-inactivating protein (RIP) from the seeds of Abrus precatorius in Taiwan, has been determined from a novel crystalline form by the molecular replacement method using the coordinates of abrin-a as the template. The structure has space group P4(1)2(1)2 with Z = 8, and been refined at 2.6 A to R-factor of 20.4%. The root-mean-square deviations of bond lengths and angles from the standard values are 0.009 A and 1.3 degrees. Primary, secondary, tertiary and quaternary structures of agglutinin have been described and compared with those of abrin-a to a certain extent. In subsequent docking research, we found that Asn200 of abrin-a may form a critical hydrogen bond with G4323 of 28SRNA, while corresponding Pro199 of agglutinin is a kink hydrophobic residue bound with the cleft in a more compact complementary relationship. This may explain the lower toxicity of agglutinin than abrin-a, despite of similarity in secondary structure and the activity cleft of two RIPs. PMID:20433687

Cheng, Jack; Lu, Tian-Huey; Liu, Chao-Lin; Lin, Jung-Yaw

2010-04-30

231

Calcite surface structure and reactivity: molecular dynamics simulations and macroscopic surface modelling of the calcite-water interface.  

PubMed

Calcite-water interactions are important not only in carbon sequestration and the global carbon cycle, but also in contaminant behaviour in calcite-bearing host rock and in many industrial applications. Here we quantify the effect of variations in surface structure on calcite surface reactivity. Firstly, we employ classical Molecular Dynamics simulations of calcite surfaces containing an etch pit and a growth terrace, to show that the local environment in water around structurally different surface sites is distinct. In addition to observing the expected formation of more calcium-water interactions and hydrogen-bonds at lower-coordinated sites, we also observed subtle differences in hydrogen bonding around acute versus obtuse edges and corners. We subsequently used this information to refine the protonation constants for the calcite surface sites, according to the Charge Distribution MUltiSite Ion Complexation (CD-MUSIC) approach. The subtle differences in hydrogen bonding translate into markedly different charging behaviour versus pH, in particular for acute versus obtuse corner sites. The results show quantitatively that calcite surface reactivity is directly related to surface topography. The information obtained in this study is not only crucial for the improvement of existing macroscopic surface models of the reactivity of calcite towards contaminants, but also improves our atomic-level understanding of mineral-water interactions. PMID:23042085

Wolthers, M; Di Tommaso, D; Du, Z; de Leeuw, N H

2012-10-08

232

High-pressure deformation of calcite marble and its transformation to aragonite under non-hydrostatic conditions  

USGS Publications Warehouse

We conducted deformation experiments on Carrara marble in the aragonite and calcite stability fields to observe the synkinematic transformation of calcite to aragonite, and to identify any relationships between transformation and deformation or sample strength. Deformation-induced microstructures in calcite crystals varied most significantly with temperature, ranging from limited slip and twinning at 400??C, limited recrystallization at 500??C, widespread recrystallization at 600 and 700??C, to grain growth at 800-900??C. Variations in confining pressure from 0.3 to 2.0 GPa have no apparent effect on calcite deformation microstructures. Aragonite grew in 10-6-10-7 s-1strain rate tests conducted for 18-524 h at confining pressures of 1.7-2.0 GPa and temperatures of 500-600??C. As in our previously reported hydrostatic experiments on this same transformation, the aragonite nucleated on calcite grain boundaries. The extent of transformation varied from a few percent conversion near pistons at 400??C, 2.0 GPa and 10-4 s-1 strain rate in a 0.8 h long experiment, to 98% transformation in a 21-day test at a strain rate of 10-7 s-7, a temperature of 600??C and a pressure of 2.0 GPa. At 500??C, porphyroblastic 100-200 ??m aragonite crystals grew at a rate faster than 8 ?? 10-1m s-1. At 600??C, the growth of aragonite neoblasts was slower, ???6 ?? 10-1 m s -1, and formed 'glove-and-finger' cellularprecipitation-like textures identical to those observed in hydrostatic experiments. The transformation to aragonite is not accompanied by a shear instability or anisotropic aragonite growth, consistent with its relatively small volume change and latent heat in comparison with compounds that do display those features. ?? 1993.

Hacker, B. R.; Kirby, S. H.

1993-01-01

233

Climate and cave control on Pleistocene/Holocene calcite-to-aragonite transitions in speleothems from Morocco: Elemental and isotopic evidence  

NASA Astrophysics Data System (ADS)

The occurrence of aragonite in speleothems has commonly been related to high dripwater Mg/Ca ratios, because Mg is known to be a growth inhibitor for calcite. Laboratory aragonite precipitation experiments, however, suggested a more complex array of controlling factors. Here, we present data from Pleistocene to Holocene speleothems collected from both a dolostone and a limestone cave in northern Morocco. These stalagmites exhibit both lateral and stratigraphic calcite-to-aragonite transitions. Aragonite fabrics are well-preserved and represent primary features. In order to shed light on the factors that control alternating calcite and aragonite precipitation, elemental (Mg, Sr, Ba, U, P, Y, Pb, Al, Ti and Th) abundances were measured using LA-ICP-MS, and analysed with Principal Component Analysis. Samples were analyzed at 100-200 ?m resolution across stratigraphic and lateral transitions. Carbon and oxygen isotope ratios were analysed at 100 ?m resolution covering stratigraphic calcite-to-aragonite transitions. Results show that the precipitation of aragonite was driven by a decrease in effective rainfall, which enhanced prior calcite precipitation. Different geochemical patterns are observed between calcite and aragonite when comparing data from the Grotte de Piste and Grotte Prison de Chien. This may be explained by the increased dripwater Mg/Ca ratio and enhanced prior aragonite precipitation in the dolostone cave versus lower dripwater Mg/Ca ratio and prior calcite precipitation in the limestone cave. A full understanding for the presence of lateral calcite-to-aragonite transitions is not reached. Trace elemental analysis, however, does suggest that different crystallographic parameters (ionic radius, amount of crystal defect sites, adsorption potential) may have a direct effect on the incorporation of Sr, Mg, Ba, Al, Ti, Th and possibly Y and P.

Wassenburg, Jasper A.; Immenhauser, Adrian; Richter, Detlev K.; Jochum, Klaus Peter; Fietzke, Jan; Deininger, Michael; Goos, Manuela; Scholz, Denis; Sabaoui, Abdellah

2012-09-01

234

Pomegranate seeds  

NSDL National Science Digital Library

This pomegranate has many seeds inside. These seeds are surrounded individually by red fruit. Some fruits also have many seeds, but all of the seeds are surrounded by the fruit instead of individually, like in an apple.

Peter N/A (None;)

2005-01-01

235

The kinetics of the ordering of 13C-18O bonds in calcite and apatite  

NASA Astrophysics Data System (ADS)

Eiler and Schauble (2004) showed that the isotopes of C and O are not randomly distributed within single phases such as CO2 gas and carbonates, and in particular, that heavy isotopes of C and O tend to bond preferentially (clump) at lower temperatures. Consequently, the measurement of the deviation from a random distribution of C and O isotope distributions in a single phase can be used as a thermometer. As with other geothermometers based on homogeneous or heterogeneous equilibria, the clumped-isotope thermometer is susceptible to resetting (e.g., if the phase is reheated or experiences slow cooling). Thus, clumped-isotope "temperatures" of phases that have experienced complex thermal histories may, in fact, be closure temperatures, the interpretation of which requires quantification of the kinetics of redistribution of C and O isotopes as a function of temperature. These kinetics have received increasing attention (Dennis and Schrag, 2010; Passey 2010), and are likely to be critical for understanding clumped-isotope temperatures of samples that have been buried for long periods of time. To better constrain these kinetics we performed experiments on natural optical calcite from Mexico and carbonate-bearing apatite from the Siilinjarvi carbonatite (Finland). For each experiment, multiple single crystal grains (~2 mm in diameter) of calcite or apatite were loaded in open Pt capsules, pressurized with Ar gas, and held at 400-700 °C, 550 bars using a rapid quench TZM apparatus for 5 min to 520 hrs. After quenching, 13C-18O clumping was measured in the samples; the change from the initial ?47 with time for each phase at each temperature was fit to simple mechanistic models of isotope exchange between sites in these phases. One conclusion of the experimental study is that resetting the internal ordering of carbonate groups proceeds more rapidly in calcites than in apatites. For example, heating apatite at 400 °C results in no change in clumping over a 24 hr period, while the ?47 of calcite changes by ~0.13% for the same conditions. We also measured clumping in cogenetic apatite and calcite from three carbonatites; these intrusive rocks cooled slowly from igneous temperatures and thus comparison of the apatite and calcite closure temperatures provides constraints on the differences in kinetics of resetting of clumping in these two phases. Apparent temperatures for apatites are 90-140 °C and are cooler than cogenetic calcites in all cases with differences of 30-60 °C. This contrasts with the expectation from the laboratory experiments we have done. Possible explanations of this include a change in the mechanism of the kinetics of reordering in calcite and apatite at temperatures lower than 300-400 °C or resetting of ?47 due to recrystalization or diagenesis for natural samples. Using the results of our models and measurements we are able to calculate activation energies and estimate expected changes in apparent temperatures of samples for geologically plausible scenarios. As such, this study has relevance for our interpretation of ancient samples that have thermal histories involving any significant heating. Additionally, it provides the groundwork for using apatites and carbonates together in paleotemperature studies to determine if thermal overprinting could be present.

Stolper, D. A.; Halevy, I.; Eiler, J. M.

2011-12-01

236

A Synthetic Calcite Standard for Determination of Relative Mn/Cr Sensitivity Factor  

NASA Astrophysics Data System (ADS)

Primitive chondrites which suffered from aqueous alteration often contain carbonates such as calcites, dolomites and breunnerites. 53Mn-53Cr decay system (half-life: 3.7 Myr) is applicable to dating their precipitation and many authors have measured their Mn-Cr ages using the SIMS. However, the relative Mn/Cr sensitivity factor: RSF (measured Mn+/Cr+ ratio divided by the true ratio) is not well established due to the absence of suitable standards, therefore the ages have systematic uncertainties. We prepared a synthetic Mn and Cr bearing calcite to evaluate the Mn/Cr RSF. Here we report the technical details of preparation for the standard and its Mn/Cr RSF. We also measured the Mn/Cr RSF of San Carlos olivine which is often assumed to be the same as that of a carbonate, and compared it with that of our synthetic calcite. The Cr-bearing calcite was produced in a N2 filled closed system by the reaction Ca2+ + CO32- = CaCO3 in an aqueous solution. The reaction proceeded by continuous addition of ammonium carbonate vapor to the solution. The crystal size of the calcite was ~300 ?m. A small amount of hydrazine was added to the solution in order to keep chromous ion from oxidation. Mn and Cr concentrations in the calcite grains were determined by the SEM-EDS. A weak, defocused beam was used due to prevent electron beam damage. In a spherical grain, radial zoning of Mn and Cr concentrations occur and they decrease towards the periphery. At the center of the grains, Mn and Cr concentrations are ~0.5 atomic % and the values of the Mn/Cr ratios are relatively constant. The Mn/Cr RSF was determined with the CAMECA NanoSIMS 50 at Ocean Research Institute of Univ. of Tokyo. A primary O- beam of ~1 nA and 5 ?m diameter was used. 43Ca+, 52Cr+, 53Cr+ and 55Mn+ ions were analyzed in a combined peak-jumping/multi-detection mode. The total measurement time was typically ~20 minutes. The measurements were started after presputtering of 15 minutes. The mass resolution power was ~3500, sufficient to resolve all relevant isobaric interferences. Several points near the center of calcite grains with constant Mn and Cr concentrations were measured. As for San Carlos olivine, a primary beam was rastered over 20 x 20 ?m2 areas. Signals of all ions decreased with time during the measurements. The 55Mn/52Cr RSF also decreased with time. Initially it was ~0.7-0.8, and approached to a nearly constant value of ~0.5-0.6 after ~20 minutes. 53Cr+/52Cr+ isotopic ratio was 0.1105 ± 0.0002 (1?), not dependent on time and space. This value corresponds to ~-26 permil fractionation from the reference value. On the other hand, the RSF of San Carlos olivine was found to be ~0.9, consistent with previous studies. The RSF of our calcite is significantly lower than the values used in previous studies and the RSF of San Carlos olivine measured in this study. This implies that Mn-Cr ages for carbonates obtained in previous studies may have systematic biases (~3 Myr).

Fujiya, W.; Ichimura, K.; Takahata, N.; Sugiura, N.; Sano, Y.

2009-12-01

237

Thermoluminescence of the green emission band of calcite  

NASA Astrophysics Data System (ADS)

The thermoluminescence (TL) characteristics of the green emission band of natural calcite are studied. The kinetic parameters of the natural and laboratory-induced defects were estimated. The ?-dose response was studied in the range of 0.01-104 Gy. The fading rate at room temperature is monitored over a period of 30 days. The results showed that calcite has a potential use as a material for ?-ray TL dosimeters for food sterilization, testing of materials and other industrial or reactor procedures.

Soliman, C.; Metwally, Saleh M.

238

Calcite-dolomite geothermometry for iron-bearing carbonates  

Microsoft Academic Search

The effect of the addition of iron on the calcite-dolomite solvus in the system CaCO3-MgCO3 is predicted from an approximate thermodynamic description of the systems CaCO3-MgCO3 and CaCO3-FeCO3. Coexisting calcites and dolomites in 15 samples from the Glockner area of the Tauern Window have been analysed to estimate metamorphic temperatures. These range from about 410 °C at the top of

M. J. Bickle; R. Powell

1977-01-01

239

H20 and OH in echinoid calcite: A spectroscopic study  

Microsoft Academic Search

Proton nuclear magnetic resonance (NMR) and visible and near-infrared (VNIR) re- flectance spectral data and heating experiments indicate that the Mg-rich calcite (com- monly referred to as high-Mg calcite and hereafter as HMC) that is produced by six different species of echinoids contains H20 with mobilities and H bond strengths ranging from those characteristic ofliquid H20 to those observed in

SUSAN J. GAFFEY

240

A new method for the study of trace element partitioning between calcium carbonate and aqueous solution: A test case for Sr and Ba incorporation into calcite  

Microsoft Academic Search

A new experimental method (evaporation method) for calcium carbonate precipitation in aqueous solution was at- tempted in order to develop a convenient and controllable experimental technique for obtaining precise trace element partition coefficients. Calcite crystals were formed by evaporation of H2O from the aqueous mother solution using a dehumidifier, and the consumed Ca ions were supplied from a refill solution

YASUTAKA TERAKADO; MAMI TANIGUCHI

2006-01-01

241

The differnces between bond lengths in biogenic and geologocal calcite.  

SciTech Connect

We used high-resolution neutron powder diffraction to accurately measure the atomic positions and bond lengths in biogenic and geological calcite. A special procedure for data analysis was developed in order to take into account the considerable amounts of magnesium present in all the investigated samples. As a result, in biogenic calcite we found some atomic bonds to have significantly different lengths as compared to those in geological calcite, after the contribution of magnesium is accounted for. The maximum effect (elongation up to 0.7%) was found for the C-O bonds. We also analyzed changes in frequencies and spectral widths of normal vibrations of carbonate groups in biogenic calcite (as compared to geological calcite) measured by Raman and Fourier transform IR techniques. Surprisingly, the frequency shifts after subtracting the magnesium contribution are close to zero. At the same time, substantial spectral broadening (up to 1.2%) in biogenic calcite as compared to geological samples was detected. Possible explanations for the experimental findings are discussed.

Zlotoyabko, E.; Caspi, E. N.; Fieramosca, J. S.; Von Dreele, R. B.; Marin, F.; Mor, G.; Politi, Y.; Addadi, L.; Weiner, S. (X-Ray Science Division); ( PNS); (Technion-Israel Inst. of Tech.); (Nuclear Research Centre); (Univ. de Bourgogne); (Weizmann Inst. of Science)

2010-01-01

242

Calcite Farming at Hollow Ridge Cave: Calibrating Net Rainfall and Cave Microclimate to Dripwater and Calcite Chemical Variability  

NASA Astrophysics Data System (ADS)

Stable isotope (?18O and ?13C) and trace element records in cave speleothems are often interpreted as climate changes in rainfall amount or source, cave air temperature, overlying vegetation and atmospheric pCO2. However, these records are difficult to verify without in situ calibration of changes in cave microclimate (e.g., net rainfall, interior ventilation changes) to contemporaneous variations in dripwater and speleothem chemistry. In this study at Hollow Ridge Cave (HRC) in Marianna, Florida (USA), cave dripwater, bedrock, and modern calcite (farmed in situ) were collected in conjunction with continuous cave air pCO2, temperature, barometric pressure, relative humidity, radon-222 activity, airflow velocity and direction, rainfall amount, and drip rate data [1]. We analyzed rain and dripwater ?D and ?18O, dripwater Ca2+, pH, ?13C and TCO2, cave air pCO2 and ?13C, and farmed calcite ?18O and ?13C to examine the relationships among rainwater isotopic composition, cave air ventilation, cave air temperature, calcite growth rate and seasonal timing, and calcite isotopic composition. Farmed calcite ?13C decreases linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of ??13C = -7‰ . A whole-cave "Hendy test" at distributed contemporaneous farming sites reveals that ventilation induces a +1.9 ± 0.96‰ ?13C offset between calcite precipitated in a ventilation flow path and out of flow paths. Farmed calcite ?18O exhibits a +0.82 ± 0.24‰ offset from values predicted by both theoretical calcite-water calculations and by laboratory-grown calcite [2]. Unlike calcite ?13C, oxygen isotopes show no ventilation effects and are a function only of temperature. Combining our data with other speleothem studies, we find a new empirical relationship for cave-specific water-calcite oxygen isotope fractionation across a range of temperatures and cave environments: 1000 ln ? = 16.1(1000/T) - 24.6 We analyzed anions, cations, and trace elements in dripwater, bedrock, and farmed calcite to examine the relationships between net rainfall, drip rates, drip water chemistry, and calcite chemistry. Dripwater Mg/Ca and Sr/Ca ratios fall on coherent mixing lines between three geochemical endmembers: rainwater, dissolved dolomite, and dissolved limestone. Dripwater Sr/Ca vs. Mg/Ca ratios are also influenced by evaporative enrichment within the epikarst as a function of net rainfall amount [3]. Farmed calcite trace Cation/Ca ratios faithfully track short-term seasonal variations in dripwater chemistry for Na, Mg, Sr, Ba and U. However, speleothem calibrations are unique to each drip site regardless of proximity to one another, suggesting that individual speleothems are unlikely to be useful as a whole-cave hydrologic proxy. [1] Kowalczk, A. J., Froelich, P. N., 2010. Cave air ventilation and CO2 outgassing by radon-222 modeling: How fast do caves breathe? Earth & Planet. Sci. Lett. 289, 209-219. [2] Tremaine, D. M., Froelich, P. N., Wang, Y., 2011. Speleothem calcite farmed in situ: Modern calibration of ?18O and ?13C paleoclimate proxies in a continuously-monitored natural cave system. Geochim. Cosmochim. Acta 75, 4929-4950. [3] Tremaine, D. M., Froelich, P. N., 2012. Speleothem trace element signatures: A modern hydrologic geochemical study of cave drip waters and farmed calcite. Geochim. Cosmochim. Acta (submitted)

Tremaine, D. M.; Kilgore, B. P.; Froelich, P. N.

2012-04-01

243

Comparison of garnet-biotite, calcite-graphite, and calcite-dolomite thermometry in the Grenville Orogen; Ontario, Canada  

Microsoft Academic Search

The Elzevir Terrane of the Grenville Orogen in southern Ontario contains metapelites and abundant graphitic marbles that\\u000a were regionally metamorphosed from the upper greenschist to upper amphibolite facies. Comparative thermometry was undertaken\\u000a with widely used calibrations for the systems garnet-biotite, calcite-dolomite, and calcite-graphite. Temperatures that are\\u000a obtained from matrix biotites paired with prograde garnet near-rim analyses are usually consistent with

Mark A. Rathmell; Margaret M. Streepey; Eric J. Essene; Ben A. van der Pluijm

1999-01-01

244

Quantum-mechanical Hartree-Fock study of calcite (CaCO 3 ) at variable pressure, and comparison with magnesite (MgCO 3 )  

Microsoft Academic Search

The static crystal energy of calcite and its structure configuration as functions of pressure were determined by ab initio all-electron periodic Hartree-Fock calculations (CRYSTAL code). Ca, O and C atoms were represented by 22, 18 and 14 atomic orbitals, respectively, in form of contracted Gaussian-type functions. Comparison between theoretical and experimental data was performed for binding energy, equilibrium unit-cell and

M. Catti; A. Pavese; E. Aprà; C. Roetti

1993-01-01

245

Preliminary experimental results for trace element uptake in carbonates: Pb2+ in calcite and U6+ in aragonite at various growth rates and temperatures  

NASA Astrophysics Data System (ADS)

The surface of a crystal in equilibrium with surrounding fluid can have a composition and structure that differs from the bulk crystal. If the growth rate is fast relative to the diffusive equilibrium time, then the crystal surface composition may be partially "captured" by the newly-formed lattice. The extent of this growth entrapment increases with crystal growth rate and is suppressed by diffusive equilibration between the surface and the bulk lattice of the crystal. Partitioning of Sr into calcite is probably affected by this entrapment mechanism. To explore the relevance of this idea to other elements and other carbonates we conducted experiments on Pb2+ in calcite and U6+ in aragonite, for which relatively few data exist. There are, however, some intriguing indications in the published data that show the concentration of Pb2+ is higher at the calcite surface relative to the bulk lattice; U6+ partitioning into aragonite decreases with increasing growth rate. Both these previous studies involved the use of bulk analytical techniques; we, in contrast, performed in situ measurements on single crystals of known growth rate. The carbonate crystals were grown by two different mechanisms: 1) precipitation of calcite on a plate (pre-coated with calcite), using a steady flow of CaCl2 - PbCl2 and Na2CO3 solutions mixed just before passage through a tube and dripped onto a plate (cave-type experiments, I=0.02); 2) growth of calcite and aragonite from a CaCl2 - NH4Cl (or NaCl) - PbCl2 (or UO2[NO3]2) solution by diffusion of CO2 from an ammonium carbonate source (drift experiments, I=0.52-0.58). The growth rate of individual crystals was determined by comparison of final size with duration of the experiment. The precipitated crystals show significant variation in size even when the runs have the same input rate of CaCO3 components. The cave-type runs at 25 ? 2, 40 ? 1, and 50 ? 1° C yielded 15-40 ? m calcites, but in the drift experiments the aragonite crystals attained sizes up to 500 ? m. Electron microprobe analysis across the large crystals shows that the concentration of U is higher in the center and decreases toward the edge. This is probably due to the cube root dependence of linear growth on volume change of the growing crystals. We also observed that increasing aragonite growth rate (V, nm/s) enhances U partitioning between the bulk crystal and the liquid, Kdbulk/liquid=(U/Ca)bulk/(U/Ca)liquid (in contrast to the previous study). Kdbulk/liquid = 0.65 to 1.91? 0.08 when log(V)=-0.73 to 0.31 at 53? 3° C in the drift-type run (I=0.58). This evidence supports the idea that U is enriched at the calcite surface relative to the bulk crystal during crystal growth. The Pb partition coefficient varies from 0.01 to 1.26. At present time we are characterizing the parameter that responsible for this broad interval of Kd variation. Uranium partitioning into aragonite decreases with increasing temperature. Our preliminary data can be described by log(Kd) versus 104/T(K) plot with the slope equal to 0.064, corresponding to ? H0 of -12.29 (kJ/mol).

Gabitov, R. I.; Watson, B. E.

