Mineralogical and chemical characteristics of complexly-zoned columbite-tantalite from the rare metal pegmatites of southern Karnataka

Detailed mineralogical and chemical studies by EPMA on the Nb-Ta minerals f rom the rare metal pegmatites of southern Karnataka have revealed wide comp ositional variations, and the presence of some new minerals like tapiolite and microlite, hitherto unreported from these pegmatites. Back scattered el ectron (BSE) images have indicated complex zoning patterns such as oscillat ory, patchy and/or their combinations, associated with replacement of colum bite-tantalite by microlite, manifested by the variations of Nb2O5, Ta2O5, FeO and MnO. Such features, although known from a few rare metal pegmatites of the world, are being recorded here for the first time in India. Oscillatory zones in columbite-tantalites are manifested as alternating dar k and light grey coloured bands, of which darker bands are Nb-rich (30-300 mum thick) and lighter bands are Ta-rich (125-450 mum thick). The wider zon es may, in turn, consist of a group of very fine sub-zones (1-50 mum) of sl ightly varying composition: Patchy zoned crystals exhibit corroded remnants of early formed columbite-tantalite (with 39.73-44.02% Ta2O5), surrounded by later formed zones which are enriched in Ta2O5 (up to 50.99 %) in columb ite-tantalite leading to the formation of microlite containing up to 73.4% Ta2O5. The zoning of the columbite-tantalites has been attributed to periodic chan ges in the composition of the major components such as Nb, Ta, Fe and Mn in the pegmatite fluid system, apparently influenced by the late stage fracti onation of volatiles. This ultimately generated fluids rich in Ta and Na, r esulting in resorption and replacement with patchy zoning of early formed, zoned columbite-tantalites.