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Nb-rich brookite from Gross Brukkaros, Namibia: substitution mechanisms and Fe2+/Fe3+ ratios

Published online by Cambridge University Press:  05 July 2018

M. Werner  and
N. J. Cook
M. Werner*
Affiliation:
Institut für Geologie der Universität Würzburg, D-97070 Würzburg, Germany
N. J. Cook
Affiliation:
Geological Survey of Norway, N-7491 Trondheim, Norway
*
*E-mail: mario.werner@mail.uni-wuerzburg.de
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Abstract

Idioblastic, mm-scale brookite is reported from SiO2-Ba-Fe-rich metasomatic rocks from the carbonatitic Gross Brukkaros Volcanic Field, Southern Namibia. Electron probe microanalysis shows significant concentrations of Nb (12–17 wt.% Nb2O5), coupled with Fe, substituting for Ti in the structure. Calculation of the analytical data indicates evolving Fe2+/(Fe2++Fe3+) ratios during crystallization and two distinct substitution mechanisms in operation: 2Ti4+ ⇋ Nb5+ + Fe3+ and 3Ti4+ ⇋ 2Nb5+ + Fe2+. The compositional patterns observed are explained in terms of initial growth in an environment which evolved from oxidizing to reducing. The outermost 10 µm rims of the crystals are characterized by oxidizing conditions, resulting from a subsequent metasomatic overprint.

Keywords

brookiteniobiumsubstitution mechanismscarbonatiteGross BrukkarosNamibia
Type
Research Article
Information
Mineralogical Magazine , Volume 65 , Issue 3 , June 2001 , pp. 437 - 440
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2001

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