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Mineral chemistry and paragenesis of astrophyllite from Egypt

Published online by Cambridge University Press:  05 July 2018

Abdel-Fattah M. Abdel-Rahman
Abdel-Fattah M. Abdel-Rahman*
Affiliation:
Department of Geological Sciences, McGill University, 3450 University Street, Montreal, Quebec, Canada, H3A 2A7
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Abstract

Astrophyllite is a rock-forming mineral (3.4 vol.%) in the metasomatized wallrocks within 2 to 3 km of the contact with the Mount Gharib A-type complex in northeastern Egypt. In the peralkaline A-type granites of the main complex, astrophyllite occurs in accessory amounts (0.1 to 0.5 vol.%), mostly in association with arfvedsonite. Eighteen chemical analyses of astrophyllite from both rock units have been used to discuss mineral chemistry, compositional variations and isomorphism.

Substitutions in the astrophyllite unit cell [(Na,K)3Fe7Ti2Si8(O,OH)31] involve F, Ca, Mn, Zn, Mg, Nb, Zr, Sn and A1. The contents of the unit cell depart from ideal stiochiometry. Unusually high Nb contents (up to 5 wt.% Nb2O5) characterize the Egyptian astrophyllitcs. The replacement of Ti by Nb is balanced by cation vacancies (Nb5+ → Ti4+ + □). The niobophyllite-astrophyllite relationship and the structure of astrophyllite in relation to mica and biopyriboles are discussed.

Astrophyllite may have been formed at the expense of alkali amphiboles by metasomatic reactions involving Ti-Nb-rich alkaline fluids and arfvedsonite;

Most astrophyllite reported in the literature occurs in association with alkali amphiboles, particularly arfvedsonite, thus suggesting that similar metasomatic reactions were responsible for the formation of astrophyllite in many other localities in the world.

Keywords

astrophylliteA-type granitearfvedsonitemetasomatic reactionsEgypt
Type
Research Article
Information
Mineralogical Magazine , Volume 56 , Issue 382 , March 1992 , pp. 17 - 26
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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Footnotes

*

Present address: Department of Geology, Concordia University, Loyola Campus, 7141 Sherbrooke Street West, Montreal, Quebec, Canada, H4B 1R6.

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