Bordet, P.; McHale, A.; Santoro, A.; Roth, R.S.
The zirconium titanates ZrTiO/sub 4/ and Zr/sub 5/Ti/sub 7/O/sub 24/ and the iron niobate FeNb/sub 2/O/sub 6/ have been investigated with the neutron diffraction powder technique and the Rietveld method. All three compounds crystallize with the symmetry of space group Pbcn and have lattice parameters a = 4.8042(2),b = 5.4825(3), c = 5.0313(2) A for ZrTiO/sub 4/; a = 14.3574(6), b = 5.3247(3), c = 5.0200(2) A for Zr/sub 5/Ti/sub 7/O/sub 24/; and a = 14.2661(2), b = 5.7334(1), c = 5.0495(1) A for FeNb/sub 2/O/sub 6/. Pure zirconium titanate, ZrTiO/sub 4/, has the ..cap alpha..- PbO/sub 2/-type structure with a random distribution of the two cations. The compounds Zr/sub 5/Ti/sub 7/O/sub 24/ and FeNb/sub 2/O/sub 6/ are ordered superstructures of ..cap alpha..-PbO/sub 2/ and in these two cases the observed distortions depend on the differences between ionic radii, leading to a fersmite-type structure in the case of Zr/sub 5/Ti/sub 7/O/sub 24/ and to a columbite-type structure in the case of FeNb/sub 2/O/sub 6/.
Mackay, Duncan A. R.; Simandl, George J.
Tantalum (Ta) and niobium (Nb) are essential metals in modern society. Their use in corrosion prevention, micro-electronics, specialty alloys and high-strength low-alloy (HSLA) steel earns them a strategic designation in most industrialised countries. The Ta market is unstable due in part to historic influx of `conflict' columbite-tantalite concentrate, or "Coltan," that caused Ta mines in Australia and Canada to be placed on care and maintenance. More recently, the growing appetite of modern society for consumer goods made of `conflict-free' minerals or metals has put pressure on suppliers. Pegmatites, rare-element-enriched granites, related placer deposits and weathered crusts overlying carbonatite and peralkaline complexes account for the majority of Ta production. Several carbonatite-related deposits (e.g. Upper Fir and Crevier, Canada) are being considered for potential co-production of Ta and Nb. Pyrochlore (Nb-Ta), columbite-tantalite (Nb-Ta), wodginite (Ta, Nb and Sn) and microlite (Ta and Nb) are the main ore minerals. Approximately 40 % of Ta used in 2012 came from Ta mines, 30 % from recycling, 20 % from tin slag refining and 10 % from secondary mine concentrates. Due to rapid industrialisation and increased use of Nb in steel making in countries such as China and India, demand for Nb is rising. Weathered crusts overlying carbonatite complexes in Brazil and one hard rock carbonatite deposit in Canada account for about 92 and 7 % of Nb world mine production, respectively. Since the bulk of the production is geographically and politically restricted to a single country, security of supply is considered at risk. Other prospective resources of Nb, beside carbonatites and associated weathered crusts, are peralkaline complexes (e.g. Nechalacho; where Nb is considered as a potential co-product of REE and zirconium). Economically, significant deposits of Ta and Nb contain pyrochlore, columbite-tantalite, fersmite, loparite and strüverite. Assuming continued