Replacement Phenomena in Tantalum Minerals from Rare-Metal Pegmatites in South Africa and Namibia

Joy R. Baldwin
Department of Geology, University of St Andrews, St Andrews, Fife, KY16 9ST, Scotland

Abstract: Manganotantalite replacement by (1) microlite and (2) ferrotantalite, and changes in composition of uranoan microlite from rare-metal pegmatites in South Africa and Namibia have been investigated with the electron microprobe. A uranmicrolite from Karibib, Namibia contained 14.35% UO2, 1.03% PbO, 56.12% Ta2O5, 13.18% Nb2O5, 0.58% Fe2O3, 6.87% CaO, 0.54% SrO, 0.59% MnO, 0.86% Na2O and 0.47% F. Analyses along traverses across a 1.3 mm uranoan microlite, Tantalite Valley, Namibia, revealed two essentially distinct compositions: a more hydrated rim area of 200 µm radius containing 7% higher Ta2O5, 10% lower CaO and 1.3% lower F than a main central area of slightly variable composition. Back-scattered electron images reveal zoning and distinctive subspheroidal structures. New data and structural features are given for radioactive uranoan microlite from Namaqualand, South Africa. These crystals contain remnants of a bismuth phase and are in various stages of replacement. In the microlites replacing manganotantalite, the microlite reflects the composition of the replaced mineral. At Rubicon Mine, Karibib, a narrow marginal zone of mangantantalite is replaced by ferrotantalite along cleavages; a zone of intermediate composition is apparent. Detailed traverses have been completed across all of these crystals.

Keywords: microlite • manganotantalite • ferrotantalite • South Africa • Namibia • replacement

Mineralogical Magazine; December 1989 v. 53; no. 373; p. 571-581; DOI: 10.1180/minmag.1989.053.373.08
© 1989, The Mineralogical Society
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