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An electron optical study of muscovite breakdown in pelitic xenoliths during pyrometamorphism

Published online by Cambridge University Press:  05 July 2018

A. J. Brearley
A. J. Brearley*
Affiliation:
Department of Geology, The University, Manchester, M13 9PL
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Abstract

Transmission electron microscopy and analytical electron microscopy have been used to study the breakdown reaction of muscovite during pyrometa-morphism. The transformation has been shown to be topotactic in nature, with the orientations of the product biotite and alkali feldspar being strongly controlled by the precursor phase. Microprobe investigations indicate that the reaction is probably isochemical with no detectable interaction with adjacent phases. Chemical analyses of the products carried out by AEM have enabled a possible balanced reaction to be calculated. It is proposed that the spatial distribution of the product minerals can be interpreted in terms of diffusion of species from one domain in the crystal to another over distances of 5 to 10 µm as a result of chemical potential gradients which develop during the reaction. The reaction probably occurred under conditions of declining temperature between 900 and 750 °C over a period of 4–5 days. This time period was insufficient to enable the reaction to reach completion as indicated by the presence of relics of muscovite coexisting with K-feldspar, corundum, biotite, hercynite, and mullite.

Keywords

muscovite breakdownelectron microscopypelitic xenolithspyrometamorphism
Type
Rates of Metamorphic Reactions
Information
Mineralogical Magazine , Volume 50 , Issue 357 , September 1986 , pp. 385 - 397
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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