2004-12-01

246

Dolomite-calcite textures in early carbonatites of the Kovdor ore deposit, Kola peninsula, Russia: their genesis and application for calcite-dolomite geothermometry  

Microsoft Academic Search

In early calcite carbonatites of the Kovdor ore deposit four morphological types of dolomite are represented. In the first type, dolomite microcrystals occur as lamellae enclosed by optically continuous calcite. In the second, dolomite microcrystals occur as segmented rods, plates and xenomorphic grains, enclosed by optically discontinuous calcite, and in the third, dolomite is represented by grains of various morphologies,

Anatoly Zaitsev; Ludmila Polezhaeva

1994-01-01

247

Epitaxial high saturation magnetization FeN thin films on Fe(001) seeded GaAs(001) single crystal wafer using facing target sputterings  

SciTech Connect

It was demonstrated that Fe-N martensite ({alpha}') films were grown epitaxially on Fe(001) seeded GaAs(001) single crystal wafer by using a facing target sputtering method. X-ray diffraction pattern implies an increasing c lattice constant as the N concentration increases in the films. Partially ordered Fe{sub 16}N{sub 2} films were synthesized after in situ post-annealing the as-sputtered samples with pure Fe{sub 8}N phase. Multiple characterization techniques including XRD, XRR, TEM, and AES were used to determine the sample structure. The saturation magnetization of films with pure Fe{sub 8}N phase measured by VSM was evaluated in the range of 2.0-2.2 T. The post annealed films show systematic and dramatic increase on the saturation magnetization, which possess an average value of 2.6 T. These observations support the existence of giant saturation magnetization in {alpha}''-Fe{sub 16}N{sub 2} phase that is consistent with a recent proposed cluster-atom model and the first principles calculation [N. Ji, X. Q. Liu, and J. P. Wang, New J. Phys. 12 063032 (2010)].

Ji Nian; Wu Yiming; Wang Jianping [Center for Micromagnetics and Information Technologies (MINT) and Department of Electrical and Computer Engineering, University of Minnesota, 200 Union St SE, 4-174 EE/CSci, Minneapolis, Minnesota 55455 (United States)

2011-04-01

248

Epitaxial high saturation magnetization FeN thin films on Fe(001) seeded GaAs(001) single crystal wafer using facing target sputterings  

NASA Astrophysics Data System (ADS)

It was demonstrated that Fe-N martensite (?') films were grown epitaxially on Fe(001) seeded GaAs(001) single crystal wafer by using a facing target sputtering method. X-ray diffraction pattern implies an increasing c lattice constant as the N concentration increases in the films. Partially ordered Fe16N2 films were synthesized after in situ post-annealing the as-sputtered samples with pure Fe8N phase. Multiple characterization techniques including XRD, XRR, TEM, and AES were used to determine the sample structure. The saturation magnetization of films with pure Fe8N phase measured by VSM was evaluated in the range of 2.0-2.2 T. The post annealed films show systematic and dramatic increase on the saturation magnetization, which possess an average value of 2.6 T. These observations support the existence of giant saturation magnetization in ?''-Fe16N2 phase that is consistent with a recent proposed cluster-atom model and the first principles calculation [N. Ji, X. Q. Liu, and J. P. Wang, New J. Phys. 12 063032 (2010)].

Ji, Nian; Wu, Yiming; Wang, Jian-Ping

2011-04-01

249

Uranyl Incorporation in Organic-Bearing Soil Calcite  

NASA Astrophysics Data System (ADS)

Phosphor imaging and fission track maps reveal a close correlation between uranium and organic-rich calcite that lines roots in an ancient caliche soil. Concordant U-Pb ages show that this uranium has not been mobilized since the soil calcite formed 298 ñ 1 Ma ago. Luminescence spectroscopic analysis of the organic-rich calcite demonstrates that uranium is present in the oxidized state. More or less comparable luminescence spectra were collected using a CW lamp at room temperature and at liquid nitrogen (LN2) temperature. Spectra for inorganic U species are generally greatly enhanced at liquid nitrogen temperatures over room temperature and thus the similarity may suggest that the U is bound to organic matter. UV-excitation Raman spectroscopy of the calcite samples reveals a vibrational signature that is, in addition to the calcite peaks, essentially identical to that of sodium humate, acquired under similar conditions and consistent with those reported previously for humate substances. Since these spectra are dominated by the carbon backbone structure, the persistence of a carbon backbone structure is noted, but persistence of the ligating functional groups cannot be proven directly. We suggest that this sample records a snapshot of the process of uranium complexation with organic acids in natural systems. In our model organic acids with active functional groups that complex uranyl are sorbed onto the calcite surface and then incorporated as humate into the mineral with further calcite precipitation. Further, the uranyl complex is preserved through near-surface reducing conditions and temperatures to the conditions associated with 2700 meters burial depths. These combined observations suggest that the encapsulating calcite not only prevents reduction of uranium in the surface environment, but also inhibits the breakdown of the humate with burial. Due to the long time scale of immobilization of uranium, evidenced by the age of the calcite, this study has profound implications for understanding sampling for U-series and U-Pb dating of carbonates as well as for developing strategies for nuclear waste management. This study also has implications for any other deposits where uranium is associated with organic matter.

Rasbury, E.; Tait, C.; Donohoe, R. J.

2003-12-01

250

Nuclear anomalies in the buccal cells of calcite factory workers  

PubMed Central

The micronucleus (MN) assay on exfoliated buccal cells is a useful and minimally invasive method for monitoring genetic damage in humans. To determine the genotoxic effects of calcite dust that forms during processing, MN assay was carried out in exfoliated buccal cells of 50 (25 smokers and 25 non-smokers) calcite factory workers and 50 (25 smokers and 25 non-smokers) age- and sex-matched control subjects. Frequencies of nuclear abnormalities (NA) other than micronuclei, such as binucleates, karyorrhexis, karyolysis and ‘broken eggs', were also evaluated. Micronuclei and the other aforementioned anomalies were analysed by two way analysis of covariance. The linear correlations between the types of micronucleus and nuclear abnormalities were determined by Spearman's Rho. There was a positive correlation between micronuclei and other types of nuclear abnormalities in accordance with the Spearman's Rho test. Results showed statistically significant difference between calcite fabric workers and control groups. MN and NA frequencies in calcite fabric workers were significantly higher than those in control groups (p < 0.05). The results of this study indicate that calcite fabric workers are under risk of significant cytogenetic damage.

2010-01-01

251

Novel TEM approaches to imaging of microstructures in carbonates: Clues to growth mechanisms in calcite and dolomite  

SciTech Connect

Relating microstructures imaged by transmission electron microscopy (TEM) to specific growth mechanisms requires imaging of both the surface microtopography on crystal faces and its corresponding microstructure in the bulk crystal. Pt-C replicas of as-grown surfaces are ideally suited to this task, as they provide a high-resolution, nearly three-dimensional image of surface topography that can be correlated with microstructures visible in sections at high angle to the as-grown surface. Ultramicrotomy enables the preparation of ultrathin sections more quickly than conventional ion-thinning and can be used to investigate chemical heterogeneities by analytical electron microscopy (AEM). The authors evaluate the potential of both techniques for the study of microstructures in calcite and dolomite. (1) TEM images were obtained from Pt-C replicas of synthetic calcite. The as-grown (10{bar 1}4) face of a Mg/Mn-doped crystal growth showed growth hillocks aligned along preferred orientations. In Pb/Mn/Sr-doped calcite, sections at high angle to as-grown faces showed a uniform microstructure in {l{underscore}brace}10{bar 1}4{r{underscore}brace} sectors. The {l{underscore}brace}01{bar 1}2{r{underscore}brace} sectors contained concentric zones that alternated from striated to uniform microstructures, suggesting periodic variations in growth rate and possibly in the growth mechanism. An oscillatory-zoned Mn-doped calcite showed fine-scale banding (30--150 mm) and periodic roughening of (10{bar 1}4) surfaces that suggest repetitive transitions between growth mechanisms.(2) In sedimentary dolomites, Pt-C replicas of surfaces cleaved parallel to a (10{bar 1}4) face showed a hillocky topography or smooth (10{bar 1}4) surfaces modified by non-equivalent facets. Surfaces produced by cleavage at high angle to a (10{bar 1}4) face exposed sectors with fine-scale banding crosscut by sharp boundaries, suggestive of closely spaced growth hillocks intercalated with non-equivalent subsectors. Ultrathin sections prepared by microtomy showed some disruption of the crystal structure but, in some dolomite samples, domains with distinctive microstructures similar in scale and shape to those seen in the Pt-C replica were identified. (3) The growth microstructures in {l{underscore}brace}10{bar 1}4{r{underscore}brace} sectors of dolomite present striking similarities to the larger scale compositional zoning patterns produced by growth spirals in calcite doped with trace elements. At this stage, Pt-C replicas and ultramicrotomy can distinguish among dolomites that are petrographically very similar.

Paquette, J.; Vali, H.; Mountjoy, E.

1999-12-01

252

Polysaccharide chemistry regulates kinetics of calcite nucleation through competition of interfacial energies  

PubMed Central

Calcified skeletons are produced within complex assemblages of proteins and polysaccharides whose roles in mineralization are not well understood. Here we quantify the kinetics of calcite nucleation onto a suite of high-purity polysaccharide (PS) substrates under controlled conditions. The energy barriers to nucleation are PS-specific by a systematic relationship to PS charge density and substrate structure that is rooted in minimization of the competing substrate–crystal and substrate–liquid interfacial energies. Chitosan presents a low-energy barrier to nucleation because its near-neutral charge favors formation of a substrate–crystal interface, thus reducing substrate interactions with water. Progressively higher barriers are measured for negatively charged alginates and heparin that favor contact with the solution over the formation of new substrate–crystal interfaces. The findings support a directing role for PS in biomineral formation and demonstrate that substrate–crystal interactions are one end-member in a larger continuum of competing forces that regulate heterogeneous crystal nucleation.

Hamm, Laura M.; Han, Nizhou; De Yoreo, James J.; Dove, Patricia M.

2013-01-01

253

Sulfation of calcitic and dolomitic lime mortars in the presence of diesel particulate matter  

NASA Astrophysics Data System (ADS)

The sulfation of four types of calcitic and dolomitic lime mortars exposed to SO2 in the presence of particulate matter from diesel vehicle exhaust emissions has been investigated. The binders mineralogy and mortars texture are the main factors influencing the formation of deleterious sulfate salts. The type of binder also influences the pore size distribution and the total porosity of the mortars: for equal aggregate (quartz or dolomite), dolomitic lime mortars have smaller pores and higher porosity than calcitic ones. During the first 24 h exposure to SO2, calcitic lime mortars undergo a higher weight increase than dolomitic ones due to rapid formation of gypsum on their surface. However, at the end of the sulfation test (10 days), dolomitic mortars show a higher weight increase due to massive formation of epsomite and gypsum, which is facilitated by their higher porosity and the high reactivity of Mg phases in the porous and partially carbonated binder. Control samples (not covered with diesel particulate matter) also develop calcium and magnesium sulfates upon long term exposure to SO2. This is due to the presence of uncarbonated Ca and Mg hydroxides that promote SO2 fixation as sulfates. However, the amount and size of sulfate crystals are significantly smaller than those observed on samples covered with diesel particulate matter. These results show that diesel particulate matter enhances the sulfation of lime mortars and demonstrate that sulfation of dolomitic lime is an important mechanism for the in situ formation of highly soluble and deleterious hydrated magnesium sulfates (epsomite and hexahydrite). The use of dolomitic limes in the conservation of monuments exposed to air pollution in urban environments may therefore pose a significant risk.

Cultrone, G.; Arizzi, A.; Sebastián, E.; Rodriguez-Navarro, C.

2008-12-01

254

Specific effects of background electrolytes on the kinetics of step propagation during calcite growth  

NASA Astrophysics Data System (ADS)

The mechanisms by which background electrolytes modify the kinetics of non-equivalent step propagation during calcite growth were investigated using Atomic Force Microscopy (AFM), at constant driving force and solution stoichiometry. Our results suggest that the acute step spreading rate is controlled by kink-site nucleation and, ultimately, by the dehydration of surface sites, while the velocity of obtuse step advancement is mainly determined by hydration of calcium ions in solution. According to our results, kink nucleation at acute steps could be promoted by carbonate-assisted calcium attachment. The different sensitivity of obtuse and acute step propagation kinetics to cation and surface hydration could be the origin of the reversed geometries of calcite growth hillocks (i.e., rate of obtuse step spreading < rate of acute step spreading) observed in concentrated (ionic strength, IS = 0.1) KCl and CsCl solutions. At low IS (0.02), ion-specific effects seem to be mainly associated with changes in the solvation environment of calcium ions in solution. With increasing electrolyte concentration, the stabilization of surface water by weakly paired salts appears to become increasingly important in determining step spreading rate. At high ionic strength (IS = 0.1), overall calcite growth rates increased with increasing hydration of calcium in solution (i.e., decreasing ion pairing of background electrolytes for sodium-bearing salts) and with decreasing hydration of the carbonate surface site (i.e., increasing ion pairing for chloride-bearing salts). Changes in growth hillock morphology were observed in the presence of Li +, F - and SO42-, and can be interpreted as the result of the stabilization of polar surfaces due to increased ion hydration. These results increase our ability to predict crystal reactivity in natural fluids which contain significant amounts of solutes.

Ruiz-Agudo, Encarnación; Putnis, Christine V.; Wang, Lijun; Putnis, Andrew

2011-07-01

255

Effect of the fluorescent indicator calcein on Mg and Sr incorporation into foraminiferal calcite  

NASA Astrophysics Data System (ADS)

The development of particular analytical methods like laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) renders it possible to determine the composition of a single chamber of foraminifers tests. This is crucial in the investigation of benthic foraminifers since the growth of new chambers under laboratory conditions does not face the difficulties associated with experiments involving reproduction. The addition of chambers can be recognized by the incorporation of the fluorescent dye calcein. When added to the culture medium, previously formed chambers are not affected, and calcein is only incorporated in calcite that is formed in its presence. However, it has never been verified whether calcein affects the element incorporation into foraminiferal calcite. In order to investigate and quantify possible effects, specimens of the benthic foraminifer Ammonia tepida were cultured in the presence and absence of calcein (15°C, salinity 33), and Mg and Sr in newly formed chambers were analyzed with LA-ICP-MS. Magnesium concentrations of cross sections and longitudinal sections of foraminifera from the same experiment were also analyzed by electron microprobe measurements. Additionally, the impact of calcein on Mg and Sr incorporation in inorganically precipitated calcium carbonate crystals was quantified. Results show that presence of calcein does not impact the incorporation of Mg and Sr into biologically and inorganically precipitated calcium carbonate.

Dissard, Delphine; Nehrke, Gernot; Reichart, Gert Jan; Nouet, Julius; Bijma, Jelle

2009-11-01

256

Kinetic inhibition of calcite (1 0 4) dissolution by aqueous manganese(II)  

NASA Astrophysics Data System (ADS)

This study presents evidence of calcite (1 0 4) dissolution inhibition by dissolved manganese(II) through detailed observations of crystal surface morphology. Rates of dissolution have been quantified by direct measurement of surface-normal retreat and etch pit growth at far-from-equilibrium conditions using vertical scanning interferometry. The approach provides new insight into the control that dissolved inorganic carbon (DIC) exerts on manganese inhibition kinetics. Our results show that at 2×10-6 molal manganese, the rate of overall calcite dissolution is suppressed to close to zero, but only when sufficient CO32- is available in solution. The near arrest of surface-normal dissolution is brought about by the likely reduction in step retreat, inferred through observed variation of etch pit and surface morphology. The inhibition mechanism likely reflects the strong affinity of Mn2+ for carbonate ion. In solution, this affinity is expressed by formation of a strong complexing ligand (MnCO30). On the (1 0 4) surface, this affinity may govern formation of a similar surface complex that stabilizes reactive sites, thereby inhibiting step movement and defect nucleation. Comparison with results from other metals suggests that inhibition may be understood in the context of the relative energetics of dehydration versus carbonation reaction steps.

Vinson, Michael D.; Arvidson, Rolf S.; Luttge, Andreas

2007-09-01

257

Field test of a calcite dissolution rate law: Fort's Funnel Cave, Mammoth Cave National Park  

SciTech Connect

The laboratory-derived calcite dissolution rate law of Plummer et al. (1978) is the most widely used and mechanistically detailed expression currently available for predicting dissolution rates as a function of water chemistry. Such rate expressions are of great use in understanding timescales associated with limestone karst development. Little work has gone into the field testing of the rate law under natural conditions. This work measured dissolution rates by a crystal weight loss experiment in Buffalo Creek within Fort's funnel Cave, which lies within a pristine, forested catchment of Mammoth Cave National Park. Continuous water chemistry sampling over the same period allowed a time-integrated prediction of the dissolution based on the Plummer et al. (1978) expression. Results indicate that the rate law overpredicted dissolution by a factor of about ten. This concurs with earlier laboratory work suggesting that the law tends to overpredict rates in solutions close to equilibrium with respect to calcite, as were the waters within this part of the groundwater flow system.

Slunder, J.S. (EMPE Inc., Nashville, TN (United States)); Groves, C.G. (Western Kentucky Univ., Bowling Green, KY (United States). Center for Cave and Karst Studies)

1994-03-01

258

EXAFS study of rare-earth element coordination in calcite  

NASA Astrophysics Data System (ADS)

Extended X-ray absorption fine-structure (EXAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd 3+ , Sm 3+ , Dy 3+ , Yb 3+ ) coprecipitated with calcite in minor concentrations from room-temperature aqueous solutions. Fitting results confirm substitution in the Ca site, but first-shell Nd-O and Sm-O distances are longer than the Ca-O distance in calcite and longer than what is consistent with ionic radii sums for sixfold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and are consistent with ionic radii sums for sixfold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggests that Nd 3+ and Sm 3+ have sevenfold coordination in a modified Ca site in calcite. This would require some disruption of the local structure, with an expected decrease in stability, and possibly a different charge compensation mechanism between Nd and Sm vs. Yb and Dy. A possible explanation for the increased coordination for the larger rare-earth elements involves bidentate ligation from a CO 3 group. Because trivalent actinides such as Am 3+ and Cm 3+ have ionic radii similar to Nd 3+ , their incorporation in calcite may result in a similar defect structure.

Elzinga, E. J.; Reeder, R. J.; Withers, S. H.; Peale, R. E.; Mason, R. A.; Beck, K. M.; Hess, W. P.

2002-09-01

259

The behavior of Ni 2+ on calcite surfaces  

Microsoft Academic Search

Transport of Ni2+ in the geosphere plays a role in the formation of ore deposits as well as in the dispersion of contaminants in the environment. Some elements (Cd2+, Zn2+, Na+, K+, and Cl?) are known to diffuse in calcite at the rate of nanometers in months, so questions arose about the ability of Ni2+ to move away from adsorption

U. Hoffmann; S. L. S. Stipp

2001-01-01

260

Age Constraints on Fluid Inclusions in Calcite at Yucca Mountain.  

National Technical Information Service (NTIS)

The (sup 207)Pb/(sup 235)U ages for 14 subsamples of opal or chalcedony layers younger than calcite formed at elevated temperature range between 1.88 (+-) 0.05 and 9.7 (+-) 1.5 Ma with most values older than 6-8 Ma. These data indicate that fluids with el...

L. A. Neymark Y. V. Amelin J. B. Paces Z. E. Peterman J. F. Whelan

2002-01-01

261

Quantitative estimation of calcite in limestones by differential thermal analysis  

Microsoft Academic Search

A quantitative method is presented for the estimation of calcite in limestones by differential thermal analysis, using calcium hydroxide as an internal standard. The effects of variations due to particle size and impurities such as quartz, iron oxide, organic material, dolomite, magnesite, gypsum and phosphates are discussed.

K. Jehan; M. A. Qaiser; A. H. Khan

1972-01-01

262

Calcite and Picocyanobacteria in Lakes: Factors Affecting Their Interaction  

Microsoft Academic Search

Calcites build large deposits which have been observed in the rock record throughout geological time at various localities around the globe. Carbonate deposits have affected atmospheric carbon dioxide concentration. As it has been generally accepted, inorganic precipitation represents a source of carbon dioxide on short geological time scales and a sink of inorganic carbon at long time scales from millions

M. Dittrich; M. Obst; D. Mavrocordatos

2003-01-01

263

Calcite as a source of excess calcium in rainwater  

Microsoft Academic Search

Snow and rain samples collected in Sendai, Japan, were analyzed for sodium, potassium, magnesium, calcium, and strontium. The chemical composition of the samples can be fairly explained by adding some calcium to diluted seawater. A low Sr\\/Ca ratio in most of the samples showed that the excess calcium over seawater contribution may be derived from calcite contained in soil dust

Masami Ichikuni

1978-01-01

264

Influence of geometry upon crack healing rate in calcite  

Microsoft Academic Search

Crack healing experiments were conducted at 780°, 815°, and 850° C in dry carbon dioxide over periods of 0.5 to 1320 h on small penny-shaped cracks of known geometry in Stahl calcite. As observed by previous investigators, healing initiates with the formation of tubes around the leading edge of the crack, followed by the pinching off of spherical bubbles from

Stephen H. Hickman; Brian Evans

1987-01-01

265

Nucleation and growth kinetics in synthesizing nanometer calcite  

Microsoft Academic Search

The size of nanometer particles uniquely depends not only on the nucleus growth rate but also on the nucleation rate. This paper presents the effect of sodium tripolyphosphate on the microimages, synthesis reaction, nucleation and growth during the synthesizing of nanometer calcite using chemical analysis, SEM technique and Rosin–Ramuler probability statistics theory. The result showed that the calcium hydroxide carbonation

Rong-yi Lin; Jia-Yun Zhang; Pei-xin Zhang

2002-01-01

266

Comparative study of nanoscale surface structures of calcite microcrystals using FE-SEM, AFM, and TEM.  

PubMed

The bulk morphology and surface features that developed upon precipitation on micrometer-size calcite powders and millimeter-size cleavage fragments were imaged by three different microscopic techniques: field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) of Pt-C replicas, and atomic force microscopy (AFM). Each technique can resolve some nanoscale surface features, but they offer different ranges of magnification and dimensional resolutions. Because sample preparation and imaging is not constrained by crystal orientation, FE-SEM and TEM of Pt-C replicas are best suited to image the overall morphology of microcrystals. However, owing to the decoration effect of Pt-C on the crystal faces, TEM of Pt-C replicas is superior at resolving nanoscale surface structures, including the development of new faces and the different microtopography among nonequivalent faces in microcrystals, which cannot be revealed by FE-SEM. In conjunction with SEM, Pt-C replica provides the evidence that crystals grow in diverse and face-specific modes. The TEM imaging of Pt-C replicas has nanoscale resolution comparable to AFM. AFM yielded quantitative information (e.g., crystallographic orientation and height of steps) of microtopographic features. In contrast to Pt-C replicas and SEM providing three-dimensional images of the crystals, AFM can only image one individual cleavage or flat surface at a time. PMID:16842642

Chien, Yung-Ching; Mucci, Alfonso; Paquette, Jeanne; Sears, S Kelly; Vali, Hojatollah

2006-08-01

267

The behavior of Ni 2+ on calcite surfaces  

NASA Astrophysics Data System (ADS)

Transport of Ni 2+ in the geosphere plays a role in the formation of ore deposits as well as in the dispersion of contaminants in the environment. Some elements (Cd 2+, Zn 2+, Na +, K +, and Cl -) are known to diffuse in calcite at the rate of nanometers in months, so questions arose about the ability of Ni 2+ to move away from adsorption sites at the surface into the bulk. Nickel incorporation into calcite is limited by its high dehydration enthalpy and by its ligand field hindrance to entering the distorted octahedra of calcite, but evidence exists that calcite can tolerate several percent Ni 2+ in the structure. Cleaved samples of Iceland spar were exposed for 1 minute to solutions of 10 -3 M and 10 -2 M Ni(ClO 4) 2, the solution was physically removed and the samples were examined using the surface sensitive techniques: X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) and Atomic Force Microscopy (AFM). XPS and TOF-SIMS showed that Ni 2+ was adsorbed while AFM confirmed that dissolution was taking place. The sample was stored in air and relative surface concentration and physical morphology were monitored for 2 years. Trends in the chemical data suggested statistically significant loss of surface Ni 2+ with time, but the decrease was very close to the limits for significance. AFM images demonstrated that surface topography of the Ni-exposed samples is modified by spontaneous recrystalization in the water layer adsorbed from air in exactly the same way that clean calcite surfaces typically rearrange. This process could bury a small amount of Ni 2+ in the bulk, explaining the very weak loss. Limited burial of Ni 2+ within the near-surface could renew calcite adsorption sites, thus increasing uptake capacity. Evidence indicates that surface recrystalization occurs even in very dry environments (<5% humidity). This means that burial could play a role in Ni 2+ mobility in unsaturated groundwater regimes or in fractures (such as in concrete) where water flow is intermittent. An important point is, however, in comparison to incorporation rates for divalent Cd and Zn, the extent of movement of Ni 2+ is extremely low. Thus, incorporation might have an effect on Ni 2+ retardation in flow paths extending over very long time scales (>10,000 years) such as would be relevant for geological processes and for long-term radioactive waste disposal. However, incorporation by burial would have negligible effect on the amount of Ni 2+ removed from groundwater by adsorption, in systems where the transport times are short (<100 years) such as for drinking water supplies from calcite-bearing porous media.

