Uncommon Nb-tantalate from the Cachoeira mine, Araçuaí pegmatite district (Minas Gerais)

Important spodumene-rich pegmatites have been mined by CBL (Companhia Brasileira de Lítio, Brazilian Lithium Co.) at the Cachoeira mine (northeastern Minas Gerais state) since the 1990's. In this deposit, black platy crystals resembling columbitetantalite are also found; they were analyzed by electronic microprobe as part of the first author's master's degree dissertation. The Cachoeira pegmatite group comprises several spodumene-rich bodies that consist of pertitic microcline, spodumene (on average 23vol%), albite, quartz and muscovite, totalizing more than 95% of the bodies' volume. Analyses on 27 samples of supposedly Nb-tantalates showed a high variation of Ta, Nb and Sn contents, which has been observed inclusive in individual crystals, causing color variation on backscattered electrons images. Surprisingly, analyses on some grains also revealed the presence of ixiolite, a mineral species that has never been described in that region; it is characterized by high SnO2 contents (up to ~16wt%). Columbite-(Fe) phase represents 42% of total Nb-tantalates analyzed samples, tantalite-( Fe) 19%, columbite-(Mn) 3% and ixiolite, 36%. Cassiterite, also observed, shows significant Ta2O5 contents, probably indicating general exsolution processes.