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A natural example of the disequilibrium breakdown of biotite at high temperature: TEM observations and comparison with experimental kinetic data

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 has been used to investigate the mechanism of natural biotite breakdown under pyrometamorphic disequilibrium conditions. Biotite in a xenolith of pelitic gneiss collected from a Tertiary dolerite sill, Isle of Mull, Scotland, shows evidence of an incipient reaction, characterised by a darkening in colour and the appearance of areas of fine-grained reaction products. TEM and analytical electron microscope data show that the reaction can be described as:

The orientations of the product phase are controlled by the crystallography of the reacting biotite, demonstrating that the transformation proceeds by a topotactic mechanism. An empirical method, based on the Mg/(Fe2+ + Fe3+) ratios of coexisting spinel and biotite from experimental data, is used to deduce that the reaction occurred above ∼ 770 °C. A comparison of the natural reaction microstructures with those produced experimentally suggest that the xenolith was probably above 800 °C for less than 48 hours and cooled to temperatures of 770 °C after ∼ 150–200 hours.

Keywords

transmission electron microscopybiotitepyrometamorphism
Type
Electron Microscopy in Mineralogy and Petrology
Information
Mineralogical Magazine , Volume 51 , Issue 359 , March 1987 , pp. 93 - 106
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1987

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Footnotes

*

Present address: Institute of Meteoritics, Dept. of Geology, University of New Mexico, Albuquerque, New Mexico 87131, USA.

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