Hoffmann, U.; Stipp, S. L. S.

2001-11-01

268

The inhibition of calcium carbonate crystal growth by the cysteine-rich Mdm2 peptide  

Microsoft Academic Search

The crystal growth of calcite, the most stable calcium carbonate polymorph, in the presence of the cysteine-rich Mdm2 peptide (containing 48 amino acids in the ring finger configuration), has been investigated by the constant composition technique. Crystallization took place exclusively on well-characterized calcite crystals in solutions supersaturated only with respect to this calcium carbonate salt. The kinetic results indicated a

E. Dalas; A. Chalias; D. Gatos; K. Barlos

2006-01-01

269

Molecular ordering of ethanol at the calcite surface.  

PubMed

To produce biominerals, such as shells, bones, and teeth, living beings create organic compounds that control the growth of the solid phase. Investigating the atomic scale behavior of individual functional groups at the mineral-fluid interface provides fundamental information that is useful for constructing accurate predictive models for natural systems. Previous investigations of the activity of coccolith-associated polysaccharides (CAP) on calcite, using atomic force microscopy (AFM) [Henriksen, K., Young, J. R., Bown, P. R., and Stipp, S. L. S. Palentology 2004, 43 (Part 3), 725-743] and molecular dynamics (MD) modeling [Yang, M., Stipp, S. L. S., and Harding, J. H. Cryst. Growth Des. 2008, 8 (11), 4066-4074], have suggested that OH functional groups control polysaccharide attachment. The purpose of this work was to characterize, using X-ray reflectivity (XR) combined with molecular dynamics (MD) simulations, the structuring on calcite of a layer of the simplest carbon chain molecule that contains an OH group, ethanol (CH(3)-CH(2)-OH). We found evidence that EtOH forms a highly ordered structure at the calcite surface, where the first layer molecules bond with calcite. The ethanol molecules stand up perpendicularly at the interface or nearly so. As a consequence, the fatty, CH(3) ends form a new surface, about 6 Å from the termination of the bulk calcite, and beyond that, there is a thin gap where ethanol density is low. Following is a more disordered layer that is two to three ethanol molecules thick, about 14 Å, where density more resembles that of bulk liquid ethanol. The good agreement between theory and experiment gives confidence that a theoretical approach can offer information about behavior in more complex systems. PMID:22060260

Pasarín, I S; Yang, M; Bovet, N; Glyvradal, M; Nielsen, M M; Bohr, J; Feidenhans'l, R; Stipp, S L S

2012-01-19

270

AFM-Study on Calcite Nucleation by Cyanobacteria  

NASA Astrophysics Data System (ADS)

Prokaryotic picoplankton plays an important role in lacustrine calcite precipitation, especially in oligotrophic lakes. The mechanisms of cyanobacteria-surface mediated precipitation reactions, however, remain poorly understood. For interpreting geochemical and isotopic information stored in sediments, it is essential to know the carbonate precipitation mechanisms and microscopical details of nucleation on the cell surface. A microscopy study was designed, based on previous results of cyanobacterial calcite precipitation. In order to investigate the nucleation of calcite on the cell surface under controlled conditions, experiments with Synechococcus (PCC 7942) were performed in a flow-through design of an atomic force microscope (AFM). The AFM was operated in `Tapping Mode' providing information on the topography and additionally on physicochemical properties by mapping the phase shift of the oscillating cantilever. For these experiments cells were immobilized on glass slides. As the basic requirement, growth experiments were performed in order to ensure that the fixed cells were alive. To our knowledge one of the first times the successive steps of a cyanobacterial cell division were observed by AFM. The growth of cell agglomerates was monitored over a period of 6 days. AFM images finally reveal the formation of a cyanobacterial biofilm on the glass surface. These observations were supported by the results obtained by epifluorescence microscopy. In a second step, calcite nucleation experiments were performed. Under carefully controlled physical and chemical conditions, nucleation of calcite was enhanced by flushing the fluid cell of the AFM with supersaturated solutions of CaCO3 (CaCl2 and NaHCO3). These solutions were 7 and 10 times supersaturated. As the result of the addition of the CaCO3-solution, changes of the microtopography and physicochemical properties of the cell surface were observed. In our study in-situ-AFM was shown to be powerful tool investigating mechanisms of biomineralization and nucleation under controlled natural conditions.

Obst, M.; Kuehn, H.; Dittrich, M.

2004-12-01

271

Study of growth of calcium carbonate crystals on chitosan film  

Microsoft Academic Search

Biomimetic heterogeneous nucleation and growth of calcium carbonate crystals on the chitosan film as the template took place in supersaturated calcium bicarbonate solution. The specimen were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, attenuated total reflectance infrared spectroscopy. The obtained crystals proved to be calcite in sheets composed of fine crystal particles. Lab results indicated that proper additives

Fengying Zhang; Jie Wang; Zhengchi Hou; Ming Yu; Leidong Xie

2006-01-01

272

Calcite-dolomite thermometry: Negative evidence from the Marmorilik Formation, West Greenland  

Microsoft Academic Search

Calcite grains from 19 composite dolomite-calcite marble samples from the Marmorilik Formation, West Greenland, have been analysed for Ca, Mg, and Fe with the microprobe in order to estimate metamorphic temperatures using the calcite-dolomite solvus geothermometer. The results show that there are large variations within single samples, and, insofar as any thermal gradient can be detected, it is the opposite

Adam A. Garde

1977-01-01

273

Modelling of the thermal dependence of structural and elastic properties of calcite, CaCO3  

NASA Astrophysics Data System (ADS)

A computational method, based on the quasiharmonic approximation, has been computer-coded to calculate the temperature dependence of elastic constants and structural features of crystals. The model is applied to calcite, CaCO3; an interatomic potential based on a C-O Morse function and Ca-O and O-O Borntype interactions, including a shell model for O, has been used. Equilibrations in the range 300 800 K reproduce the experimental unit-cell edges and bond lengths within 1%. The simulated thermal expansion coefficients are 22.3 (//c) and 2.6 (? c), against 25.5 and-3.7×10-6K-1 experimental values, respectively. The thermal coefficients of elastic constants tend to be underestimated; for the bulk modulus, -2.3 against-3.7×10-4K-1 is obtained.

Pavese, A.; Catti, M.; Parker, S. C.; Wall, A.

1996-03-01

274

Stability of basalt+anhydrite+calcite at HP-HT: implications for Venus, the Earth and Mars  

NASA Astrophysics Data System (ADS)

“Canali” observed at Venus’ surface by Magellan are evidence for very long melt flows, but their composition and origin remain uncertain. The hypothesis of water-rich flow is not reasonable regarding Venus’ surface temperature. The length of these channels could not be explained by a silicate melt composition but more likely, by a carbonate-sulfate melt which has a much lower viscosity (Kargel et al 1994). One hypothesis is that calcite CaCO3 and anhydrite CaSO4 - which are alteration products of basalts - melted during meteorite impacts. A famous example recorded on the Earth (Chicxulub) produced melt and gas rich in carbon and sulfur. Calcite and sulfate evaporites are also present on Mars surface, associated with basalts. An impact on these materials may release C- and S-rich melt or fluid. Another type of planetary phenomenon (affecting only the Earth) might provoke a high pressure destabilization of basalt+anhydrite+calcite. Very high contents of C and S are measured in some Earth’s magmas, either dissolved or in the form of crystals (Luhr 2008). As shown by the high H content and high fO2 of primary igneous anhydrite-bearing lavas, the high S content in their source may be explained by the subduction of an anhydrite-bearing oceanic crust, either directly (by melting followed by eruption) or indirectly (by release of S-rich melt or fluid that metasomatize the mantle). Calcite is a major product of oceanic sedimentation and alteration of the crust. Therefore, sulfate- and calcite-rich material may be subducted to high pressures and high temperatures (HP-HT) and release S- and C-rich melts or fluids which could influence the composition of subduction zone lavas or gases. Both phenomena - meteorite impact and subduction - imply HP-HT conditions - although the P-T-time paths are different. Some HP experimental/theoretical studies have been performed on basalt/eclogite, calcite and anhydrite separately or on a combination of two. In this study we performed piston-cylinder experiments at 1 GPa between 950 and 1700°C using a mixture of 70wt% tholeiitic basalt + 15wt% anhydrite + 15wt% calcite. Up to ~1440°C, an ultracalcic (CaO>13.5 wt%; CaO/Al2O3>1 wt%) picrobasaltic (SiO2~40-45 wt%; Na2O+K2O<2 wt%) melt containing up to 5 wt% SO3 and up to 5.3 wt% CO2+H2O (calculated by difference) is present in equilibrium with clinopyroxene, anhydrite, spinel-chromite, a CAS-phase and a gas composed mainly of CO and an aliphatic thiol (CH2)4SH. Hydrogen was incorporated either by contact between the starting material and air or by diffusion through the capsule during the experiments. The S content in the gas increases with temperature and run duration, implying that gases with various C/S ratios might be released during an impact or at subduction zones, depending on the P-T-t path and on the H content. Above ~1440°C, a Ca-rich carbonate-sulfate melt forms (in equilibrium with the picrobasaltic melt) which contains a few percents of Na and K. Such melt is not expected to form at Earth’s subduction temperatures. If it forms by meteorite impact, it might crystallize too fast to explain long flows like Venus’ canali. A different basalt/anhydrite/calcite ratio might, however, decrease its formation temperature.

Martin, A. M.; Righter, K.; Treiman, A. H.

2010-12-01

275

Pore-Scale Investigation of Mixing-Induced Calcite Precipitation in Silicon-Based Micromodels  

NASA Astrophysics Data System (ADS)

CO2 sequestration in geologic formations is increasingly being studied as a strategy for limiting CO2 emission to the atmosphere, but there are potentials for geochemical reactions in the reservoirs and uncertainties associated with the environmental impacts. Reactions between dissolved CO2 and cations in groundwater (e.g., Ca2+) may cause mineral precipitation, and hence reductions in permeability. In this study, microfluidic pore structures etched into silicon wafers were used as two-dimensional model groundwater systems (i.e., micromodels) to study the mechanisms of mineral precipitation relevant to CO2 sequestration. Solutions containing CaCl2 and Na2CO3 were introduced through two separate inlets and they mixed along the center of the micromodel. Images of the micromodel were taken using a microscope equipped with a digital camera at selected time points. Calcite formation rates and morphology along the mixing zone are determined from images and the impacts of water flowrates and solution concentrations on reaction rates are investigated. Results indicate calcite formation rates and the morphology of precipitates along the mixing zone depends on the reactant concentrations. Large crystals were observed at lower concentration. At higher concentrations, amorphous precipitates were formed along the center of transverse mixing zone with some crystals forming off center. Impacts of pore-scale processes, i.e., mixing induced reaction, permeability changes, diffusion and dispersion, will be discussed. Current and future work on pore-scale study of caprock seal integrity and trapping mechanism of supercritical CO2 will also be discussed.

Zhang, C.; Dehoff, K.; Werth, C. J.; Valocchi, A. J.; Oostrom, M.; Wietsma, T. W.

2009-12-01

276

Microbes Caught in the Act: Disentangling the Role of Biofilms in the Formation of Low Mg Calcite Ooids in a Freshwater Lake  

NASA Astrophysics Data System (ADS)

Biofilms and molds of cyanobacteria infilling depressions within the outermost cortex of ooids have been previously described in recent ooidal sands of Lake Geneva (Switzerland). Detailed sedimentological and mineralogical analyses of these ooids further indicated a low-Mg calcite composition of their cortex. Observed ooidal morphological features called for a fundamental role of biofilms triggering carbonate precipitation. A detailed microbiological study was much needed, however, in order to decipher the relative role of organic versus purely physicochemical processes during ooid formation. An experimental device consisting of frosted microscope slides was set at 2.50 m water depth in western Lake Geneva. These slides provide an attractive substrate for the microorganisms involved in low-Mg calcite precipitation allowing the in-situ harvesting of biofilms on a regular basis during more than three years. The inspection of the frosted slides showed the development of biofilms on their surface containing coccoid and filamentous cyanobacteria, heterotrophic bacteria and diatom frustules. Microscopical observations under natural light and autoflorescence show a close association between freshly low Mg-calcite precipitates and biofilms containing at least five species of filamentous and coccoid cyanobacteria. Carbonate precipitation peaks at early spring and late summer, and low Mg calcite crystals are always in close association with mostly cyanobacteria filaments (e.g., Tolipothrix, Oscillatoria). Ultra high-resolution elemental analyses performed immediately after recovering the samples confirmed the microscopical observations. Further SEM inspection of the samples revealed a clear seasonal pattern of carbonate precipitation identifying low Mg- calcite with crystal shapes varying from poorly to not crystallized compact aggregates; subautomorph to automorph rhomboedric crystals; and snowy cluster in which particles are very little or not crystallized. Liquid and solid cultures were further developed in the laboratory from the harvested biofilms allowing the determination of the microbial community using a PCR-DGGE approach. Two different primers were used to target all bacteria, and cyanobacteria and diatoms (341F-GC/907RM and 359F-GC/781 (a+b), respectively). Initial results indicate a similar microbial diversity between the sampled natural biofilms and those from BG11 enriched cultures. These data will allow us to design further laboratory experiments on low-Mg calcite precipitation including changes in pH, temperature and light intensity that mimic the natural biological and physicochemical cycle of the modern lake water throughout the year. Hence, this first dataset illustrates the significance of the in situ experiment to validate previous observations. Combined with the ongoing microbial cultures under laboratory-controlled conditions the outcome of our investigations will bring new light behind the role of biofilms in freshwater ooids formation.

Plee, K.; Ariztegui, D.; Sahan, E.; Martini, R.; Davaud, E.

2006-12-01

277

Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: III. Impact of salinity/ionic strength  

PubMed Central

In order to apply Sr/Ca and 44Ca/40Ca fractionation during calcium carbonate (CaCO3) formation as a proxy to reconstruct paleo-environments, it is essential to evaluate the impact of various environmental factors. In this study, a CO2 diffusion technique was used to crystallize inorganic calcite from aqueous solutions at different ionic strength/salinity by the addition of NaCl at 25 °C. Results show that the discrimination of Sr2+ versus Ca2+ during calcite formation is mainly controlled by precipitation rate (R in ?mol/m2/h) and is weakly influenced by ionic strength/salinity. In analogy to Sr incorporation, 44Ca/40Ca fractionation during precipitation of calcite is weakly influenced by ionic strength/salinity too. At 25 °C the calcium isotope fractionation between calcite and aqueous calcium ions (?44/40Cacalcite-aq = ?44/40Cacalcite ? ?44/40Caaq) correlates inversely to log R values for all experiments. In addition, an inverse relationship between ?44/40Cacalcite-aq and log DSr, which is independent of temperature, precipitation rate, and aqueous (Sr/Ca)aq ratio, is not affected by ionic strength/salinity either. Considering the log DSr and ?44/40Cacalcite-aq relationship, Sr/Ca and ?44/40Cacalcite values of precipitated calcite can be used as an excellent multi-proxy approach to reconstruct environmental conditions (e.g., temperature, precipitation rate) of calcite growth and diagenetic alteration.

Tang, Jianwu; Niedermayr, Andrea; Kohler, Stephan J.; Bohm, Florian; K?sakurek, Basak; Eisenhauer, Anton; Dietzel, Martin

2012-01-01

278

Biological Control of Crystal Texture: A Widespread Strategy for Adapting Crystal Properties to Function  

Microsoft Academic Search

Textures of calcite crystals from a variety of mineralized tissues belonging to organisms from four phyla were examined with high-resolution synchrotron x-ray radiation. Significant differences in coherence length and angular spread were observed between taxonomic groups. Crystals from polycrystalline skeletal ensembles were more perfect than those that function as single-crystal elements. Different anisotropic effects on crystal texture were observed for

Amir Berman; Jonathan Hanson; Leslie Leiserowitz; Thomas F. Koetzle; Stephen Weiner; Lia Addadi

1993-01-01

279

A data-driven model of the global calcite lysocline  

NASA Astrophysics Data System (ADS)

Gridded maps of sediment calcium carbonate (calcite) concentration and overlying water saturation state [Archer, 1996] are combined with maps of benthic oxygen fluxes and sediment accumulation rates from Jahnke [1996] and Cweink [1986] to drive a diagenetic model of calcium carbonate preservation in deep-sea sediments. The only model input for which we cannot draw a detailed map is the rain rate of calcite to the seafloor, so I use the model to calculate the calcite rain rate required to simulate the observed distribution of calcite concentration on the seafloor. The predictive power of the model can be checked by searching for input parameters to which the model is sensitive and comparing the model requirements with independent data. The model is sensitive to the ratio of organic carbon to calcite rain rates (the rain ratio) and, within the constraint of pelagic sediment trap rain ratio data, is unable to reproduce the calcite field without respiratory dissolution, the promotion of calcium carbonate dissolution by the oxic degradation of organic carbon. However, relative to variability in sediment trap data, the required model rain ratio is insensitive to the extent of anoxic respiration, the stoichiometric ratio of O:C during respiration, the bioturbation rate, the dissolution rate constant, the effect of borate chemistry, and small offsets in the saturation state for CaCO3. The model is sensitive to the accumulation rate of non-CaCO3 material and predicts an Atlantic/Pacific difference in non-CaCO3 rain rate which is consistent with observations. The model predicts that the dissolution flux of CaCO3 from sediments is 25-40 × 1012 mol yr-1, roughly half of the total deep sea CaCO3 dissolution rate estimated from the water column alkalinity distribution [Broecker and Peng, 1987; Archer and Maier-Reimer, 1994; Wollast, 1994]. The model also predicts that only 20-30% of the flux of CaCO3 to the seafloor globally escapes dissolution.

Archer, David

1996-09-01

280

The solubility of Fish Produced High Magnesium Calcite in Seawater  

NASA Astrophysics Data System (ADS)

Fish have been shown to produce high 10 to 30% magnesium calcite as part of the physiological mechanisms responsible for maintaining salt and water balance. The importance of this source to the marine carbon cycle is only now being considered. In this paper we report the first measurements of the solubility of this CaCO3 in seawater. The resulting solubility (pKsp = 5.85 ± 0.07) is more than two times higher than aragonite and similar to the high magnesium calcite generated on the Bahamas Banks (pKsp = 5.90). This high solubility of fish produced CaCO3 renders it soluble in near surface (<2000 m) waters contributing to the input of carbonate to surface ocean waters, and at least partially explaining the increase in total alkalinity above the aragonite saturation horizon.

Woosley, R. J.; Millero, F. J.; Grosell, M.

2011-12-01

281

Incorporation of Mg and Sr in calcite of cultured benthic foraminifera: impact of calcium concentration and associated calcite saturation state  

NASA Astrophysics Data System (ADS)

We investigated the effect of the calcium concentration in seawater and thereby the calcite saturation state (?) on the magnesium and strontium incorporation into benthic foraminiferal calcite under laboratory conditions. For this purpose individuals of the shallow-water species Heterostegina depressa (precipitating high-Mg calcite, symbiont-bearing) and Ammonia tepida (low-Mg calcite, symbiont-barren) were cultured in media under a range of [Ca2+], but similar Mg/Ca ratios. Trace element/Ca ratios of newly formed calcite were analysed with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and normalized to the seawater elemental composition using the equation DTE=(TE/Cacalcite)/(TE/Caseawater). The culturing study shows that DMg of A. tepida significantly decreases with increasing ? at a gradient of -4.3×10-5 per ? unit. The DSr value of A. tepida does not change with ?, suggesting that fossil Sr/Ca in this species may be a potential tool to reconstruct past variations in seawater Sr/Ca. Conversely, DMg of H. depressa shows only a minor decrease with increasing ?, while DSr increases considerably with ? at a gradient of 0.009 per ? unit. The different responses to seawater chemistry of the two species may be explained by a difference in the calcification pathway that is, at the same time, responsible for the variation in the total Mg incorporation between the two species. Since the Mg/Ca ratio in H. depressa is 50-100 times higher than that of A. tepida, it is suggested that the latter exhibits a mechanism that decreases the Mg/Ca ratio of the calcification fluid, while the high-Mg calcite forming species may not have this physiological tool. If the dependency of Mg incorporation on seawater [Ca2+] is also valid for deep-sea benthic foraminifera typically used for paleostudies, the higher Ca concentrations in the past may potentially bias temperature reconstructions to a considerable degree. For instance, 25 Myr ago Mg/Ca ratios in A. tepida would have been 0.2 mmol/mol lower than today, due to the 1.5 times higher [Ca2+] of seawater, which in turn would lead to a temperature underestimation of more than 2 °C.

Raitzsch, M.; Dueñas-Bohórquez, A.; Reichart, G.-J.; de Nooijer, L. J.; Bickert, T.

2010-03-01

282

Interaction of copper with the surface of calcite  

SciTech Connect

The interaction of Cu ions in solution with the surface of calcite has been studied in a range of solutions from pure water to seawater. Observations of the uptake of Cu from solution onto calcite indicates that the process is rapid and strong in both distilled water and seawater. In distilled water, Cu uptake is directly proportional to the concentration of Cu in solution; Cu/sub s/ = K/sub s/Cu/sub 1/. This Cu/sub s/ dependence on Cu/sub 1/ is linear over the entire Cu concentration range studied (0.1 to 200 ..mu..M). Results do not indicate the formation of a precipitate of either malachite or copper carbonate. In seawater, the uptake of Cu is also directly proportional to the concentration of Cu/sub 1/ up to a limiting value of approximately 13 ..mu..M. The average value for K/sub s/ in seawater, 0.24 +- 0.06 (Cu/sub 1/ less than or equal to 13 ..mu..M), is approximately an order of magnitude less than in distilled water. This is probably the result of smaller Cu/sub 1/ activity coefficients and increased site competition by other ions in seawater. Attempts to increase the Cu/sub 1/ concentration above 13 ..mu..M resulted in the additional Cu being deposited on the surface of the calcite. A possible explanation for this behavior is the formation of a precipitate of malachite on the calcite surface. The value of K/sub s/ decreased slightly with increasing solid to solution ratios in seawater.

Franklin, M.L.; Morse, J.W.

