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Foordite-thoreaulite, Sn²⁺Nb₂O₆–Sn²⁺Ta₂O₆: compositional variations and alteration products

Pavel Uher^1,*, Petr ern² and Ron Chapman²

¹ Department of Mineral Deposits, Faculty of Natural Sciences, Comenius University, Mlynská dolina G, 842 15 Bratislava, Slovakia
² Department of Geological Sciences, University of Manitoba, Winnipeg, MB, Canada R3T 2N2

^* Corresponding author, e-mail: puher{at}fns.uniba.sk

The foordite-thoreaulite series of Sn²⁺-bearing Nb, Ta-oxide phases is a rare constituent of complex, rare-element, Li-Cs-Ta-rich (LCT-family) granitic pegmatites with local low fO₂ environment. In this study, detailed electron-microprobe analyses (EMPA) reveal a broad range of nearly continuous foordite-thoreaulite solid solution: at. Ta/(Ta+Nb) =0.23–0.92. Valence equilibration of the formulae suggests up to 6 at.% of total Sn in Sn⁴⁺ state occupying the octahedral Nb, Ta-populated B-site. Three substitution mechanisms dominate the foordite-thoreaulite chemistry: NbTa_–1, Pb²⁺Sn_–1²⁺ and Sb³⁺Sn⁴⁺Sn_–1²⁺(Nb, Ta) _–1⁵⁺; Pb²⁺ and Sb³⁺ occupy 21.5 at. % and 7.6 at. % of the A-site position, respectively. Sb shows positive correlation with Ta/(Ta+Nb). Primary large foordite-thoreaulite crystals are compositionaly homogeneous, only areas of secondary alteration show small zones of recrystallized foordite-thoreaulite and grains with diffuse or patchy zoning, variable Nb/Ta ratio and locally increased Pb content. Influx of late-magmatic to hydrothermal fluids and/or alkali elements under higher fO₂ causes breakdown of foordite-thoreaulite and production of cassiterite and numerous Nb, Ta-oxide minerals. At Lutsiro pegmatite, foordite is replaced by mosaic fine-grained aggregate of secondary foordite-thoreaulite + columbite-tantalite + Ta-rich cassiterite. Simpsonite is present in Manono and Maniema pegmatites. Local replacement of foordite-thoreaulite by alkali-bearing Nb-Ta phases is widespread; irregular veinlets and zones of lithiotantite, calciotantite, irtyshite/natrotantite, cesplumtantite, a mineral with composition (Na, Cs)₂(Pb, Sb³⁺)₃Ta₈O₂₄, rankamaite, fersmite, and pyrochlore-group minerals occur.

Key-words: foordite, thoreaulite, cassiterite, columbite, tantalite, simpsonite, alkali-bearing Nb-Ta minerals, granitic pegmatites.