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Amphiboles from the Younger Granites of Nigeria. Part I. Chemical classification

Published online by Cambridge University Press:  14 March 2018

G. D. Borley
G. D. Borley*
Affiliation:
Department of Geology, Imperial College, London, S.W. 7
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Summary

The Younger Granites of Nigeria contain varied assemblages of iron-rich ferromagnesian minerals in which amphiboles are an important component. Eighteen of these amphiboles (separated from rocks belonging to nine complexes) have been chemically analysed and the results are presented and discussed. The amphiboles are shown to fall into two groups, a lime-alkali group (comprised of ferrohastingsite of general formula NaCa2Fe″4Fe‴Al2Si6O22(OH)2) and an alkali group (of general formula (Na, K, Ca)2.5–3.0(Fe″, Fe‴)5(Al, Si)3O22(OH)2). The alkali amphiboles are characterized by their extremely low magnesia content, by the presence of Li and F (both in significant amounts), and are of specialinterest because of the unusual amount of ZnO they contain. Optical properties of the amphiboles have been determined but it has not been found possible to use these properties to determine accurately the chemical composition of the alkali amphiboles. It is suggested that, according to current classifications of amphiboles, the analysed alkali amphiboles should be called riebeckitic arfvedsonites or arfvedsonites. The relationship between the hastingsites and the alkali amphiboles is not yet clear.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 33 , Issue 260 , March 1963 , pp. 358 - 376
Copyright
Copyright © 1963, The Mineralogical Society

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