1981-05-01

283

Planktic foraminiferal sedimentation and the marine calcite budget  

NASA Astrophysics Data System (ADS)

The vertical flux and sedimentation rate of planktic foraminiferal tests are quantified and a global planktic foraminiferal CaCO3 budget is presented. Test and calcite flux rates are calculated according to the distribution of species obtained from multinet and sediment trap samples. Modern planktic foraminiferal population dynamics are discussed as a prerequisite for the quantification of the calcite budget, highlighting the importance of ecological, autecological (e.g., reproduction), and biogeochemical conditions that determine the presence or absence of species. To complete the open-marine, particulate CaCO3 inventory, the contribution of coccolithophores, pteropods, and calcareous dinophytes is discussed. Based on the studied regions, the global planktic foraminiferal calcite flux rate at 100 m depth amounts to 1.3-3.2 Gt yr-1, equivalent to 23-56% of the total open marine CaCO3 flux. The preservation of tests varies on a regional and temporal scale, and is affected by local hydrography and dissolution. During most of the year (off-peak periods), many tests dissolve above 700-m water depth while settling through the water column, with on average only 1-3% of the initially exported CaCO3 reaching the deep-seafloor. Pulsed flux events, mass dumps of fast settling particles, yield a major contribution of tests to the formation of deep-sea sediments. On average, ˜25% of the initially produced planktic foraminiferal test CaCO3 settles on the seafloor. The total planktic foraminiferal contribution of CaCO3 to global surface sediments amounts to 0.36-0.88 Gt yr-1, ˜32-80% of the total deep-marine calcite budget.

Schiebel, Ralf

2002-12-01

284

Deformation twinning in calcite, dolomite, and other rhombohedral carbonates  

Microsoft Academic Search

Transmission electron microscope (tem) observations of single and multiple twins in calcite and dolomite are presented, and the results are analysed by means of selected area diffraction and trace analysis. Simple twinning in rhodochrosite and kutnahorite is also analysed. It is shown that the ordered carbonates, such as dolomite, have a common twinning plane {01\\u000a$$\\\\bar 1$$\\u000a2} and this

D. J. Barber; H.-R. Wenk

1979-01-01

285

Crystallographic reorganization of the calcitic prismatic layer of oysters  

Microsoft Academic Search

The calcitic columnar prisms of pteriomorphian bivalves have the crystallographic c-axis oriented perpendicular to the shell surface and the a-axes rotated without any preferential orientation. In oysters, SEM, XRD and EBSD analyses show that individual prisms initially have their a-axes randomly oriented but are able to progressively orient them parallel to those of their neighbors. This ability is apparently confined

Antonio G. Checa; Francisco J. Esteban-Delgado; Joaquín Ramírez-Rico; Alejandro B. Rodríguez-Navarro

2009-01-01

286

Use of coupled passivants and consolidants on calcite mineral surfaces  

SciTech Connect

Deterioration of monuments, buildings, and works of art constructed of carbonate-based stone potentially can be arrested by applying a combination of chemical passivants and consolidants that prevent hydrolytic attack and mechanical weakening. The authors used molecular modeling and laboratory synthesis to develop an improved passivating agent for the calcite mineral surface based on binding strength and molecular packing density. The effectiveness of the passivating agent with and without a linked outer layer of consolidant against chemical weathering was determined through leaching tests conducted with a pH-stat apparatus at pH 5 and 25 C. For the range of molecules considered, modeling results indicate that the strongest-binding passivant is the trimethoxy dianionic form of silylalkylaminocarboxylate (SAAC). The same form of silylalkylphosphonate (SAP) is the second strongest binder and the trisilanol neutral form of aminoethylaminopropylsilane (AEAPS) is ranked third. Short-term leaching tests on calcite powders coated with the trisilanol derivative of SAAC, the triethoxy neutral form of SAP, and the trimethoxy neutral form of AEAPS show that the passivant alone does not significantly slow the dissolution rate. However, all passivants when linked to the sol consolidant result in decreased rates. Combined AEAPS plus consolidant results in a coating that performs better than the commercial product Conservare{reg_sign} OH and at least as well as Conservare{reg_sign} H. The modeling results indicate that there may be a threshold binding energy for the passivant above which the dissolution rate of calcite is actually enhanced. More strongly-binding passivants may aid in the dissolution mechanism or dissociate in aqueous solution exposing the calcite surface to water.

Nagy, K.L.; Cygan, R.T.; Brinker, C.J.; Ashley, C.S. [Sandia National Labs., Albuquerque, NM (United States); Scotto, C.S. [Naval Research Lab., Washington, DC (United States). Optical Sciences Div.

1997-02-01

287

Calcite and dolomite discrimination using airborne SWIR imaging spectrometer data  

Microsoft Academic Search

The SWIR full spectrum imager (SFSI) is a hyperspectral push-broom imager, acquiring imagery in 120 0.010 micrometers wide bands simultaneously covering the 1.20 micrometers to 2.47 micrometers spectral region. During the first flights of the instrument hyperspectral imagery was acquired over a calcite quarry and a dolomite quarry. Both these minerals show distinctive carbonate absorption features in the 1.7 to

Neil Rowlands; Robert A. Neville

1996-01-01

288

A re-evaluation of aragonite versus calcite seas  

Microsoft Academic Search

Some workers have argued that the mineralogy of ancient carbonates may have been different from that of modern sediments,\\u000a with calcite being considered the dominant mineral during the Ordovician, Devonian-mid Carboniferous, and Jurassic-Cretaceous\\u000a to Early\\/Middle Cenozoic (e.g. Sandberg 1983; Wilkinson and Algeo 1989). Variation in carbonate mineralogy has been related\\u000a to the position of global sea level (emergent or submergent

Mohammad H. Adabi

2004-01-01

289

Arsenic uptake by natural calcite: An XAS study  

NASA Astrophysics Data System (ADS)

As-bearing travertine rocks from Tuscany (Italy), where previous studies suggested the existence of a CO 32- ? AsO 33- substitution in the calcite lattice, were investigated with X-ray Absorption Spectroscopy (XAS) at the As K-edge (11,867 eV). In two of the studied samples, XANES indicates that As is in the 5+ oxidation state only, and EXAFS analysis reveals a local environment typical of arsenate species. For these samples, the lack of detectable second shell signals suggests a poorly ordered environment, possibly corresponding to an adsorption onto oxide and/or silicate phases. On the other hand, in the third sample XANES reveals a mixed As oxidation state (III and V). This sample also presents evident next nearest neighbor coordination shells, attributed to As-Ca and As-As contributions. The occurrence of next neighbor shells is evidence that part of As is incorporated in an ordered lattice. Furthermore, the local structure revealed by EXAFS is compatible with As incorporation in the calcite phase, as further supported by DFT simulations. The observation of next neighbors shells only in the As(III)-rich sample suggests the substitution of the arsenite group in place of the carbonate one (CO 32- ? AsO 33-). The conclusion of this work is that uptake of As by calcite is in general less favored than adsorption onto iron oxhydroxides, but could become environmentally important wherever the latter phenomenon is hindered.

Bardelli, F.; Benvenuti, M.; Costagliola, P.; Di Benedetto, F.; Lattanzi, P.; Meneghini, C.; Romanelli, M.; Valenzano, L.

2011-06-01

290

Calcite deposition at Miravalles geothermal field, Costa Rica  

SciTech Connect

The calcite deposition problem at Miravalles has been studied since it was observed in the first three wells drilled on the slopes of the Miravalles Volcano. Long-term tests have been carried out to study reservoir characteristics. The change in the production behavior of the wells with the restriction imposed by the deposited calcite has been studied trying to evaluate and quantify the scaling problem. Work is being done on predictions of the deposition rate, location and distribution of the deposited mineral inside the wells. This work was compared with real data obtained from caliper logs of the wells before and after production. The feasibility of the first 55 MW power plant has been demonstrated. It was considered that the solution for the calcite problem is the reaming during discharge of the wells trying at the same time to minimize the cleaning interventions with a new well design. It is believed, due to the thermodynamics and chemical characteristics of the extracted fluids, that it is possible to find a non-deposition zone which will permit the drilling of wells without a scaling problem.

Vaca, L.; Alvarado, A.; Corrales, R. (Instituto Costarricense de Electricidad, San Jose (Costa Rica))

1989-01-01

291

Avocado Seeds  

NSDL National Science Digital Library

This Science Update considers the fruit in its natural state -- investigating how the great big avocado seed manages to "get around." This story investigates how the avocado seed is spread, providing an interesting introduction to the larger topic of seed dispersal.

Science Update;

2001-05-15

292

Solvothermal crystal growth of functional materials  

Microsoft Academic Search

Hydrothermal crystal growth was associated with the development of ? -quartz single crystals for devices based on piezoelectric materials. This technology was then used to elaborate single crystals of different functional materials such as quartz-like oxides (AlPO4, GaPO4, GaAsO4, etc.), calcite CaCO3, etc. During the last few years two materials, GaN (using non-aqueous solvent) and ZnO, have been particularly investigated

Alain Largeteau; Stephane Darracq; Graziella Goglio; Gerard Demazeau

2008-01-01

293

Sr isotopic and elemental characteristics of calcites in the Chinese deserts: Implications for eolian Sr transport and seawater Sr evolution  

Microsoft Academic Search

Calcite content, Sr concentrations, and isotopes of calcites in the Chinese deserts are systematically studied in this paper. Calcite contents, which are calculated according to acid-soluble Ca contents in the deserts, are generally higher in the sandy deserts than in the sandy lands and decrease roughly from northwest to northeast of China. Acid-soluble Sr is well correlated with calcite in

Wenbo Rao; Jun Chen; Jiedong Yang; Junfeng Ji; Gengxin Zhang

2009-01-01

294

Interaction of copper with the surface of calcite  

SciTech Connect

The interaction of Cu ions in solution with the surface of calcite has been studied in a range of solutions from pure water to seawater. Observations of the uptake of Cu from solution onto calcite indicates that the process is rapid and strong in both distilled water and seawater. In distilled water, Cu uptake is directly proportional to the concentration of Cu in solution; Cu/sub s/ = K/sub s/Cu/sub 1/. The average value for K/sub s/ is 3.5 +- 1.7. The Cu/sub s/ dependence on Cu/sub 1/ is linear over the entire Cu concentration range studied (0.1 to 200 ..mu..M). Results do not indicate the formation of a precipitate of either malachite or copper carbonate. A precipitate of the form Cu/sub x/Ca/sub 1-x/CO/sub 3/ may be deposited onto the calcite surface in distilled water. The value of K/sub s/ in distilled water decreased sharply over the solid to solution ratio range of 0.1 to 2 g CaCO/sub 3/ 1/sup -1/. This was followed by a small change in K/sub s/ for solid to solution ratios in the range of 2 to 10 g CaCO/sub 3/ 1/sup -1/. In seawater, the uptake of Cu is also directly proportional to the concentration of Cu/sub 1/ up to a limiting value of approximately 13 ..mu..M. The average value for K/sub s/ in seawater, 0.24 +- 0.06 (Cu/sub 1/ less than or equal to 13 ..mu..M), is approximately an order of magnitude less than in distilled water. This is probably the result of smaller Cu/sub 1/ activity coefficients and increased site competition by other ions in seawater. Attempts to increase the Cu/sub 1/ concentration above 13 ..mu..M resulted in the additional Cu being deposited on the surface of the calcite. A possible explanation for this behavior is the formation of a precipitate of malachite on the calcite surface. The value of K/sub s/ decreased slightly with increasing solid to solution ratios in seawater.

Franklin, M.L.; Morse, J.W.

1981-12-01

295

Addition of Calcite Reduces Iron’S Bioavailability in the Pennsylvania Coals—Potential Use Of Calcite for the Prevention Of Coal Workers’ Lung Diseases  

Microsoft Academic Search

In the present study, a hypothesis was tested that the addition of calcite into the Pennsylvania coals may reduce levels of bioavailable iron (BAI), an important component in the mixed coal dust that may contribute to coal workers’ lung diseases. Predetermined proportions of calcite (0, 1, 2, 5, 10% w\\/w) were added into three PA coals. After suspending the mixtures

Qi Zhang; Xi Huang

2005-01-01

296

Speciation of As in calcite by micro-XAFS: Implications for remediation of As contamination in groundwater  

NASA Astrophysics Data System (ADS)

To evaluate the role of calcite as a host phase of arsenic (As) in As-contaminated groundwater, distribution behavior of Asbetween natural calcite and groundwater in deep underground was investigated based on As oxidation state. Speciation analyses of As in natural calcite by ?-XRF-XAFS analyses showed (i) preferentialarsenate uptake by calcite, and (ii) promptness of arsenate uptake by minor iron (Fe) carbonate minerals coprecipitated with calcite. These findings suggest that the effect of calcite on As remediation of the As-contamination systems stronglydepends on arsenite to arsenate ratio (i.e., redox condition) in groundwater, and maybe governed bythe amount of Fe coprecipitated with calcite.

Yokoyama, Y.; Iwatsuki, T.; Terada, Y.; Takahashi, Y.

2013-04-01

297

Reactive uptake of acetic acid on calcite and nitric acid reacted calcite aerosol in an environmental reaction chamber.  

PubMed

The heterogeneous chemistry of gas-phase acetic acid with CaCO(3)(calcite) aerosol was studied under varying conditions of relative humidity (RH) in an environmental reaction chamber. Infrared spectroscopy showed the loss of gas-phase reactant and the appearance of a gaseous product species, CO(2). The acetic acid is observed to adsorb onto the calcite aerosol through both a fast and a slow uptake channel. While the fast channel is relatively independent of RH, the slow channel exhibits enhanced uptake and reaction as the RH is increased. In additional experiments, the calcite aerosol was exposed to both nitric and acetic acids in the presence of water vapor. The rapid conversion of the particulate carbonate to nitrate and subsequent deliquescence significantly enhances the uptake and reaction of acetic acid. These results suggest a possible mechanism for observed correlations between particulate nitrate and organic acids in the atmosphere. Calcium rich mineral dust may be an important sink for simple organic acids. PMID:18075693

Prince, Amy Preszler; Kleiber, Paul D; Grassian, Vicki H; Young, Mark A

2007-11-21

298

Crystallization of CaCO3 in the presence of sulfate and additives: experimental and molecular dynamics simulation studies.  

PubMed

The effects of sulfate and BHTPMP (Bis (hexamethylene) triaminepentakis (methylene phosphonic acid)) on the crystallization rate, phase composition and morphology of calcium carbonate have been studied. It was observed that sulfate reduces the nucleation rate and favors the formation of aragonite form in the calcium carbonate precipitate. Moreover, in the presence of sulfate the rhombohedral morphology of the calcite crystals is modified, and during the formation of calcite, the development of {104} faces are more significantly prohibited than {110} faces. In the presence of sulfate together with BHTPMP, the crystallization process is inhibited and the modified morphology and the dominant calcite form are observed in the solid. The results from molecular dynamics simulations show the more strong combination of sulfate with calcite surface, in particular the {104} face, in comparison with the aragonite surface. The strong interaction of BHTPMP with sulfate and the aragonite surface favors the formation of the dominant calcite phase in the precipitate. PMID:22487230

Tang, Yongming; Zhang, Fan; Cao, Ziyi; Jing, Wenheng; Chen, Yizhong

2012-03-23

299

Interrogating coccolith calcite: paleoceanographic proxy data from nannofractions, foraminifera and single coccolith specimens  

NASA Astrophysics Data System (ADS)

Nannofossils are the calcareous cell-wall coverings of oceanic phytoplankton and the most abundant but smallest routinely studied fossil group. Small size (typically <8 microns and <400 pg of calcite per lith) has limited their use as geochemical proxies because it is difficult to separate them by mechanical picking or using centrifuge or filtration techniques. However, the <63µm fraction of marine sediments is often implied to represent the bulk nannofossil signal in geochemical studies, but actually represents a spectrum of oceanic carbonate, from parts of larger organisms (potentially benthic, nektonic and planktonic) to micarb (of unknown, possibly non-biological, origin), consequently obfuscating the true nannofossil signal. Additionally, it is known that certain nannofossil taxa are more responsive to paleoenvironmental change than others, and so a bulk nannofossil signal is potentially misleading. Consequently, there has been a strong incentive to develop techniques that overcome these difficulties, as coccoliths represent a potentially invaluable and inexhaustible geochemical archive of paleoenvironmental information. Most importantly, coccoliths are hugely abundant in marine sediments deposited above the calcite compensation depth (billions of specimens per gram) and their chemical composition is resistant to the modifying effects of diagenesis since they are relatively non-porous structures formed from a small number of solid calcite crystals. Here we compare a range of paleoceanographic geochemical proxies (element ratios, stable isotopes) measured from (a) bulk sediment, single foraminifers and sieved near-monotaxic nannofossil fractions using wet chemistry and (b) single coccolith specimens using an ion microprobe SIMS. The data come from the PETM interval of ODP Leg 207 Site 1259 (Demerara Rise, western equatorial Atlantic) and enable comparisons between paleontological, assemblage-based productivity proxies and geochemical proxies based on nannofraction samples and single-species/single-nannofossil specimens. The latter technique is particularly informative, with low Mg and very low B values demonstrating that coccolithophores exert a far greater selectivity on incorporation of trace elements during calcification compared with, for example, foraminifera. Sr values, however, reveal clear and distinct taxon-specific ranges that are consistent with paleoecological interpretations inferred by other means (e.g. regarding eutrophs vs. oligotrophs). The lack of significant change in the Sr values into and through the PETM, despite major nannofossil assemblage shifts and inferred paleoenvironmental change, suggests that production of nannoplankton was not greatly affected at this site. However, Sr measured in nannofraction samples and bulk showed distinctly different trends that require explanation.

Bown, P. R.; Lees, J. A.; Young, J.; Robinson, S. A.; MacLeod, K. G.; Grocke, D. R.; Schmidt, D. N.; Hinton, R.

2009-12-01

300

Fluid-enhanced low-temperature plasticity of calcite marble: Microstructures and mechanisms  

NASA Astrophysics Data System (ADS)

Postmetamorphic carbonate fault rocks from the Damara orogen, Namibia, show the following features: (1) obvious contrasts between centimeter-scale grains in the clasts and nanometer- to micrometer-scale grains in the matrix, and between macroscopic cataclastic and microscopic mylonitic microstructures; (2) coexistence of tangled dislocations and organized dislocation walls; (3) occurrence of subgrains along margins of clasts and their transition into dynamically recrystallized grains in the matrix; and (4) new grains in grain sizes of a few nanometers to 3 ?m in diameter. The coexistence of brittle and ductile microstructures is attributed to comprehensive intragranular twinning, and kinking, fracturing, and subsequent dislocation remobilization or reorganization and recrystallization. Fracturing is triggered by dislocation pileups due to dynamic loading, twinning, and kinking. It also generates free dislocations and tangled dislocations. Fractures provide fluid paths, increase fluid-rock interfaces, and enhance the possibility of fluid-rock interaction. Fracturing is subsequently accommodated by low-temperature plasticity that is attributed to hydrolytic weakening, i.e., fluid-enhanced recovery and dynamic recrystallization due to the infiltration of fluids into the deforming grains. During hydrolytic weakening, remobilized free dislocations and tangled dislocations climb toward incoherent boundary-like fractures. The dislocations are reorganized into dislocation walls that commonly constitute parts of subgrains developing into new grains. We conclude that: (1) fluids may increase the rate of dislocation glide and dislocation climb and may also enhance the recovery of strain-hardened rocks to accommodate fracturing processes in calcite marbles at low temperatures; and (2) calcite marble may have low-temperature plasticity and may undergo crystal plastic deformation due to hydrolytic weakening at shallow crustal levels.

Liu, Junlai; Walter, Jens M.; Weber, Klaus

2002-09-01

301

Calcite-bearing dolomite carbonatite dykes from Veseloe, North Transbaikalia, Russia and possible Cr-rich mantle xenoliths  

NASA Astrophysics Data System (ADS)

Carbonatites were discovered at Veseloe, North Transbaikalia, Russia in 2002. They are fluorapatite and calcite-bearing dolomite carbonatites and chemically classified as magnesiocarbonatites with high P, Ti, Cr and Ni. The Cr content in some parts of the Veseloe carbonatites is 3-4 times higher than average magnesiocarbonatite and results from the presence of partially resorbed polymineralic 3-5 mm diameter aggregates of Cr-rich minerals (magnetite, rutile, titanite, ilmenite, phengite, chlorite, chromite, titanite and talc). These aggregates may well be relict mantle xenoliths. They contain a rutile-magnetite intergrowth observed for the first time in nature and have compositions consistent with minerals from eclogite, kimberlite and mantle xenoliths. In the Veseloe carbonatites dolomite crystallized first, and then calcite, which is the inverse of the order of crystallization seen in most carbonatite complexes. Strontium isotope ratios are consistent with a Riphean mantle source. Carbon and O isotope ratios rule out some processes of alteration (metamorphic dehydration and high temperature hydrothermal alteration by meteoric waters) but do not plot in the accepted box for primary igneous carbonatite. Mineral geothermometers, inclusion measurements and the relict mantle xenoliths suggest petrogenesis by independent production of magnesiocarbonatite melt in the mantle rather than crustal differentiation of a carbonated-silicate parental melt.

Doroshkevich, A. G.; Wall, F.; Ripp, G. S.

2007-05-01

302

Phosphoprotein modulation of apatite crystallization  

Microsoft Academic Search

Summary  Several phosphoprotein preparations (phosvitin, rat incisor and fetal calf molar dentin phosphoproteins) all inhibit apatite\\u000a growth\\/replication from pre-existing crystal seeds in metastable solutions. Two stages of the crystal growth process were\\u000a inhibited by these phosphoproteins. First, an initial lag period was induced, probably associated with seed surface phenomena.\\u000a This period was prolonged indefinitely when a combination of phosphoprotein precoated seeds

J. D. Termine; E. D. Eanes; K. M. Conn

1980-01-01

303

Kinetic Inhibition of Calcium Carbonate Crystal Growth in the Presence of Natural and Synthetic Organic Inhibitors  

Microsoft Academic Search

Addition of carboxylate-containing polymeric materials to a metastable supersaturated calcium carbonate solution greatly reduced\\u000a calcite crystal growth rates at constant supersaturation and pH = 8.5. Calcite crystallization rates were decreased to half\\u000a their value in pure solutions by a tannic acid concentration of about 0.3 ppm (parts per million); a fulvic acid concentration\\u000a of about 0.2 ppm; and a poly(acrylic

Zahid Amjad; Jeff Pugh; Michael M. Reddy

304

Vaterite crystals contain two interspersed crystal structures.  

