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Replacement phenomena in tantalum minerals from rare-metal pegmatites in South Africa and Namibia

Published online by Cambridge University Press:  05 July 2018

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

microlitemanganotantaliteferrotantaliteSouth AfricaNamibiareplacement
Type
Non-silicate Mineralogy
Information
Mineralogical Magazine , Volume 53 , Issue 373 , December 1989 , pp. 571 - 581
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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