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Barian-titanian micas from Ilha da Trindade, South Atlantic

Published online by Cambridge University Press:  05 July 2018

J. C. Greenwood
J. C. Greenwood*
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
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Abstract

Extremely high abundances of TiO2 (up to 12.57 wt.%) and BaO (up to 7.11 wt.%) are reported from dark reddish-brown phlogopite-biotite rims around paler phlogopite cores in a lamprophyric dyke from Ilha da Trindade, South Atlantic. Both TiO2 and BaO contents increase towards the rims of the micas and in groundmass crystals. This is accompanied by corresponding increases in FeO*, MnO and Mg number; whereas SiO2, Al2O3, Cr2O3, MgO and K2O abundances all decrease. The substitution of Ba into the biotite structure can be explained by the scheme Ba + Al = K + Si. Ti-substitution schemes however are much more complex, as no general equation can account for all the Ti incorporated into the structure and therefore several schemes are proposed.

Keywords

micatitaniumbariumIlha da Trindade
Type
Research Article
Information
Mineralogical Magazine , Volume 62 , Issue 5 , October 1998 , pp. 687 - 695
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1998

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