PubMed

Calcite, aragonite, and vaterite are the three anhydrous polymorphs of calcium carbonate, in order of decreasing thermodynamic stability. Although vaterite is not commonly found in geological settings, it is an important precursor in several carbonate-forming systems and can be found in biological settings. Because of difficulties in obtaining large, pure, single crystals, the crystal structure of vaterite has been elusive for almost a century. Using aberration-corrected high-resolution transmission electron microscopy, we found that vaterite is actually composed of at least two different crystallographic structures that coexist within a pseudo-single crystal. The major structure exhibits hexagonal symmetry; the minor structure, existing as nanodomains within the major matrix, is still unknown. PMID:23620047

Kabalah-Amitai, Lee; Mayzel, Boaz; Kauffmann, Yaron; Fitch, Andrew N; Bloch, Leonid; Gilbert, Pupa U P A; Pokroy, Boaz

2013-04-26

305

Calcite production by coccolithophores in the south east Pacific Ocean  

NASA Astrophysics Data System (ADS)

BIOSOPE cruise covered an oceanographic transect through the centre of the South Pacific Gyre (SPG) from the Marquesas archipelago to the Peru-Chile upwelling (PCU). Water samples from 6 depths in the euphotic zone were collected at 20 stations. The concentrations of suspended calcite particles, coccolithophores cells and detached coccoliths were estimated together with size and weight using an automatic polarizing microscope, a digital camera, and a collection of softwares performing morphometry and pattern recognition. Some of these softwares are new and described here for the first time. The coccolithophores standing stocks were usually low and reached maxima west of the PCU. The coccoliths of Emiliania huxleyi, Gephyrocapsa spp. and Crenalithus spp. (Order Isochrysidales) represented more than 30% of all the suspended calcite particles detected in the size range 0.1 46 ?m (22% of PIC in term of calcite weight). These species grew preferentially in the Chlorophyll maximum zone. In the SPG their maximum cell concentrations were recorded between depth of 150 and 200 m, which is unusually deep for these taxa. The weight of coccoliths and coccospheres were correlated to their size. Large and heavy coccoliths and coccospheres were found in regions with relatively high fertility in the Marquises Island and in the PCU. Small and light coccoliths and coccospheres were found west of the PCU. This distribution is strongly related to ocean chemistry in particular to alkalinity and to carbonate ions concentration. The biotic (coccolithophores production) influence on calcification is mainly driven at the local scale (depth) whereas the abiotic (carbonate chemistry) plays its most important role at the regional (horizontal) level. Here 94% of the variability of coccolith and coccosphere weight can be explained by a change in 7 environmental variables.

Beaufort, L.; Couapel, M.; Buchet, N.; Claustre, H.; Goyet, C.

2008-08-01

306

Calcite saturation in the River Dee, NE Scotland.  

PubMed

The spatial and temporal variations in calcite (calcium carbonate) solubility within the Dee basin of NE Scotland were assessed using water chemistry data gathered from a network of 59 sites monitored for water quality from June 1996 to May 1997. Calcite solubility, expressed in terms of a saturation index (SIcalcite), was determined from measured streamwater pH, Gran alkalinity and calcium concentrations and water temperature. In general, the waters of the Dee system are undersaturated with respect to calcite, though the saturation index is higher during the summer months indicating a dependency on flow conditions and biological activity. Under low-flow conditions, the streamwaters are dominated by water derived from the lower soil horizons and deeper groundwater stores and therefore, ions such as Gran alkalinity and calcium are at their highest concentrations as they are derived mainly from bedrock weathering. The influence of biological activity on the carbonate system is also evident as the observed pH and estimated EpCO2 values indicate strong seasonal patterns, with the highest pH and lowest EpCO2 values occurring during the summer low-flow periods. Only at three sites in the lowland region of the catchment, during the summer low-flow period, are the waters oversaturated. As such, the Dee system represents an extreme 'end-member' case when compared to many UK rivers that span both under- and oversaturated conditions during the year. Regression analysis highlights a systematic change in the SIcalcite-pH relationship in a broad east-west direction across the Dee system. At sites draining the relatively impermeable upland areas, the regression of SIcalcite against pH gives a straight line with a gradient in the range 1.6-2.4. Correspondingly, under the most extreme alkaline conditions found at sites draining lowland agricultural areas, a straight-line relationship exists but with a gradient of unity. It is concluded that these changes in the SIcalcite-pH relationship are due to variations in the bicarbonate system induced by the flow conditions and biological activity. Given the waters are undersaturated, then calcite precipitation and hence phosphorus co-precipitation cannot occur within the water column. PMID:11846077

Wade, A J; Neal, C; Smart, R P; Edwards, A C

2002-01-23

307

The role of dislocations and surface morphology in calcite dissolution  

Microsoft Academic Search

The authors have measured the dissolution rate for undeformed (ρ â¼ 10³ dislocations{center dot}cm⁻²) calcite to be 3.1 à 10⁻¹° mol{center dot}cm⁻²{center dot}s⁻¹ in free-drift rotating disk experiments at 1,160 rpm, 25C and pH 8.6 in 0.7 M KCl solution far from equilibrium. The rate increased by a factor of â¼ 2.3 for a strained sample (ρ = 6 Ã

IAN N. MACINNIS; SUSAN L. BRANTLEY

1992-01-01

308

Strong kinetic effects on Sr\\/Ca ratios in the calcitic bivalve Pecten maximus  

Microsoft Academic Search

Although Sr\\/Ca ratios in abiogenic calcite are strongly controlled by precipitation rates, such a kinetic effect has never been demonstrated in calcitic bivalve shells. Therefore, we report Sr\\/Ca ratios together with daily growth rates in the calcitic shells of four individuals of the bivalve Pecten maximus (age class I). Ratios of Sr\\/Ca were found to be variable among individuals that

Anne Lorrain; David P. Gillikin; Yves-Marie Paulet; Laurent Chauvaud; Alain Le Mercier; Jacques Navez; Luc André

2005-01-01

309

High-magnesian calcite: leaching of magnesium in the deep sea.  

PubMed

The high-magnesian calcite fraction of a shallow-water carbonate sand was converted to low-magnesian calcite after transport to the deep sea; strontium was also leached from the carbonate. Oxygen isotopic ratios indicate that loss of magnesium and strontium took place during recrystallization of the carbonate in the deep sea; this process did not alter textures of skeletal fragments. Previously, high-magnesian, calcite was thought only to dissolve in the deep sea. PMID:17731593

Gomberg, D N; Bonatti, E

1970-06-19

310

A balance analysis of phosphorus elimination by artificial calcite precipitation in a stratified hardwater lake  

Microsoft Academic Search

Elimination of phosphorus was achieved by artificial calcite precipitation in the Dagowsee, a stratified eutrophic hardwater lake. The artificial calcite precipitation was induced by hypolimnetic injection of CaO in enclosures during the summer stagnation. The hypolimnetic injection of CaO was combined with aeration for 1 day. The coprecipitation of phosphorus with calcite was shown by energy dispersive X-ray analysis. The

M. Dittrich; T. Dittrich; I. Sieber; R. Koschel

1997-01-01

311

Utilization of calcite and waste glass for preparing construction materials with a low environmental load.  

PubMed

In this study, porous calcite materials are hydrothermally treated at 200 °C using powder compacts consisting of calcite and glasses composed of silica-rich soda-lime. After treatment, the glasses are converted into calcium aluminosilicate hydrates, such as zeolite phases, which increase their strength. The porosity and morphology of new deposits of hydrothermally solidified materials depend up on the chemical composition of glass. The use of calcite and glass in the hydrothermal treatment plays an important role in the solidification of calcite without thermal decomposition. PMID:21794973

Maeda, Hirotaka; Imaizumi, Haruki; Ishida, Emile Hideki

2011-07-26

312

Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO 2 concentrations  

NASA Astrophysics Data System (ADS)

Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO 2 components—atmospheric CO 2 and CO 2 from in situ oxidation of organic matter. The ? 13C values measured for the Fe(CO 3)OH component in solid solution in these Permian goethites are -13.5‰ for the Lower Leonardian (˜283 Ma BP) paleosol (MCGoeth) and -13.9‰ for the Upper Leonardian (˜270 Ma BP) paleosol (SAP). These goethites contain the most 13C-rich Fe(CO 3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO 2 components. ? 13C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of -20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric P CO2 value of about 1 × PAL, but the scatter in the measured ? 13C values of organic matter permits a calculated maximum P CO2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO 3)OH in MCGoeth and SAP correspond to soil CO 2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO 2 concentrations are similar to modern soils in warm, wet environments. The average ? 13C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from -6.5‰ to -4.4‰, with a mean ? 13C value for all profiles of -5.4‰. Thus, the value of ? 13C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric P CO2 and organic matter ? 13C values were the same for both paleosol types. Furthermore, the atmospheric P CO2 calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric P CO2 (permitted by one standard deviation of the organic matter ? 13C value) of ˜5 × PAL. If, however, measured average ? 13C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of 13C-depleted, labile organic compounds, calculated Permian atmospheric P CO2 using these 13C-enriched organic values would underestimate the actual atmospheric P CO2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO 2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO 2. The Fe(CO 3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO 3)OH in goethites from soils in which there are mixtures of two isotopic CO 2 components. Field-relationships and the ? 13C value (-1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO 2 components at the time of goethite crystallization. The three components were probably atmospheric CO 2, CO 2 from in situ oxidation of organic matter and CO 2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO 2 components, as recorded by Fe(CO 3)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric P CO2.

Tabor, Neil J.; Yapp, Crayton J.; Montañez, Isabel P.

2004-04-01

313

AFM Observations of Weathering and Microbiological Alterations on the Surface of Calcite Buried in Arctic Soil (Spitsbergen)  

NASA Astrophysics Data System (ADS)

This study focused on the direct observation of chemical weathering and biological activity on mineral surfaces in the newly forming arctic soil of West Spitsbergen . Chemical weathering and soil forming processes associated with glaciers may affect several geochemical cycles including global carbon cycle and as a result have negative feedbacks on the global climate. Study areas are the foreland of the Werenskiold glacier, continuously retreating by several meters a year. Several samples of freshly cleaved calcite had been buried in the soils for one year. Samples were analyzed with the use of Atomic Force Microscopy (AFM). Results of AFM investigation show changes observed on a calcite samples located respectively about 2500 meters (sample calcite 1) and 100 m (sample calcite 2) from the glacier front as compared to a control sample calcite 0, that has never been exposed to glacier environment. Samples calcite 1 and calcite 2 were recovered from Spitsbergen after 1 year. Compared to the control sample calcite 0, which displays sharp edges and smooth surfaces, both field-treated samples calcite 1 and calcite 2 display rounded edges, irregular surfaces, numerous dissolution features and rounded pitches associated with bacterial activities. The observations suggest that both samples calcite 1 and 2 undergo intensive and rapid chemical and biological weathering when exposed to relatively unsaturated with respect to calcite glacial meltwaters. Several types of analyses have been applied to various regions and lines on the calcite surface. Selected regions on the calcite surface included (a) the entire area of the observed surface (b) top step region roughness, and (c) bottom step region roughness. Selected line parameters have been calculated along: (a) three randomly selected parallel lines, (b) top step line roughness, and (c) bottom step line roughness. Both surface area roughness and line roughness are calculated as the mean deviation of the height. Significant differences have been observed between the samples in calculated roughness parameters, with increase of these parameters ranging from 28% to 241% in calcite 1 located 2500 m from the glacier front and from 100% to 486% in calcite 2 located 100m from the glacier front, as compared to calcite 0. Roughness of the entire surface area for the control calcite 0 was 5.73nm, which increased by 28% in calcite 1 and by 100% in calcite 2. Top step edge roughness increases from calcite 0 (1.82 nm) by 241% to 6.2 nm in calcite 1 and by 486% to 8.84 nm in calcite 2. Bottom step edge roughness increases from 2.08 in calcite 0 by 98% to 4.52nm in calcite 1 and by 149% to 5.67 nm in calcite 2. Line roughness for calcite 0 is 3.26, which increased by 102% in calcite 1 and by 217% in calcite 2. Top step line roughness increases from 2.04 nm in calcite 0 by 191 % to 5.93 nm in calcite 1 and by 276% to 7.67nm in calcite 2. Bottom step line roughness increases from 2.28 nm in calcite 0 by 198% to 4.52 nm in calcite 1and by 249% to 5.67 nm in calcite 2. In calcite 0

Summers, S.; Matyjasik, M.; Inglefield, C.; Manecki, M.; Plonka, A.; Paget, C.; Park, C.

2008-12-01

314

Isotopic analysis for degradation diagnosis of calcite matrix in mortar.  

PubMed

Mortar that was used in building as well as in conservation and restoration works of wall paintings have been analysed isotopically (delta(13)C and delta(18)O) in order to evaluate the setting environments and secondary processes, to distinguish the structural components used and to determine the exact causes that incurred the degradation phenomena. The material undergoes weathering and decay on a large proportion of its surface and in depth, due to the infiltration of water through the structural blocks. Mineralogical analysis indicated signs of sulphation and dissolution/recrystallisation processes taking place on the material, whereas stable isotopes provided information relative to the origin of the CO(2) and water during calcite formation and degradation processes. Isotopic change of the initial delta(13)C and delta(18)O in carbonate matrix was caused by alteration of the primary source of CO(2) and H(2)O in mortar over time, particularly by recrystallisation of calcite with porewater, evaporated or re-condensed water, and CO(2) from various sources of atmospheric and biogenic origin. Human influence (surface treatment) and biological growth (e.g. fungus) are major exogenic processes which may alter delta(18)O and delta(13)C in lime mortar. PMID:19784637

Dotsika, E; Psomiadis, D; Poutoukis, D; Raco, B; Gamaletsos, P

2009-09-26

315

Hydrothermal replacement of calcite by sphalerite in a temperature gradient  

SciTech Connect

The replacement of calcite by sphalerite under near-neutral pH's was investigated experimentally and by numerical modeling. The results showed maximum solubility of Zn at 375/sup 0/C, because at higher temperatures Na-Cl and Ca-Cl ion-pairing becomes nearly as strong as the Zn-Cl complexing, effectively scavenging the available chloride. Solubility decreases below 375/sup 0/C because Zn-Cl complexes become less stable. These calculations were confirmed experimentally: ZnS-saturated chloride solutions were pumped through various pure marble sands and gravels down a temperature gradient. CO/sub 2/-CaCO/sub 4/ reactions were used to buffer the input solution at temperatures 15/sup 0/ above those used for the temperature gradients during replacement. In runs cooler than 375/sup 0/C, sphalerite was deposited when the solution cooled to cause ZnS super-saturation. In runs above 375/sup 0/C, no sphalerite precipitated until the temperature fell well below 375/sup 0/C. Such delayed replacement characteristically has sphalerite disseminated as isolated grains, and the marble showed evidence of dissolution. This process opened void space in solid marbles, permitting increased flow of sulfide-depositing solutions. Thus temperature changes may cause replacement of calcite by sphalerite even where there is no change in the reactivity of the host rock, or its accessibility to ore-forming solutions.

Gould, W.W.; Barnes, H.L.

1985-01-01

316

Calcite and dolomite discrimination using airborne SWIR imaging spectrometer data  

NASA Astrophysics Data System (ADS)

The SWIR full spectrum imager (SFSI) is a hyperspectral push-broom imager, acquiring imagery in 120 0.010 micrometers wide bands simultaneously covering the 1.20 micrometers to 2.47 micrometers spectral region. During the first flights of the instrument hyperspectral imagery was acquired over a calcite quarry and a dolomite quarry. Both these minerals show distinctive carbonate absorption features in the 1.7 to 2.5 micrometers region. These absorption features are centered at wavelengths approximately 0.1 micrometers shorter in dolomite than in calcite. The two minerals are clearly distinguished using a single Gaussian fit to the prominent carbonate absorption feature near approximately 2.3 micrometers in the quarry reflectance data from the SFSI test flights. The spectral resolution of the airborne spectra also allowed the 2.3 micrometers absorption feature to be resolved into tow closely spaced features. Both these absorption features show the mineralogical shift in band center. This has also been seen in laboratory spectra and a detailed comparison with this data is made.

Rowlands, Neil; Neville, Robert A.

1996-11-01

317

Environmental controls on the Emiliania huxleyi calcite mass  

NASA Astrophysics Data System (ADS)

Although ocean acidification is expected to impact (bio)calcification by decreasing the seawater carbonate ion concentration, [CO32-], there exists evidence of non-uniform response of marine calcifying plankton to low seawater [CO32-]. This raises questions on the role of environmental factors other than acidification and on the complex physiological responses behind calcification. Here we investigate the synergistic effect of multiple environmental parameters, including temperature, nutrient (nitrate and phosphate) availability, and seawater carbonate chemistry on the coccolith calcite mass of the cosmopolitan coccolithophore Emiliania huxleyi, the most abundant species in the world ocean. We use a suite of surface (late Holocene) sediment samples from the South Atlantic and southwestern Indian Ocean taken from depths lying well above the modern lysocline. The coccolith calcite mass in our results presents a latitudinal distribution pattern that mimics the main oceanographic features, thereby pointing to the potential importance of phosphorus and temperature in determining coccolith mass by affecting primary calcification and possibly driving the E. huxleyi morphotype distribution. This evidence does not necessarily argue against the potentially important role of the rapidly changing seawater carbonate chemistry in the future, when unabated fossil fuel burning will likely perturb ocean chemistry beyond a critical point. Rather our study highlights the importance of evaluating the combined effect of several environmental stressors on calcifying organisms to project their physiological response(s) in a high CO2 world and improve interpretation of paleorecords.

Horigome, M. T.; Ziveri, P.; Grelaud, M.; Baumann, K.-H.; Marino, G.; Mortyn, P. G.

2013-06-01

318

Using Clumped Isotopes To Help Understand Isotopic Sector Zoning In Calcite  

NASA Astrophysics Data System (ADS)

Debated among scientists for decades, observations of compositional differences between different crystallographic faces within a same growth zone -called sector zoning- represents a major challenge for geochemistry, and in particular for isotope geochemistry. Nowadays, convincing evidence for the existence of sector zoning has been reported in the literature. However, no consensus on the mechanisms triggering sector zoning has been reached. Understanding how and why sector zoning occurs is essential because the presence of sector zoning could skew the isotopic characterization of a carbonate mineral depending on the area being sampled. This would result in an ambiguous interpretation of the data. The emphasis of this study, which is part of the Qatar Carbonates and Carbon Research Center (QCCSRC), is thus placed on deciphering different processes leading to isotopic sector zoning in calcite, with the help of a new tool: the clumped isotopes (i.e. isotopologues). Large calcite crystals with clearly visible growth zones have been collected from fracture infills in Oman. The minerals growth zones and sectors are recognized via cathodoluminescence microscopy, and the trace element content of the different zones in several sectors is estimated via electron microprobe. These different sectors are then sampled by a micro-drill following single growth zones and analyzed for their oxygen and carbon isotopic compositions. The approach highlighted above allows for the recognition of the presence of sector zoning, but does not constrain the type of mechanism(s) possibly affecting isotopic fractionation. To gain further insight, we turn to clumped isotopes (multiply substituted isotopologues) which are molecules with distinct chemical and physical properties. This should theoretically lead to distinctive fractionations. Clumped isotopes have been used recently to measure the temperature of precipitation of carbonate minerals. If each of the crystalline phases of a given crystal were precipitated under thermodynamic equilibrium, one would expect a constant clumped isotope value between sectors. We intend to use clumped isotopes as a discriminative geochemical tool to check if any differences in the ?47 values between sectors exist. If this is the case, isotopologues could provide new insights on sector zoning and would give an extra-dimension to the quest of the different factors resulting in these type of isotopic zonations. Combining compositional, stable isotopes and clumped isotopes information could ultimately permit to reveal the secrets of sector zoning.

Jourdan, A.; John, C. M.; Inchenko, A.; Davis, S.

2010-12-01

319

Relationship between surface structure, growth mechanism, and trace element incorporation in calcite  

NASA Astrophysics Data System (ADS)

Crystal growth and coprecipitation experiments demonstrate the manner in which surface structure and, in turn, crystal structure influence growth mechanism and trace element incorporation in calcite. Dominant {10 4} faces grow by the spiral mechanism, producing asymmetric polygonized growth hillocks comprised of two pairs of nonequivalent vicinal faces. Trace elements Mg, Mn, and Sr are differentially incorporated into structurally distinct growth steps that comprise the nonequivalent vicinal faces. The resulting trace element distributions represent intrasectoral zoning patterns and are found to be consistent with face symmetry. Lateral spreading rates are also different for nonequivalent growth steps at a given degree of supersaturation, and the rate anisotropy is dependent on the Ca 2+ :CO 3 2- ratio in the growth solution. A rounding transition associated with changes in kink site density occurs preferentially on only one pair of equivalent growth steps, further demonstrating the importance of step-specific kinetics and affinities on {10 4} faces. Growth on other forms, including {01 2}, {11 0}, and {0001}, does not result in differential partitioning of trace elements and intrasectoral zoning, which is consistent with each of their surface symmetries and allowed growth mechanisms. However, sectoral zoning of Mg, Mn, and Sr occurs between these nonequivalent sectors, as well as {10 4}. We present a model detailing the geometry and coordination of elementary kink sites to explain both the differential incorporation and the rate anisotropy between nonequivalent growth steps on individual {10 4} faces. The model, constrained by face symmetry, accounts for affinities of different trace elements among four structurally distinct kink sites at which incorporation is preferred. The model also explains the observed differences in growth step velocity, as well as the in situ observations of growth step velocities by atomic force microscopy. Trace element incorporation on the dominant {10 4} faces of calcite is controlled by the detailed structure of the interface, which varies spatially on a face, as well as with external conditions. Consequently, our observed trace element distributions violate equilibrium partitioning, and it is likely that many trace element distributions in natural carbonates also may not reflect equilibrium.

Paquette, Jeanne; Reeder, Richard J.

1995-02-01

320

Seed proteomics.  

PubMed

Rather than providing a single specific protocol, the inclusive area of seed proteomics is reviewed; methods are described and compared and primary literature citations are provided. The limitations and challenges of proteomics as an approach to study seed biology are emphasized. The proteomic analysis of seeds encounters some specific problems that do not impinge on analyses of other plant cells, tissues, or organs. There are anatomic considerations. Seeds comprise the seed coat, the storage organ(s), and the embryonic axis. Are these to be studied individually or as a composite? The physiological status of the seeds must be considered; developing, mature, or germinating? If mature, are they quiescent or dormant? If mature and quiescent, then orthodox or recalcitrant? The genetic uniformity of the population of seeds being compared must be considered. Finally, seeds are protein-rich and the extreme abundance of the storage proteins results in a study-subject with a dynamic range that spans several orders of magnitude. This represents a problem that must be dealt with if the study involves analysis of proteins that are of "normal" to low abundance. Several different methods of prefractionation are described and the results compared. PMID:24136535

Miernyk, Ján A

2014-01-01

321

The Effects of High Magnetic Fields on the Isothermal Dissolution and Growth Rates of Fe(NH4)2(SO4)2.6H2O and KA(SO4)2.12H2O Seed Crystals.  

National Technical Information Service (NTIS)

A homogeneous magnetic field H increases both the isothermal rates of dissolution and growth R of a paramagnetic seed crystal Fe(NH4)2(SO4)2.6H2O in isothermal experiments. The magnitude of the increasement of R at 140 kOe and 292K is equivalent to the ef...

M. Schieber

1967-01-01

322

Crystal Structures of the Ribonuclease MC1 from Bitter Gourd Seeds, Complexed with 2?UMP or 3?UMP, Reveal Structural Basis for Uridine Specificity  

Microsoft Academic Search

Ribonuclease MC1 (RNase MC1) isolated from seeds of bitter gourd (Momordica charantia) consists of 190 amino acids and is characterized by a preferential cleavage at the 5?-side of uridine. This uridine specificity distinguishes RNase MC1 from other enzymes belonging to the RNase T2 family. The three-dimensional structures of RNase MC1, in a complex with either 2?-UMP or 3?-UMP, were determined

Akio Suzuki; Min Yao; Isao Tanaka; Tomoyuki Numata; Singo Kikukawa; Nobuyuki Yamasaki; Makoto Kimura

2000-01-01

323

Scattering Seeds  

NSDL National Science Digital Library

In this lesson, students will: 1) Investigate how seeds are dispersed. 2) Understand the variety of seed dispersal that exists in the plant kingdom. 3) Understand how factors in the natural environment can effect how far a seed is dispersed. Included in this one to two day lesson plan are the objectives, needed materials, procedures, adaptations for older students, discussion questions, an evaluation, extensions, suggested reading, links to other sites, vocabulary, and academic standards. Students will collect a variety of seeds and design a wind-dispersed seed mechanism. They can click on a vocabulary word to hear its pronunciation and a sentence using the word. Teachers can purchase the video, The Private Life of Plants 5-Pack (Branching Out) and download comprehension questions and answers. They can link to Teaching Tools to create custom worksheets, puzzles, and quizzes. A printable version of this lesson plan can be downloaded.

Discoveryschool.com; Cahill, Mary

2007-01-01

324

Impact of thin SrTiO3 seed layer to achieve low-temperature crystallization below 300 °C and ferroelectricity of lead zirconate titanate thin film  

NASA Astrophysics Data System (ADS)

The authors report low-temperature preparation of Pb(Zr0.6,Ti0.4)O3 (PZT) thin films below 300 °C using SrTiO3 seed layers on (111)Pt/IrO2/SiO2/(001)Si substrates by metal-organic chemical vapor deposition. The critical parameters contributing low-temperature preparation for PZT were crystallinity and orientation of SrTiO3 seeds and deposition rate of PZT films. Using 1-2 nm of SrTiO3 with (h00) and (hh0) mixed orientations, polycrystalline PZT films could be obtained at 290 °C with deposition rate of 0.5-1.0 nm/min. The remanent polarization (2Pr) and coercive field (2Ec) of PZT films on SrTiO3 seeds with deposition rate of 0.5 nm/min were 42.4 ?C/cm2 and 256 kV/cm, respectively.

Moon, Ji-Won; Tazawa, Shogo; Shinozaki, Kazuo; Wakiya, Naoki; Mizutani, Nobuyasu

2006-11-01

325

Planktic Foraminiferal Contribution To The Marine Calcite Budget  

NASA Astrophysics Data System (ADS)

The vertical flux and sedimentation of planktic foraminiferal tests is quantified and a global planktic foraminiferal CaCO3 budget is presented. The eastern North Atlantic, the Arabian Sea, and the Caribbean were identified as key areas for this investiga- tion, covering a wide spectrum of ecological conditions and faunal compositions. Test and calcite flux rates are calculated according to the distribution of species obtained from multinet and sediment trap samples. Population dynamics of modern plank- tic foraminifers are discussed as a prerequesite for the quantification of the calcite budget, highlighting the importance of ecological, autecological (e.g., reproduction), and biogeochemical conditions that determine the presence or absence of species. To complete the open-marine, particulate CaCO3 inventory, the contribution of coccol- ithophores, pteropods, and calcareous dinophytes is discussed. It is assumed that the global planktic foraminiferal calcite flux at 100 m depth amounts to 1.3-3.2 Gigatons per year, equivalent to 23-56% of the total open marine CaCO3 flux. The preservation of tests varies on a regional and temporal scale, and is affected by local hydrogra- phy and dissolution susceptibility of planktic foraminiferal tests. Throughout most of the year (off-peak periods) a major portion of tests is dissolved above 700 m water depth while individually settling through the water column, with on average only 1- 3% of the initially exported CaCO3 reaching the deep sea floor. Pulsed flux events, mass dumps of fast settling particles, yield a major contribution of the tests to the formation of deep-sea sediments. On an annual average, about one fourth of the ini- tially produced planktic foraminiferal test CaCO3 may settle on the sea floor. The total planktic foraminiferal contribution of CaCO3 to global deep-marine surface sediments amounts to 0.36-0.88 Gt per year, which is 32-80% of the total CaCO3 budget that is estimated at 1.1 Gt per year (e.g., Milliman et al., 1999).

Schiebel, R.

326

AN OPTIMIZATION OF CALCITE GRINDING USING THE TAGUCHI METHOD WITH MULTIPLE PERFORMANCE CHARACTERISTICS  

Microsoft Academic Search

The purpose of this study is to demostrate how the application of Taguchi method can be used to develop an effective optimization process for calcite grinding. In order to observe the influencing degree of contol factors in dry calcite grinding, three control factors namely mill speed, ball size and grinding time on lowest product 80% passing size (d80) were studied

N. Aslan

327

Effect of Solute Impurity of Mg on the Microstructure of Calcite  

Microsoft Academic Search

We produced synthetic marbles by hot isostatic pressing (HIPing) mixtures of calcite and dolomite powders for different intervals (2 to 30 hrs) at 850°C and 300 MPa confining pressure. The HIP treatment resulted in macroscopically homogeneous calcite aggregates with Mg content ranging from 0.5 to 17 mol%. We performed creep tests on samples after HIP at differential stresses from ~

L. Xu; B. J. Evans; J. Renner

2006-01-01

328

Sr Isotope Tracing of Differences in the Effects of Microorganisms on Weathering of Calcite and Apatite  

Microsoft Academic Search

To study differences in the effects of microorganisms on weathering of calcite and apatite, one strain of Aspergillus niger (A. niger) and one strain of Penicillium glaucum (P. glaucum), which respectively contain the mixture of calcite and apatite were cultivated for 24 days in the sucrose-potato culture medium, supernatant was taken every three days from the culture medium, followed by

S. Chen; B. Lian; H. Y. Zheng; F. Q. Dong

2012-01-01

329

Solubility of calcite and aragonite in seawater at atmospheric pressure and 34. 5% salinity  

Microsoft Academic Search

Direct measurement of the solubility of calcite in seawater is difficult and irreproducible because of kinetic factors that inhibit the attainment of true, reversible equilibrium. To avoid this problem, the solubility product constant for calcite in seawater was determined in the present study by calculation from new measurements of the aragonite solubility product constant in seawater and the differential solubility

Berner

1976-01-01

330

Zonation and geochemical patterns of burial calcite cements: Upper Smackover Formation, Clarke County, Mississippi  

Microsoft Academic Search

Cathodoluminescence (CL), trace element concentrations, and isotope compositions of luminescently zoned poikilitic and mosaic calcite cements of the upper Smackover Formation are studied in the Harmony, Pachuta Creek, East Nancy, Goodwater, and Garland Creek fields in Clarke County, Mississippi. These calcites precipitated during burial in a depth range of 100 m to 3 km, a temperature range of 30-100 C,

E. Heydari; C. H. Moore

1993-01-01

331

Interactions between calcite precipitation (natural and artificial) and phosphorus cycle in the hardwater lake  

Microsoft Academic Search

The influence of calcite precipitation on the phosphorus cycle in stratified hardwater lake was studied before and during experiments with a new restoration technique. Surveys of the chemical composition of water column and monitoring of settling particles of Lake Luzin (North–East) showed that calcite precipitation occurs each year over 2–3 periods during spring and summer. The change of the phosphorus

Maria Dittrich; Rainer Koschel

2002-01-01

332

ONTARIO'S EXPERIMENTAL LAKE NEUTRALIZATION PROJECT: CALCITE ADDITIONS AND SHORT-TERM CHANGES IN LAKE CHEMISTRY  

Microsoft Academic Search

This paper presents a preliminary analysis of chemical changes associated with the addition of 84 tonnes of fine calcite (mean diameter 9 ?m) in dry form during Aug. 11–16, 1983, to Bowland Lake, a remote and highly acidic lake near Sudbury, Ontario. A Canso water bomber was used to apply the calcite to the lake surface. Immediately following liming, the

L. A. Molot; J. G. Hamilton; G. M. Booth

1984-01-01

333

Silicon Crystal Pulling  

NSDL National Science Digital Library

This Quicktime animation shows the first step in the silicon wafer manufacturing process. Under 15 seconds in length, the animation shows the melting of polysilicon and the addition of minute amounts of electrically active elements to form the "seed" of the silicon crystal. The crystal is then extracted from the crystal-growing equipment.The next animation in this sequence about Silicon Ingot Rod Grinding can be seen here.

2009-10-23

334

Physical behavior of purified and crude wax obtained from Sunflower ( Helianthus annuus ) seed oil refineries and seed hulls  

Microsoft Academic Search

The sunflower seed waxes obtained from two sources (i) seed hull as astandard and (ii) crude wax from oil refineries were studied for theircrystallization, melting characteristics and morphology of crystals. Theresults of differential scanning calorimetry of wax obtained from seed hullsshowed the melting temperature range of 13.18 °C with the onsetat 62.32 °C, for purified wax, compared to the melting

T. C. Sindhu Kanya; K. Udaya Sankar; M. C. Shamnathaka Sastry

2003-01-01

335

Age constraints on fluid inclusions in calcite at Yucca Mountain  

SciTech Connect

The {sup 207}Pb/{sup 235}U ages for 14 subsamples of opal or chalcedony layers younger than calcite formed at elevated temperature range between 1.88 {+-} 0.05 and 9.7 {+-} 1.5 Ma with most values older than 6-8 Ma. These data indicate that fluids with elevated temperatures have not been present in the unsaturated zone at Yucca Mountain since about 1.9 Ma and most likely since 6-8 Ma. Discordant U-Pb isotope data for chalcedony subsamples representing the massive silica stage in the formation of the coatings are interpreted using a model of the diffusive loss of U decay products. The model gives an age estimate for the time of chalcedony formation around 10-11 Ma, which overlaps ages of clay minerals formed in tuffs below the water table at Yucca Mountain during the Timber Mountain thermal event.

Neymark, Leonid A.; Amelin, Yuri V.; Paces, James B.; Peterman, Zell E.; Whelan, Joseph F.

2001-04-29

336

Solute-controlled dissolution thresholding at near-equilibrium calcite-water interfaces  

NASA Astrophysics Data System (ADS)

Direct measurements at the aqueous solution-calcite (104) interface were performed using in situ atomic force microscopy (AFM) at 50—70 °C to reveal the influence of solution saturation state and magnesium concentration on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, time-sequential AFM images demonstrated a linear relationship between step speed and solution saturation state, ?calcite. Dissolved Mg2+ brought negligible inhibitory effects on calcite dissolution at [Mg2+] ? 10-4 molal. Upon the introduction of 10-3 molal Mg2+, however, ?calcite acted as a “switch” for magnesium inhibition. That is, no significant changes in step kinetics were observed at ?calcite ? 0.1, whereas a sudden inhibition from Mg2+ was activated at ?calcite ? 0.2. The presence of the ?-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. Etch pits formed in Mg-free near-equilibrium solutions exhibited a nearly-triangle shape, while in the presence 10-3 molal Mg2+ the etch pits displayed a unique distorted rhombic profile, different from those formed under far-from-equilibrium conditions. Such inconsistency in calcite step morphologies may be associated with the increased influence of the backward reaction and the anisotropy in net kink detachment rates. The calcite dissolution results presented in this work demonstrate a need to consider fundamental surface interactions that may lead to highly nonlinear near-equilibrium behavior and serve as a foundation for the development of robust geochemical models for the prediction of long-term calcite dissolution kinetics in natural water systems.

Xu, M.; Knauss, K. G.; Higgins, S. R.

2010-12-01

337

Diel cycles in calcite production and dissolution in a eutrophic basin  

SciTech Connect

Calcite production is understood largely as a longer-term phenomenon (e.g., seasonal whitings) that can occur in hardwater lakes, and is significant ecologically because it can slow the rate of eutrophication by reducing, through adsorption, the availability of nutrients to primary producers. In this study the authors show that rapid changes in concentration of dissolved CO{sub 2} by photosynthesis and respiration within a eutrophic basin generated strong day-to-night cycles in calcite production and dissolution. Diel cycles in calcite production and dissolution were large enough that they could drive secondary diel cycles in the availability of metals that strongly sorb to the surfaces of calcite particles. They explored the possibility of the secondary diel cycling of metals by intensive 7-d in situ monitoring of water-quality conditions in a shallow, eutrophic spill-control basin near an industrial facility in eastern Tennessee; inspecting data from a 7-year record of water-quality parameters for this basin; analyzing physicochemical characteristics and mineralogic composition of sediments in the basin; and conducting laboratory experiments to characterize the interaction of calcite with Cd, under solid-liquid nonequilibrium conditions. The authors found that the basin accumulated and stored calcite. In situ monitoring showed that calcite was produced during daylight, and tended to dissolve again at night; the calcite production and dissolution processes seemed to be modulated by dissolved-phase CO{sub 2} dynamics, in concert with large diel fluctuations in pCa, pH, and Po{sub 2}. Laboratory experiments showed a rapid interaction ({lt}6 h) of Cd with calcite, in response to dissolved CO{sub 2} changes. Thus, concentrations of dissolved Cd can vary over daily cycles, mediated by diel changes in calcite production and dissolution. Thermodynamic considerations suggest that other metals, such as Zn, Sr, Ni, and Ba, may demonstrate this behavior as well.

Cicerone, D.S.; Stewart, A.J.; Roh, Y.

1999-10-01

338

Distinguishing Phosphate Structural Defects From Inclusions in Calcite and Aragonite by NMR Spectroscopy (Invited)  

NASA Astrophysics Data System (ADS)

Variations in the concentration of minor and trace elements are being studied extensively for potential use as proxies to infer environmental conditions at the time of mineral deposition. Such proxies rely fundamentally on a relationship between the activities in the solution and in the solid that would seem to be simple only in the case that the species substitutes into the mineral structure. Other incorporation mechanisms are possible, including inclusions (both mineral and fluid) and occlusion of surface adsorbate complexes, that might be sensitive to other factors, such as crystallization kinetics, and difficult to distinguish analytically. For example, it is known from mineral adsorption studies that surface precipitates can be nanoscopic, and might not be apparent at resolutions typical of microchemical analysis. Techniques by which a structural relationship between the substituting element and the host mineral structure are needed to provide a sound basis for geochemical proxies. NMR spectroscopy offers methods for probing such spatial relationship. We are using solid-state NMR spectroscopy to investigate phosphate incorporation in calcium carbonate minerals, including calcite speleothems and coral skeletal aragonite, at concentrations of the order 100 ?g P g -1. In 31P NMR spectra of most samples, narrow peaks arising from crystalline inclusions can be resolved, including apatite in coral aragonite and an unidentified phase in calcite. All samples studied yield also a broad 31P signal, centered near chemical shifts of +3 to +4 ppm, that could be assigned to phosphate defects in the host mineral and from which the fraction of P occurring in the carbonate mineral structure can be determined. To test this assignment we applied rotational-echo double-resonance (REDOR) NMR techniques that probe the molecular-scale proximity of carbonate groups to the phosphate responsible for the broad 31P peak. This method measures dipole-dipole coupling between 31P of phosphate and carbonate carbon, which varies with the inverse-cube of the internuclear distance. 31P{13C} REDOR NMR results for synthetic phosphate/(13C)-aragonite coprecipitates show that the broad peak is closely associated with carbonate, exhibiting a 31P-13C dipolar coupling qualitatively consistent with phosphate occupying an anion structural site (i.e., 6 C at 0.32 nm). 31P-detected 1H NMR spectra, which contain signal only from H located near P, show that structural water molecules help accommodate phosphate in the structure. Similar methods can be applied to other elements of potential paleo-proxy interest having NMR-active isotopes, including B, Mg, and Cd.

Phillips, B. L.; Mason, H. E.

2010-12-01

339

Orientation with a Viking sun-compass, a shadow-stick, and two calcite sunstones under various weather conditions.  

PubMed

It is widely accepted that Vikings used sun-compasses to derive true directions from the cast shadow of a gnomon. It has been hypothesized that when a cast shadow was not formed, Viking navigators relied on crude skylight polarimetry with the aid of dichroic or birefringent crystals, called "sunstones." We demonstrate here that a simple tool, that we call "shadow-stick," could have allowed orientation by a sun-compass with satisfying accuracy when shadows were not formed, but the sun position could have reliably been estimated. In field tests, we performed orientation trials with a set composed of a sun-compass, two calcite sunstones, and a shadow-stick. We show here that such a set could have been an effective orientation tool for Vikings only when clear, blue patches of the sky were visible. PMID:24085076

Bernáth, Balázs; Blahó, Miklós; Egri, Adám; Barta, András; Kriska, György; Horváth, Gábor

2013-09-01

340

Interactions between Acidic Proteins and Crystals: Stereochemical Requirements in Biomineralization  

NASA Astrophysics Data System (ADS)

Acidic matrix macromolecules are intimately involved in biological crystal growth. In vitro experiments, in which crystals of calcium dicarboxylate salts were grown in the presence of aspartic acid-rich proteins, revealed a stereochemical property common to all the interacting faces. Calcite crystals are nucleated on stereochemically analogous faces when proteins are adsorbed onto a rigid substrate. The importance of this property in biomineralization is discussed.

Addadi, L.; Weiner, S.

1985-06-01

341

Synthesis of Calcium Carbonate Crystals Using Cells of Plant Leaves  

Microsoft Academic Search

In this paper, calcium carbonate (CaCO3) crystals were synthesized by using the leaves of the plant Scindapsus aureum. The resultant CaCO3 crystals were characterized by scanning electron microscopy and electron diffraction. The biomolecules of Scindapsus aureum was confirmed by using UV-vis analysis. The results showed that the elongated rhombohedral calcite crystals were produced in the cells of Scindapsus aureum leaves.

Long Chen; Xiaoping Fang; Shineng Zhang; Xiayu Wu; Jingjing Wu; Qinqin Cui; Liqi Yu; Qingqing Wang

2010-01-01

342

Many Seeds: Estimating Hidden Seeds  

NSDL National Science Digital Library

In this activity, learners will estimate how many seeds are in a fruit or vegetable, then count to find out. The result: mix estimation with healthy eating. Vary the level of difficulty by using different foods: for younger learners, choose something with up to about 15 seeds inside (e.g. apples, snow peas); for a medium level of difficulty, try melon slices or cucumbers; for more challenge, use pomegranates or mini-pumpkins. Available as a web page or downloadable pdf.

Terc

2010-01-01

343

Kinetics of Cd sorption, desorption and fixation by calcite: A long-term radiotracer study  

NASA Astrophysics Data System (ADS)

Time-dependent sorption and desorption of Cd on calcite was studied over 210 days utilizing 109Cd as a tracer to distinguish between 'labile' and 'non-labile' forms of sorbed Cd. Stabilizing the calcite suspensions for 12 months under atmospheric P CO2 and controlled temperature was necessary to reliably follow Cd dynamics following initial sorption. Results revealed time-dependant Cd sorption and marked desorption hysteresis by calcite under environmentally relevant conditions. Data obtained were fitted to a first-order kinetic model and a concentric shell diffusion model. Both models described the progressive transfer of Cd 2+ to a less reactive form within calcite and subsequent desorption of Cd subject to different initial contact times. The kinetic model provided a better fit to the combined sorption and desorption data ( R2 = 0.992). It differentiates between two 'pools' of sorbed Cd 2+ on calcite, 'labile' and 'non-labile', in which labile sorbed Cd is in immediate equilibrium with the free Cd 2+ ion activity in solution whereas non-labile Cd is kinetically restricted. For the diffusion model ( R2 = 0.959), the rate constants describing Cd dynamics in calcite produced a half-life for Cd desorption of ˜175 d, for release to a 'zero-sink' solution. Results from this study allow comment on the likely mechanisms occurring at the calcite surface following long-term Cd sorption.

Ahmed, Imad A. M.; Crout, Neil M. J.; Young, Scott D.

2008-03-01

344

Top-seed solution growth and characterization of AlSb single crystals for gamma-ray detectors. Final report, 1 October 1994--30 September 1995  

SciTech Connect

The ultimate objective of the conducted research is to ascertain the potential of AlSb (in single crystal form) for application as {gamma}-detector material operating at room temperature. To this end approaches to crystal growth were to be developed which permit control of growth parameters affecting critical application specific properties of AlSb. The research was focused on exploration of the effectiveness of the Czochralski method and on the development of methods and procedures leading to AlSb crystals with low free carrier concentration and a high mobility-lifetime product. Conventional melt growth of AlSb by the Czochralski technique (from stoichiometric charges) generally yielded material with high net carrier concentrations and low mobility-lifetime products. Significant improvement in crystal properties was achieved, when operating with non-stoichiometric melts, containing Sb in excess at levels of 3 to 10 mol%, further improvements were obtained when changing ambient argon pressure from atmospheric to 300 psi, and using high purity alumina crucibles which were inductively heated with a graphite susceptor CVD coated with silicon-carbide. Initial efforts to reduce evaporative loss of Sb through application of the LEC technique (liquid encapsulated Czochralski) with conventional encapsulants (B{sub 2}O{sub 3}, LiF, CaF{sub 2}) failed because of their interaction with the crucible and the AlSb melt. Compensation techniques (based on extrinsic doping) were found to lead to the desired reduction of free carriers in AlSb. Such material, however, exhibits a significant decrease of charge carrier mobility and lifetime. Early termination of this research program prevented optimization of critical materials properties in AlSb and precluded at this time a realistic assessment of the potential of this material for solid state detector applications.

Witt, A.F.; Becla, P.; Counterman, C.; DiFrancesco, J.; Landahl, G.; Morse, K.; Sanchez, J.

1996-01-26

345

Aqueous synthesis and transmission electron microscopy observation of seed-grown spherical ferrite nanoparticles  

NASA Astrophysics Data System (ADS)

Uniform-sized spherical iron ferrite nanoparticles grew on seed crystals in an aqueous solution containing sucrose. Using the seed crystals which were highly dispersed in acidic or alkaline seed-crystal suspension without relation to pH of the suspension, we widely controlled the particle diameter in the range 20-200 nm by changing the additive amount of the seed crystals. By transmission electron microscopy observation and X-ray diffraction analysis, it indicated that the particles were highly crystalline but not amorphous. Selected area diffraction patterns of the particles by using transmission electron microscope revealed that the particles were composed of one to several crystals. Thus we provided the evidence that the particles grew on clusters composed of one to several seed crystals to which those of several dozen seed crystals were disintegrated.

Tanaka, T.; Tada, M.; Nakagawa, T.; Abe, M.

2012-03-01

346

Direct visualization of single ions in the Stern layer of calcite.  

PubMed

Calcite is among the most abundant minerals on earth and plays a central role in many environmental and geochemical processes. Here we used amplitude modulation atomic force microscopy (AFM) operated in a particular regime to visualize single ions close to the (1014) surface of calcite in solution. The results were acquired at equilibrium, in aqueous solution containing different concentrations of NaCl, RbCl, and CaCl(2). The AFM images provide a direct and atomic-level picture of the different cations adsorbed preferentially at certain locations of the calcite-water interface. Highly ordered water layers at the calcite surface prevent the hydrated ions from directly interacting with calcite due to the energy penalty incurred by the necessary restructuring of the ions' solvation shells. Controlled removal of the adsorbed ions from the interface by the AFM tip provides indications about the stability of the adsorption site. The AFM results show the familiar "row pairing" of the carbonate oxygen atoms, with the adsorbed monovalent cations located adjacent to the most prominent oxygen atoms. The location of adsorbed cations near the surface appears better defined for monovalent ions than for Ca(2+), consistent with the idea that Ca(2+) ions remain further away from the surface of calcite due to their larger hydration shell. The precise distance between the different hydrated ions and the surface of calcite is quantified using MD simulation. The preferential adsorption sites found by MD as well as the ion residence times close to the surface support the AFM findings, with Na(+) ions dwelling substantially longer and closer to the calcite surface than Ca(2+). The results also bring new insights into the problem of the Stern and electrostatic double layer at the surface of calcite, showing that parameters such as the thickness of the Stern layer can be highly ion dependent. PMID:23339738

Ricci, Maria; Spijker, Peter; Stellacci, Francesco; Molinari, Jean-Francois; Voïtchovsky, Kislon

2013-02-05

347

Low-magnesium uranium-calcite with high degree of crystallinity and gigantic luminescence emission.  

PubMed

Cabrera (Madrid) low-Mg calcites exhibit: (i) an unusual twofold elevation in X-ray diffraction pattern intensity; (ii) a 60-fold elevation of luminescence emission, compared to six common natural calcites selected for comparison purposes; (iii) a natural relatively high radiation level of circa 200 nSvh(-1) not detected in 1300 other calcites from the Natural History Museum of Madrid. Calcites were analysed by the X-ray diffraction powder method (XRD), cathodo-luminescence spectroscopy in scanning electron microscopy (CL-SEM), thermoluminescence (TL), differential thermal analysis (DTA), X-ray fluorescence spectrometry (XRF) and particle size distribution (PSD). The Cabrera calcite study shows: (i) helicoidally distributed steps along the (0001) orientation; (ii) protuberance defects onto the (0001) surface, observed by SEM; (iii) XRF chemical contents of 0.03% MgO, 0.013% of Y(2)O(3), and 0.022% of U(3)O(8), with accessory amounts of rare earth elements (REE); (iv) DTA dissociation temperature of 879 degrees C; (v) TL maxima peaks at 233 and 297 degrees C whose areas are 60 times compared to other calcites; (vi) spectra CL-SEM bands at 2.0 and 3.4 eV in the classic structure of Mn(2+) activators; (vii) a twofold XRD pattern explained given that sample is a low-Mg calcite. The huge TL and CL emissions of the Cabrera calcite sample must be linked with the uranyl group presence. This intense XRD pattern in low-Mg calcites could bring into being analytical errors. PMID:17011199

Valle-Fuentes, Francisco-Jose; Garcia-Guinea, Javier; Cremades, Ana; Correcher, Virgilio; Sanchez-Moral, Sergio; Gonzalez-Martin, Rafael; Sanchez-Muñoz, Luis; Lopez-Arce, Paula

2006-09-29

348

The effect of liquid phase chemistry on growth of lead magnesium niobate-lead titanate single crystals by seeded polycrystal conversion  

NASA Astrophysics Data System (ADS)

The effect that liquid phase content and chemistry has on single crystal growth was investigated in this study. This was accomplished by determining the single crystal growth mechanism as well as studying the effect of hot pressing environment. Characterization techniques played a large role in deciphering the relevance of each effect. One such technique was used because x-ray absorption of oxygen is significant in thin specimens of Pb(Mg1/3Nb2/3 )O3-35 mol% PbTiO3 [PMN-35PT] due to the presence of Pb and Nb. Using only XEDS data, the zeta-factor method provides absorption corrected compositional information, which was helpful to fully characterize the liquid phase. It was shown that the compositional data were in very good agreement with the nominal values for PMN-35 PT, whereas the uncorrected data underestimated the oxygen content by 300%. This technique linked the swelling of samples to changes in the oxygen to lead ratio of the excess PbO due to hot-pressing in air. This observation corroborates the microstructural and x-ray diffraction data that was obtained. The influence of excess PbO on {001} single crystal growth was studied in the range of 0-10 vol% PbO. It was found that up to 1.5 vol% PbO, the amount of single crystal and grain growth increased with increasing PbO additions. At higher PbO contents, however, the growth becomes independent of liquid fraction for all annealing times. In addition since the matrix grains were faceted and the growth best fit parabolic kinetics, interface reaction control was deemed the most likely growth mechanism. It was also shown that the chemistry of the liquid phase was dynamic as characterized in an Analytical Electron Microscope. In fact, MgO was found to precipitate out of the system due the saturation of MgO in the liquid, which in turn altered the liquid/solid surface energy. By changing the energy of the system, the matrix grains become more faceted, i.e. the frequency of {100} surfaces increases. This reduces the driving force for growth of the single crystal, which appears to be responsible for reducing the growth rate during annealing.

Gorzkowski, Edward P., III

349

EXPERIMENTAL STUDY OF THE CALCITE PRECIPITATION BASED ON THE UREASE PRODUCTION BACTERIUM ISOLATED FROM PEAT  

NASA Astrophysics Data System (ADS)

In this paper, authors proposed the newly calcite precipitation method for peat. This method can be isolated the urease production bacterium. The main outcomes of this research were: (1) Proposed method can be isolated the urease production bacterium from peat. (2) Urease production bacterium from peat can be accelerate the calcite precipitation at the high pH and high chlorine conditions. (3) Calcite precipitation speed was slower than the B. pasteurii . (4) Proposed method can accelerate the soil strength (Over 400kN/m2 -1D compression test) after 2 week cultivation.

Hata, Toshiro; Sato, Atsuko; Kawasaki, Satoru; Abe, Hirofumi

350

Permeability and Storativity of Calcite-Quartz Aggregates During Deformation  

NASA Astrophysics Data System (ADS)

Permeability, storage capacity and volumetric strain were measured in situ during triaxial deformation on hot-pressed calcite-quartz aggregates containing 10, 20 and 30 wt% quartz. The tests were performed at confining pressure of 300 MPa, pore pressure 50 to 290 MPa, temperature from 298K to 873K and at strain rates of 3.0\\times10-5, 8.3\\times10-5 or 3.0\\times10-4. Either argon gas or water were used as pore fluid. The initial porosities of the starting samples vary from 5% to 9%, with higher porosity correlated to higher amount of quartz. Under pure hydrostatic loading the compaction rate covaries with porosity and increases with increasing of effective pressure. The compaction-induced permeability reduction is partly irreversible and probably owes to plastic processes. Substantial amounts of porosity(4-6%) remain under hydrostatic loading at temperature up to 873K. From the microstructural observation, two kinds of pores can be found: cracks along interface and spherical voids within calcite matrix. Variation in pore structure may influence the permeability. The sensitivity of permeability and storage capacity to effective pressure are higher in the samples with 20% and 30% quartz. During dilatant deformation, permeability is linearly related to storage capacity enhancement. During compactive deformation, all samples show reductions in storage capacity and permeability, but the 20% and 30% samples show smaller reductions in storage capacity. The pore aspect ratio a/c can be estimated at (Fisher and Paterson, 1992): $(a)/(c)=1+(? c)/(?_m ) (? c)/(?_c ?_f )-1&) where a and c are the maximicrons and minimicrons dimensions of the pore respectively, ?_m and ?_f are compressibilities of the mineral and the pore fluid respectively, and ?_c and ?_c are the storativity and porosity of the sample respectively. The experiments suggest that the ratio a/c tend to increase as porosity decreases for 20% and 30% samples, which perhaps indicating that the pores become less equant. Observations of microstructure do show that many spherical pores are eliminated by plastic deformation.

Xiao, X.; Evans, B.

2001-12-01

351

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate  

PubMed Central

Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC ? less metastable hydrated ACC?anhydrous ACC ? biogenic anhydrous ACC?vaterite ? aragonite ? calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO2 sequestration.

Radha, A. V.; Forbes, Tori Z.; Killian, Christopher E.; Gilbert, P. U. P. A.; Navrotsky, Alexandra

2010-01-01

352

Stress remagnetization in pyrrhotite-calcite synthetic aggregates  

NASA Astrophysics Data System (ADS)

Stress-induced remagnetization has been applied to multidomain pyrrhotite-calcite synthetic aggregates in a triaxial rig. Experimental deformation used 150MPa confining pressure, a constant strain rate of 10-5 s-1 and applied differential stresses of up to 70MPa. New components of magnetization, parallel to the direction of the pressure vessel field, were added to the pre-deformational magnetization. The intensity of remagnetization (M'-M0) increases with the intensity of the applied differential stress and affects the coercivity fraction below 15mT. Bulk shortening is less than 8 per cent, thus grain rotation cannot explain selective remagnetization of the low-coercivity fraction. Remagnetization is thus attributed to deformational viscous remanent magnetization (DVRM). It is observed that high-coercivity (>15mT) grains do not remagnetize. There is, however, slight progressive rotation of pre-deformational magnetization with increasing strain up to 8 per cent of bulk shortening. The lack of piezoremanent magnetization in the high-coercivity range may be due to defects introduced in pyrrhotite during sample preparation. Experiments using synthetic pyrrhotite, expected to show low dislocation densities, would be necessary to test this effect.

Robion, Philippe; Borradaile, Graham J.

2001-01-01

353

Effects of Dolomitic and Calcitic Limes on Strength Development in Mixtures with Two Clay Minerals.  

National Technical Information Service (NTIS)

Conflicting reports about the relative effectiveness of two principal lime types, dolomitic monohydrated and calcitic hydrated, in soil stabilization have resulted in a need to re-evaluate the response to lime stabilization of the reactive components in s...

W. C. Ormsby E. B. Kinter

1972-01-01

354

Studies on cyanobacterial extracellular polymeric substances: functional groups, calcite biomineralization and formation of capsular polymeric substances  

NASA Astrophysics Data System (ADS)

Extracellular polymeric substances (EPS) of microbial origin are an important class of polymeric materials which have been involved in different processes such as biofilm development or mineral precipitation. Cyanobacteria have been known as potential EPS producers for a long time. Despite their ubiquitous distribution, there is still a great lack of knowledge concerning the diversity of EPS binding sites of different picocyanobacterial strains on the one hand and the specific components of EPS which are responsible for calcite precipitation and crystal morphology on the other hand. It is generally accepted that capsular extracellular polymeric substances are the main components of biofilm matrixes. In this context, it is important to understand under which conditions cyanobacteria produce surface polysaccharides. In a recent study, we characterized the binding sites of EPS of three unicellular autotrophic picocyanobacterial strains of the Synechococcus-type. Potentiometric titrations were conducted to determine different types of functional groups present at the various sites. Precipitation experiments with EPS of different strains allowed for estimating the potential of EPS to precipitate calcium carbonate and the impact of functional groups composition on crystal morphology. In order to clarify the conditions under which cyanobacteria formed capsular EPS, we performed growth experiments in nutrients medium with different phosphorus concentrations (0.4, 4.1, 8.2 and 41 mgP/l). Cyanobacterial cells produced capsular EPS under phosphorus concentrations of 0.4, 4.1 and 8.2 mgP/l, while no capsular EPS were observed for the highest P concentration (41 mgP/l). At this concentration, however, calcium rich storage products were detected in the cells. The results thus suggest that both extracellular and intracellular products are regulated through phosphorus concentrations in growth solutions. Titrations reveal five or six distinct sites on surfaces of picocyanobacterial strains. The protonation constants of the different strains are rather similar but the site densities vary within an order of magnitude. The EPS of the phycoerythrin-rich strain had a higher number of binding sites than the phycocyanin-rich strain and PCC 7942. Precipitation experiments demonstrated that EPS of all strains were able to precipitate calcium carbonate, which always showed a similar morphology. This result demonstrates that minor differences in compositions of functional groups do not impact crystal morphology.

Dittrich, M.; Sibler, S.; Matsko, N.

2006-12-01

355

Sock Seeds  

NSDL National Science Digital Library

The experiment demonstrated in this ZOOMSci video segment--wearing an old sock through a grassy or weedy field, planting the sock, and watching what grows--will not only give young scientists a better idea of the kinds of plants growing in their area, but will also help them begin to think about the evolutionary strategies of plants to disperse seeds.

Foundation, Wgbh E.

2009-07-23

356

Seed Bargains  

NSDL National Science Digital Library

Children as well as adults are usually intrigued by the ideas of bargains. More seeds for less money is often enough to convince them to buy a larger, cheaper package. In this case, the children are savvy enough to question the value of either package. On

Konicek-Moran, Richard

2008-04-01

357

Surface speciation models of calcite and dolomite\\/aqueous solution interfaces and their spectroscopic evaluation  

Microsoft Academic Search

The composition and density of surface hydroxyl and carbonate groups on calcite and dolomite after contact at 25 C with solutions of different pH (3 to 12) and carbonate concentration (10⁻⁴ â¤Î£COâ⤠0.1 M) were monitored by means of diffuse reflectance infrared (DRIFT) spectroscopy. Both for calcite and dolomite, broad high-intensity absorbance bands at about 3,400 and 1,600 cm⁻¹ were

O. S. Pokrovsky; J. A. Mielczarski; O. Barres; J. Schott

2000-01-01

358

Effect of calcitic and dolomitic lime on physicochemical properties of a Chilean Andisol  

Microsoft Academic Search

Changes in surface reactivity produced by different types of liming in a Chilean Andisol were determined. Barros Aranas soil with 53% aluminum (Al) saturation and 4.8 pH was incubated with different amounts of calcitic and dolomitic lime. Each cmol of calcitic or dolomitic lime applied per kg of soil increased the pH with 0.13 and 0.16 units, respectively. Consequently, the

M. L. Mora; G. Baeza; C. Pizarro; R. Demanet

1999-01-01

359

Geochemical preservation potential of high-grade calcite marble versus dolomite marble: implication for isotope chemostratigraphy  

Microsoft Academic Search

The Neoproterozoic Hekkelstrand Marble from the North-Central Norwegian Caledonides contains a calcite marble unit hosting a commercially exploited white dolomite marble deposit, which underwent polyphase amphibolite-facies, Scandian metamorphism. Major and trace element geochemistry, ?18O, ?13C and 87Sr\\/86Sr ratios have been used to define isotopic differences between interbedded calcite and dolomite marbles and to assess their post-depositional isotopic resetting. A high

V. A. Melezhik; D. Roberts; A. E. Fallick; I. M. Gorokhov; A. B. Kusnetzov

2005-01-01

360

Neutralization of sulfuric acid solutions by calcite dissolution and the application to anoxic limestone drain design  

Microsoft Academic Search

Batch reactor (BR) experiments were conducted to measure the effect of hydrodynamics and gypsum coatings on calcite neutralization rates. A factorial array of BR experiments measured the H+ concentration change by calcite dissolution over a pH range of 1.5–3.5 and Na2SO4 concentrations of 0–1M. The rate of H+ concentration change with time was determined by numerical differentiation of H+ concentration

Danielle M. C. Huminicki; J. Donald Rimstidt

2008-01-01

361

Coprecipitation of Uranium(VI) with Calcite: XAFS, micro-XAS, and luminescence characterization  

Microsoft Academic Search

X-ray absorption and luminescence spectroscopies have been used to characterize the local structure and coordination of uranium(VI) species coprecipitated with calcite (CaCO3) from room-temperature aqueous solutions. Different solution chemistries and pHs are found to result in differences in the equatorial coordination of the uranyl species (UO22+) incorporated in the calcite, with multiple coordination environments of uranyl evident in one sample.

Richard J. Reeder; Melissa Nugent; C. Drew Tait; David E. Morris; Steve M. Heald; Kenneth M. Beck; Wayne P. Hess; Anthony Lanzirotti

2001-01-01

362

Coprecipitation of Uranium (VI) with Calcite: XAFS, Micro-XAS,and Luminescence Characterization  

SciTech Connect

X-ray absorption and luminescence spectroscopies have been used to characterize the local structure and coordination of uranium (VI) species coprecipitated with calcite (CaCO3) in room-temperature aqueous solutions. Different solution chemistries and pHs are found to result in defferences in the equatorial coordination of the uranyl species (UO2/2+) in the calcite, with multiple coordination environments of uranyl evident in one sample.

Reeder, Richard J.; Nugent , Melissa; Tait, C DREW.; Morris, David E.; Heald, Steve M.; Beck, Kenneth M.; Hess, Wayne P.; Lanzirotti, Anthony

2001-04-06

363

Synthetic polycrystalline aragonite to calcite transformation kinetics: experiments at pressures close to the equilibrium boundary  

Microsoft Academic Search

Summary ¶This study experimentally investigated the transformation kinetics of synthetic polycrystalline aragonite to calcite at four temperature\\/pressure conditions (330?°C\\/200?MPa, 380?°C\\/325?MPa, 430?°C\\/580?MPa, and 480?°C\\/875?MPa), close to the calcite-aragonite equilibrium phase boundary. The extents of transformation measured as a function of time in a synthetic system, using in-situ annealed, high purity samples, are consistent with the kinetic model for grain-boundary nucleation and

W.-L. Huang

2003-01-01

364

Trace element distribution in calcite-dolomite carbonatites from Spitskop: inferences for differentiation of carbonatite magmas and the origin of carbonates in mantle xenoliths  

NASA Astrophysics Data System (ADS)

We have studied trace element distribution in intrusive calcite-dolomite carbonatites from South Africa, in which primary igneous textures are remarkably well preserved. These rocks contain few non-carbonate minerals and show no effects of crustal contamination. The trace element compositions of carbonates and fine-grained interstitial aggregates were determined in situ by laser ablation inductively coupled plasma mass spectrometry while whole rock samples were analyzed by solution inductively coupled plasma mass spectrometry. A range of rare earth element (REE) contents and various distribution patterns have been found for minerals in each rock. Calcite phenocrysts typically have low REE abundances (1-20×primitive mantle) and nearly flat primitive mantle-normalized distribution patterns with no or minor enrichments in light REE. In comparison, interstitial calcite in the same rocks is strongly enriched in light over heavy REE (La/YbN~200). Fine-grained interstitial material (apatite-bearing) has high contents of REE and of highly incompatible trace elements. These in situ analyses have documented significant chemical differences between distinct textural groups of carbonate minerals (phenocrysts and interstitial grains) which must be related to their crystallization sequence. These results indicate incompatible behavior of REE during fractional crystallization of carbonatite liquids and suggest strong enrichments of residual liquids in highly incompatible elements. Sr abundances show relatively little variation, consistent with carbonate/melt partition coefficients higher than for REE but close to or less than unity. The REE abundances in calcite phenocrysts are nearly as low as those reported for individual carbonate grains from mantle peridotite xenoliths and are lower than in composite carbonate-rich pockets found in some xenoliths. These results strongly indicate that the carbonates in mantle xenoliths are crystal cumulates from primitive carbonate-rich melts rather than quenched carbonatite liquids. Furthermore, relatively low REE abundances (and high Sr) may be common in primary mantle-derived carbonate liquids. Strong enrichments in highly incompatible elements found in many carbonatites may require complex fractionation of initial liquids during ascent as well as derivation from highly enriched mantle domains.

Ionov, Dmitri; Harmer, Robin E.

2002-05-01

365

Mixing-induced calcite precipitation and dissolution kinetics in micromodel experiments.  

SciTech Connect

Dissolved CO2 from geological CO2 sequestration may react with dissolved minerals in fractured rocks or confined aquifers and cause mineral precipitation. The overall rate of reaction can be limited by diffusive or dispersive mixing, and mineral precipitation can block pores and further hinder these processes. Mixing-induced calcite precipitation experiments were performed by injecting solutions containing CaCl2 and Na2CO3 through two separate inlets of a micromodel (1-cm x 2-cm x 40-microns); transverse dispersion caused the two solutions to mix along the center of the micromodel, resulting in calcite precipitation. The amount of calcite precipitation initially increased to a maximum and then decreased to a steady state value. Fluorescent microscopy and imaging techniques were used to visualize calcite precipitation, and the corresponding effects on the flow field. Experimental micromodel results were evaluated with pore-scale simulations using a 2-D Lattice-Boltzmann code for water flow and a finite volume code for reactive transport. The reactive transport model included the impact of pH upon carbonate speciation and calcite dissolution. We found that proper estimation of the effective diffusion coefficient and the reaction surface area is necessary to adequately simulate precipitation and dissolution rates. The effective diffusion coefficient was decreased in grid cells where calcite precipitated, and keeping track of reactive surface over time played a significant role in predicting reaction patterns. Our results may improve understanding of the fundamental physicochemical processes during CO2 sequestration in geologic formations.

Valocchi, Albert J.; Dewers, Thomas A.; Dehoff, Karl; Yoon, Hongkyu; Werth, Charles J.

2010-12-01

366

On the mechanism of dislocation creep of calcite at high temperature: Inferences from experimentally measured pressure sensitivity and strain rate sensitivity of flow stress  

NASA Astrophysics Data System (ADS)

Previous laboratory experiments on coarse grained calcite materials at intermediate conditions of stress and temperature have shown low strain rate sensitivity of flow stress, expressed by standard power law stress exponents n ? 7. This conflicts with conventional models for creep controlled by dislocation climb (n = 3-4.5), a recovery mechanism widely used to explain steady-state dislocation creep. This paper addresses the question whether dislocation cross slip rather than climb is the mechanism controlling creep of calcite at intermediate conditions. New uniaxial compression tests were performed on calcite single crystals and Carrara marble at T = 800-1000°C, ? = 10-4-2 × 10-7 s-1, and P = 100-600 MPa. Emphasis was on the pressure (P) and strain rate (?) sensitivity of flow stress, because these form potential ways for discriminating between climb and cross slip models. Both single crystals and Carrara marble demonstrated a small increase in flow stress with increasing pressure (˜1.6% per 100 MPa) at constant strain rate and temperature. The low strain rate sensitivity of flow stress was confirmed, though n gradually changes with stress or temperature rather than taking a constant value. Evaluation of the experimental data against microphysical models indicated that cross slip controlled by dissociation of dislocations offers the best explanation for the observed mechanical behavior. If the associated creep equation is used to estimate flow stresses under natural conditions, the resulting values at T < 500°C are substantially less than those following from previous flow laws. The influence of pressure on the behavior of marble, however, can be safely ignored for most practical purposes.

de Bresser, J. H. P.

2002-12-01

367

Microphysical mechanisms controlling slip stability in calcite-rich fault gouges  

NASA Astrophysics Data System (ADS)

It is well established that for an earthquake rupture to nucleate on a natural fault the sliding medium within the fault, i.e. the fault gouge, must exhibit velocity weakening frictional properties. Fault frictional behavior is usually modeled empirically using Rate-and-State-dependent Friction (RSF) "laws". However the deformation mechanisms controlling the velocity dependence of slip are still not well understood. We have investigated the microphysical processes leading to velocity weakening slip in calcite(-rich) fault gouges. We performed dry and wet direct shear friction experiments at a constant effective normal stress (50 MPa) at sliding velocities of 0.1-10 µm/s, at temperatures ranging from ~20 to 150°C. Our results show a transition from velocity strengthening to velocity weakening slip above ~100°C. The presence of water does not affect this transition. After each experiment we analyzed ultrathin sections prepared from the sheared gouges using light microscopy. The sheared samples generally show pervasive R1- and, less common, R2- and P-shears, plus sharply defined boundary shears. The boundary and R1 shear bands are characterized by grain comminution to sub-micron size plus a strong Lattice Preferred Orientation (LPO) in the shear band cores. At the light microscopy scale (>1 µm), no differences were found between the microstructures of the velocity weakening and velocity strengthening samples. Focused Ion Beam SEM work is ongoing in an attempt to establish any finer scale differences. Nonetheless, the results suggest that the processes controlling friction and slip stability operate on the nanometer-scale, and involve crystal plasticity in combination with cataclasis and grain size reduction, with compaction via plasticity perhaps competing with dilatant granular flow. Our results, especially the effect of temperature, have important implications for shallow focus seismicity in tectonically active carbonate-bearing terrains, providing possible constraints on the upper limit of the seismogenic zone.

Verberne, B. A.; Niemeijer, A. R.; Spiers, C. J.; He, C.-R.; Drury, M. R.

2012-04-01

368

Adsorption and modification of calcium salt crystal growth by anionic peptides and spermine  

Microsoft Academic Search

Synthetic polyanions, including peptide analogs of naturally occurring proteins, have been shown to inhibit the nucleation and growth of calcium salt crystals. The binding characteristics of polyaspartate and aspartate-serine copolymers to calcium carbonate (calcite) and hydroxyapatite (HAP) are presented here. The binding is related to dosedependent inhibition of crystal growth measured by constant composition assays. Peptide phosphorylation had little effect

Erich Mueller; C. Steven Sikes

1993-01-01

369

Crystal Growth and the Role of the Organic Network in Eggshell Biomineralization  

Microsoft Academic Search

A model based on geometrical crystal growth considerations is proposed for the deposition of the crocodilian, testudinian and avian eggshells. In each shell column, crystal deposition is initiated at a single location, from which growth fans out at all angles to the shell normal. In both calcitic and aragonitic shells, growth is in the [001] direction, resulting in an increase

Heather Silyn-Roberts; R. M. Sharp

1986-01-01

370

Microstructural Character and Stable Isotope Signatures of Calcite Veins in the Mis and SMS Sedimentary Rock Cores, Victoria Land Basin, Antarctica  

NASA Astrophysics Data System (ADS)

The two drilling projects in the ANDRILL portfolio, the McMurdo Ice Shelf (MIS) project and the Southern McMurdo Sound (SMS) project, successfully recovered 1285 and 1138 m respectively of Neogene sedimentary core from the Victoria Land rift basin in Antarctica. The MIS core contains 1475 faults, veins, and clastic dikes, with veins making nearly half of this population. Of the 1050 natural fractures logged in the SMS core, just over half of them were characterized as veins. This study combines microscopic study of textures and crosscutting relationships between veins, cements, and microstructures to determine the evolution of fluid sources relative to the deformation history during rifting. Isotopic analysis of carbonate fracture fills has been completed in order to discriminate whether fluids precipitated from the glacial melt-waters or from some other hydrological regime. Opening-mode fibrous veins, pressure shadows on clast margins and small and diffuse, anastomosing vein swarms are common. Most veins are filled by calcite, although zeolite and late-stage pyrite are also common. Many calcite veins are associated with discrete faults and have conjugate geometries, dip-slip slickenfibers, and calcite filled voids along pull-aparts, indicating veins formed during fault displacement. In the MIS core, fibrous opening-mode are commonly wavy to tightly folded, indicating the host sediment was cohesive enough to fracture but was not fully lithified, and accommodated vein buckling during compaction. Veins commonly exhibit multiple generations of calcite fill and many are strongly twinned. Cathodoluminescence microscopy shows minor changes in color and intensity and minimal zoning of calcite vein fill, suggesting relatively rapid crystallization of fluids of similar chemistry. However, stable isotope analyses reveal large variations in values: in the MIS core, oxygen values range from -5.35 to -11.97 (VPDB) and carbon values range from -7.15 to -21.91 (VPDB); the SMS isotopic data shows a slightly wider range of values, with oxygen ranging from -6.33 to -14.00 (VPDB) and carbon ranging from 4.14 to -16.88 (VPDB). Integrated analysis of these data will help to identify the hydrological regime associated with the fluids, and the relative influence of glacial and tectonic processes on fluid circulation and evolution in this glaciated rift basin.

Millan, C.; Wilson, T. J.; Paulsen, T. S.

2010-12-01

371

BULK SEED TENDERS FOR HANDLING PEANUT SEED  

Technology Transfer Automated Retrieval System (TEKTRAN)

Planting is a very labor intensive operation requiring both skilled and unskilled labor. Intense manual labor is required to load seed hoppers on the planter. A seeding rate of 112 kg/ha, requires 11.2 t of seed per 100 ha planted. Bulk seed tenders have been used in the Midwestern United States to ...

372

Calcite Fluid Inclusion, Paragenetic, and Oxygen Isotopic Records of Thermal Event(s) at Yucca Mountain, Nevada  

SciTech Connect

Yucca Mountain, Nevada, is under consideration as a potential high-level radioactive waste repository situated above the water table in 12.7 Ma tuffs. A wealth of textural and geochemical evidence from low-temperature deposits of calcite and silica, indicates that their genesis is related to unsaturated zone (UZ) percolation and that the level of the potential repository has never been saturated. Nonetheless, some scientists contend that thermal waters have periodically risen to the surface depositing calcite and opal in the tuffs and at the surface. This hypothesis received some support in 1996 when two-phase fluid inclusions (FIs) with homogenization temperatures (Th) between 35 and 75 C were reported from UZ calcite. Calcite deposition likely followed closely on the cooling of the tuffs and continues into the present. The paragenetic sequence of calcite and silica in the UZ is early stage calcite followed by chalcedony and quartz, then calcite with local opal during middle and late stages. Four types of FIs are found in calcite assemblages: (1) all-liquid (L); (2) all-vapor (V); (3) 2-phase with large and variable V:L ratios; and (4) a few 2-phase with small and consistent V:L ratios. Late calcite contains no FI assemblages indicating elevated depositional temperatures. In early calcite, the Th of type 4 FIs ranges from {approx} 40 to {approx} 85 C. Such temperatures (sub-boiling) and the assemblage of FIs are consistent with deposition in the UZ. Some delta 18O values < 10 permil in early calcite support such temperatures. Type 4 FIs, however, seem to be restricted to the early calcite stage, during which either cooling of the tuffs or regional volcanism were possible heat sources. Nonetheless, at present there is no compelling evidence of upwelling water as a source for the calcite/opal deposits.

B. Peterman; R. Moscati

2000-08-10

373

Bacterially Induced Calcite Precipitation and Strontium Co-Precipitation under Flow Conditions in a Porous Media System  

NASA Astrophysics Data System (ADS)

The process of in situ carbonate mineral formation has implications in many environmental applications, including, but not limited to aquifer decontamination, enhancement of soil stability, and carbon capture and storage (CCS). The high stability of carbonates and the potential for co-precipitation of contaminants within carbonates are attractive attributes for several potential engineering applications. Ureolytic precipitation of calcium and strontium carbonates by Sporosarcina pasteurii was examined in two-dimensional flat plate porous media reactors. Complete reactor plugging due to biofilm formation and calcium carbonate precipitation was achieved in Sr-free systems after 14 hours and in Sr-inclusive systems after 15 hours. Comparison of the reactor influent and effluent after 11 hours indicated that Ca2+ concentrations in the Sr-free reactor effluent were reduced to approximately 0.48% of the influent concentration while the Ca2+ and Sr2+ concentrations of the Sr-inclusive effluent were reduced to 0.64% and 2.34% of the influent concentration indicating a slight inhibitory effect of strontium on calcium carbonate precipitation . Despite this slight inhibition, more than 98% of the Ca2+ entering the reactors was precipitated. Calcite was identified as the main mineral formed and a larger mean crystal size and density were observed near the reactor influent. Homogenous partition coefficients calculated from extracted precipitates suggest higher Sr2+ partitioning near the inlet region, where higher precipitation kinetics exist. Results confirm the possibility of effective calcite-based co-precipitation of Sr2+ under flow conditions and contributes towards the development of field-scale calcium carbonate mineral-based immobilization strategies.

Gerlach, Robin; Mitchell, Andrew C.; Schultz, Logan N.; Cunningham, Al B.

2010-05-01

374

Bacterially Induced Calcite Precipitation and Strontium Co-Precipitation under Flow Conditions in a Porous Media System  

NASA Astrophysics Data System (ADS)

The process of in situ carbonate mineral formation has implications in many environmental applications, including, but not limited to aquifer decontamination, enhancement of soil stability, and carbon capture and storage (CCS). The high stability of carbonates and the potential for co-precipitation of contaminants within carbonates are attractive attributes for several potential engineering applications. Ureolytic precipitation of calcium and strontium carbonates by Sporosarcina pasteurii was examined in two-dimensional flat plate porous media reactors. Complete reactor plugging due to biofilm formation and calcium carbonate precipitation was achieved in Sr-free systems after 14 hours and in Sr-inclusive systems after 15 hours. Comparison of the reactor influent and effluent after 11 hours indicated that Ca2+ concentrations in the Sr-free reactor effluent were reduced to approximately 0.48% of the influent concentration while the Ca2+ and Sr2+ concentrations of the Sr-inclusive effluent were reduced to 0.64% and 2.34% of the influent concentration indicating a slight inhibitory effect of strontium on calcium carbonate precipitation . Despite this slight inhibition, more than 98% of the Ca2+ entering the reactors was precipitated. Calcite was identified as the main mineral formed and a larger mean crystal size and density were observed near the reactor influent. Homogenous partition coefficients calculated from extracted precipitates suggest higher Sr2+ partitioning near the inlet region, where higher precipitation kinetics exist. Results confirm the possibility of effective calcite-based co-precipitation of Sr2+ under flow conditions and contributes towards the development of field-scale calcium carbonate mineral-based immobilization strategies.

Gerlach, R.; Mitchell, A. C.; Schultz, L.; Cunningham, A.

2009-12-01

375

Top seeded growth and joining of bulk YBCO.  

SciTech Connect

We report (i) systematic studies of the growth rate for melt textured YBCO, (ii) top seeding growth techniques to determine the minimum seed size, and (iii) joining techniques for melt textured YBCO, enabling the fabrication of large single domain structures of arbitrary shape. Seeded growth of YBCO occurs in a narrow temperature window about 20 C below the peritectic decomposition temperature. Successful top seeding depends on the size of the NdBCO seed crystal. Small seeds are eventually dissolved in the melt before nucleation occurs, while large seeds regularly produce single domain monoliths. Joining techniques based on seeding of low melting point Tm123/Y211 filler material by neighboring YBCO are described. Magneto-optical images of the YBCO/TmBCO/YBCO assembly show no detectable penetration of magnetic field at the joints.

Zheng, H.; Veal, B. W.; Paulikas, A.; Nikolova, R.; Welp, U.; Claus, H.; Crabtree, G. W.

1999-10-26

376

Bio-Inspired Approaches to Crystals with Composite Structures  

NASA Astrophysics Data System (ADS)

Advances in technology demand an ever-increasing degree of control over material structure, properties and function. As the properties of monolithic materials are necessary limited, one route to extending them is to create a composite by combining contrasting materials. The potential of this approach is beautifully illustrated by the formation of biominerals where organic macromolecules are combined with brittle minerals such as calcite to create crystals with considerable fracture toughness. This talk will discuss how bio-inspired approaches can be used to generate single crystals with composite crystals through a simple one-pot method. By precipitating calcite crystals in the presence of ``occlusion species'' ranging from latex particles, to organic and inorganic nanoparticles and finally small molecules we demonstrate that high amounts of foreign species can be incorporated through control over the additive surface chemistry, and that this can lead to an enhancement of the mechanical properties of the calcite. Occlusion of 20 nm anionic diblock copolymer micelles was achieved at levels of over 13 wt%, and the properties of the resuktant composite calcite crystals were measured using a range of techniques including IR spectroscopy, high resolution powder XRD and high resolution TEM. Incorporation of these macromolecules leads to crystals with structures and mechanical properties similar to those of biominerals. With sizes in the range of some intracrystalline proteins, the micelles act as ``pseudo-proteins'', thereby providing an excellent model system for investigation of the mechanism of macromolecule insertion within biominerals. Extension of these studies to the incorporation of small molecules (amino acids) again demonstrated high levels of incorporation without any change in the crystal morphology. Further, occlusion of these small molecules within the calcite lattice again resulted in a significant increase in the hardness of the calcite, a result which appears to derive from an increase in lattice strain on molecular occlusion. Finally, the generality of this strategy is demonstrated by its extension to the incorporation of inorganic particles such as magnetite and gold within calcite, leading to the formation of inorganic-inorganic composites.

Meldrum, Fiona

2013-03-01

377

Effect of Mg on the Grain Growth and Dislocation Creep of Calcite  

NASA Astrophysics Data System (ADS)

We tested the effect of variations in the amount of the solute impurity (Mg) on grain growth and strength of calcite aggregate. Synthetic marbles were produced by hot isostatic pressing mixtures of powders of calcite and dolomite at 850° C and 300 MPa confining pressure for different intervals (2 to 30 hrs). The HIP treatment resulted in homogeneous aggregates of calcite with Mg content from 0.5 to 17 mol%. Stress stepping tests and constant strain rate tests were used to examine the effect of Mg content on the dislocation creep of calcite. The grain growth rate under static conditions was decreased with Mg content from 7 to 17 mol%, indicating perhaps that grain boundary mobility is suppressed by the solute drag effect. In the diffusion creep at stresses below 40 Mpa, the strength of calcite decreases with increasing Mg content owing to the difference in grain size at 800° C and 300 MPa confining pressure. The contribution of dislocation creep increases with increasing stress, and the transition between diffusion and dislocation creep occurs at higher stresses for the samples with higher magnesium content and smaller grain size. The creep data were fit assuming a composite flow law consisting of a linear combination of diffusion and dislocation creep and a single-valued grain size. The best agreement was obtained by using a dislocation creep law with exponential dependence of strain rate on stress (e.g. Peierls law). More evidence from microstructure is needed to identify the dominant deformation mechanism conclusively. Most of the samples were compressed up to strains of 0.25; small recrystallized grains are formed resulting in a bimodal grain size distribution in some of the deformed samples. Preliminary data shows that the recrystallized grain sizes are smaller for Mg-calcite compared with that of pure calcite. This study will help to understand the effect of impurities on grain-growth kinetics and strain weakening in localized shear zones.

Xu, L.

2004-12-01

378

Growth of Gray Tin Crystals  

Microsoft Academic Search

A method is proposed for growing large a-Sn single crystals without their disintegration as a result of the ß ? a phase transition. a-Sn crystals are prepared by freezing the closed system water–Sn–seed in such a way that the pressure exerted by the expanding ice minimizes the amount of a-Sn nuclei and reduces the growth rate.

A. D. Styrkas

2003-01-01

379

Seed Treatment. Bulletin 760.  

ERIC Educational Resources Information Center

|This manual gives a definition of seed treatment, the types of seeds normally treated, diseases and insects commonly associated with seeds, fungicides and insecticides used, types of equipment used for seed treatment, and information on labeling and coloring of treated seed, pesticide carriers, binders, stickers, and safety precautions. (BB)|

Lowery, Harvey C.

380

Variable export fluxes and efficiencies for calcite, opal, and organic carbon in the Atlantic Ocean: A ballast effect in action?  

Microsoft Academic Search

Latitudinal variability in export fluxes and efficiencies (ThE) of calcite, opal, and particulate organic carbon (POC) were examined during a basin-scale Atlantic Ocean cruise. A clear relationship between integrated euphotic zone POC and calcite export combined with similarities in average ThE for calcite (0.26), opal (0.31), and POC (0.29) implies a potential association between biomineral and POC export. However, such

Sandy J. Thomalla; Alex J. Poulton; Richard Sanders; Robert Turnewitsch; Patrick M. Holligan; Mike I. Lucas

2008-01-01

381

Monitoring bacterially induced calcite precipitation in porous media using magnetic resonance imaging and flow measurements  

NASA Astrophysics Data System (ADS)

A range of nuclear magnetic resonance (NMR) techniques are employed to provide novel, non-invasive measurements of both the structure and transport properties of porous media following a biologically mediated calcite precipitation reaction. Both a model glass bead pack and a sandstone rock core were considered. Structure was probed using magnetic resonance imaging (MRI) via a combination of quantitative one-dimensional profiles and three-dimensional images, applied before and after the formation of calcite in order to characterise the spatial distribution of the precipitate. It was shown through modification and variations of the calcite precipitation treatment that differences in the calcite fill would occur but all methods were successful in partially blocking the different porous media. Precipitation was seen to occur predominantly at the inlet of the bead pack, whereas precipitation occurred almost uniformly along the sandstone core. Transport properties are quantified using pulse field gradient (PFG) NMR measurements which provide probability distributions of molecular displacement over a set observation time (propagators), supplementing conventional permeability measurements. Propagators quantify the local effect of calcite formation on system hydrodynamics and the extent of stagnant region formation. Collectively, the combination of NMR measurements utilised here provides a toolkit for determining the efficacy of a biological–precipitation reaction for partially blocking porous materials.

Sham, E.; Mantle, M. D.; Mitchell, J.; Tobler, D. J.; Phoenix, V. R.; Johns, M. L.

2013-09-01

382

Effects of phosphate ions on intergranular pressure solution in calcite: An experimental study  

NASA Astrophysics Data System (ADS)

Intergranular pressure solution (IPS) is a coupled chemical-mechanical process of widespread importance that occurs during diagenesis and low-temperature deformation of sedimentary rocks. Laboratory experiments on IPS in halite, quartz, and calcite have largely concentrated on the mechanical aspects of the process. In this study, we report the effects of pore fluid chemistry, specifically varying phosphate ion concentration, on the mechanical compaction by IPS of fine-grained calcite powders at room temperature and 1 to 4 MPa applied effective stress. Phosphate was investigated because of its importance as a biogenic constituent of sea and pore waters. Increasing the pore fluid phosphate concentration from 0 to 10 -3 mol/L systematically reduced compaction strain rates by up to two orders of magnitude. The sensitivity of the compaction strain rate to phosphate concentration was the same as the sensitivity of calcite precipitation rates to the addition of phosphate ions reported in the literature, suggesting that the rate of IPS in phosphate-bearing samples was controlled by calcite precipitation on pore walls. The results imply that IPS and associated porosity/permeability reduction rates in calcite sediments may be strongly reduced when pore fluids are enriched in phosphates, for example, through high biologic productivity or a seawater origin. Future modeling of IPS-related processes in carbonates must therefore take into account the effects of pore fluid chemistry, specifically the inhibition of interfacial reactions.

Zhang, Xiangmin; Spiers, Christopher J.

2005-12-01

383

Pseudospherulitic fibrous calcite from the Quaternary shallow lacustrine carbonates of the Farafra Oasis, Western Desert, Egypt: A primary precipitate with possible bacterial influence  

NASA Astrophysics Data System (ADS)

Pseudospherulitic fibrous calcite (PFC) has been found as a major constituent (85-90%) within thin massive limestone beds of the Quaternary mudflat-shallow lacustrine facies association (1.5-2 m thick) that forms part of combined facies associations of the Quaternary clastic-carbonate unit (25-30 m thick) at Bir-Karawein area in the Farafra Oasis, Western Desert, Egypt. The thin massive limestone beds (2-5 cm thick) are devoid of pedogenic features and marine fossils. They form a rhythmic cyclic succession with thin massive mudrocks (5-10 cm thick). The mudflat-shallow lacustrine facies association herein occurs within a depositional sequence of distal alluvial-floodplain (6-12 m thick) and palustrine (1.5-4.5 m thick) facies associations.The PFC is a composed of loosely packed rounded to sub-rounded single low-Mg-calcite crystals (150-250 ?m-sized) with intracrystalline fibrous microfabric marked by fibers (150-250 ?m long and 10-20 ?m wide) radiating from the center of the individual crystals and displaying irregular internal growth with lobate pattern. The PFC crystals show non-planar to highly irregular intercrystalline boundaries. Under SEM, the individual crystal fibers group of PFC form ellipsoid to sub-globular bodies. Each PFC crystal exhibits successive zones of thick non-luminescence and thin brightly orange to dull luminescence. The matrix (10-15%) between the PFC crystals is mainly a honeycomb-like smectite.The PFC is postulated to be a primary precipitate. This concept is reached because the PFC: (i) does not display the criteria of typical Microcodium structures, root-calcification, speleothem structures, calcite spherulites of laminar calcretes, and calcitization of precursor dolomite or aragonite, (ii) possesses homogenous compositional and textural characteristics, and (iii) occurs within limestone beds that lie in between impermeable massive mudrock beds that dampen diagenesis. A role for possible bacterial contribution in crystallization of the PFC is assumed in the view of its internal microfabric characteristics (pseudospherulitic and lobate internal growth patterns), and morphological features (ellipsoidal to subglobular appearance), in addition to its slightly negative values of ?13C (-0.51‰ to -2.19‰). The low concentration of Na (0.11-0.20%), Sr (70-110 ppm) and Mn (0.04-0.31%), in addition to the negative values of ?18O (-4.65‰ to -5.96‰) in the PFC reflect its deposition from oxygenated freshwater. In addition, the absence of covariance between ?13C and ?18O values (r = -0.202) of the PFC indicates precipitation in a hydrologically-open, short-lived lake setting. In summary, the PFC is of low-Mg type and formed in a hydrologically-open, short-lived, freshwater lake as a primary precipitate with possible bacterial contribution.

Wanas, H. A.

2012-04-01

384

Seed Magazine  

NSDL National Science Digital Library

Seed is an online (and print) magazine dedicated to increasing the scientific awareness and knowledge of the general public. The site is updated six days a week and features news articles, original-content articles on a variety of science topics, columns, and reviews. "Cribsheets," a downloadable feature, provide brief tutorials on scientific issues and innovations that may occur in everyday conversation. The magazine also features a collection of multimedia materials, such as podcasts, slide shows, and video content. The web site's main page features articles from the latest issue; back issues may be browsed by category, author, or date. Information about subscribing to the print version is also provided.

385

Blind Seed Disease.  

National Technical Information Service (NTIS)

In blind seed disease, infertilized or developing seed of susceptible grasses are colonized by the fungus Gloeotinia temulenta. Infection results in loss of seed germination. About 56 species of grasses are colonized by the fungus Gloeotinia temulenta. In...

S. C. Alderman

2001-01-01

386

Solution Top-Seeding: Growth of Germanium Dioxide Polymorphs.  

National Technical Information Service (NTIS)

Single crystals of both tetragonal (rutile) germanium dioxide and hexagonal (quartz) germanium dioxide have been grown by top-seeding of appropriate molten salt solutions. Tetragonal germanium dioxide is grown from a sodium oxide solution (92.3 molar per ...

J. W. Goodrum

1972-01-01

387

Seeded directional growth of superconducting bulk Y123  

NASA Astrophysics Data System (ADS)

We report a fabrication process of textured YBCO grown by seeded directional solidification using a MgO crystal as a seed, which alignes YBCO grains along c-axis with a texturing degree reduction as the distance from the seed increases. Measurements of magnetic hysteresis cycles were performed by means of vibrating sample magnetometry and the critical current density was estimated using Bean's critical state model.

Boffa, M.; Boutet, M.; di Trolio, A.; Pace, S.; Sparvieri, N.; Testa, A. M.; Vecchione, A.

1997-08-01

388

Structure of supersaturated solution and crystal nucleation induced by diffusion  

NASA Astrophysics Data System (ADS)

The effect of a seed crystal on nucleation of L-alanine from a quiescent supersaturated solution was investigated. When a seed crystal was not used, nucleation did not occur at least for 5h. When a seed crystal was introduced into the supersaturated solution with careful attention to avoid convection of the solution, fine crystals appeared at the place far from the seed crystal. At that time, there was no convection at the place that fine crystals appeared. Namely, there was no possibility that those fine crystals came from the surface of seed crystal. We supposed that nucleation was induced by directional diffusion of solute molecules caused by growth of the seed crystal. In order to prove this hypothesis, we designed an experiment using an apparatus composed of two compartments divided by a dialysis membrane that L-alanine molecules could freely permeate. Two supersaturated solutions having a supersaturation ratio of 1.2 and a smaller ratio were placed in the two compartments in the absence of seed crystals. This apparatus allowed the directional diffusion of solute molecules between two solutions. Nucleation occurred within 30min. The frequency of nucleation among 7-times repeated experiments was in proportion to the difference of supersaturation ratio between the two solutions. This result poses a new mechanism of the secondary nucleation that the directional diffusion caused by growth of existing crystals induces nucleation.

Ooshima, Hiroshi; Igarashi, Koichi; Iwasa, Hideo; Yamamoto, Ren

2013-06-01

389

Structure of the (1014) Surfaces of Calcite, Dolomite, and Magnesite under Wet and Dry Conditions  

SciTech Connect

Atomistic computer simulation methods have been employed to model the structure of the (10{bar 1}4) surfaces of calcite, dolomite and magnesite. The authors calculations show that under vacuum conditions, calcite undergoes the greatest degree of surface relaxation with rotation and distortion of the carbonate group accompanied by movement of the calcium ion. The magnesite surface is the least distorted of the three carbonates, with dolomite being intermediate to the two end members. When water molecules are placed on the surface to produce complete monolayer coverage, the calcite surface is stabilized and the amount of relaxation is substantially reduced. In contrast, the dolomite and magnesite surfaces are destabilized by hydration as indicated by a significant increase in the surface energies relative to the dry surface.

WRIGHT,KATE; CYGAN,RANDALL T.; SLATER,BEN

2000-06-12

390

Bean Seed Imbibition.  

ERIC Educational Resources Information Center

|Enables students to examine the time course for seed imbibition and the pressure generated by imbibing seeds. Provides background information, detailed procedures, and ideas for further investigation. (DDR)|

Hershey, David R.

1998-01-01