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Garnet-quartz intergrowths in graphitic pelites: the role of the fluid phase

Published online by Cambridge University Press:  05 July 2018

Kevin W. Burton
Kevin W. Burton*
Affiliation:
Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
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Abstract

Garnets with an unusual inclusion pattern of cylindrical quartz intergrowths have been found to develop exclusively in the presence of graphite. The intergrowths consist of quartz rods, 1–5 µm in diameter, originating at the sector-zone interfaces in the garnet with the long axes normal to the crystal faces. The lattice orientation and continuity of the quartz suggests that the interphase boundaries between the quartz and garnet are epitaxially related and that new material was added to the tube as the crystal face of the garnet grew. In the presence of a C-O-H fluid, at the temperatures and pressures recorded, (P = 6.5 kbar, T = 500°C), the amount of CO2 present restricts the solubility of SiO2 in the intergranular fluid phase, where the oxygen fugacity (fo2) is below the Quartz-fayalite-magnetite (QFM) buffer, and within the stability field of graphite. The reduced solubility will lower the concentration of SiO2 in solution, and hence restrict its ease of transport via the fluid, resulting in an excess of SiO2at the site of garnet growth. Under such conditions the SiO2 is incorporated in the growing garnet in the form of the cylindrical quartz intergrowths.

Keywords

garnetquartzintergrowthsgraphitic pelites
Type
Silicate mineralogy
Information
Mineralogical Magazine , Volume 50 , Issue 358 , December 1986 , pp. 611 - 620
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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Footnotes

*

Present address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ.

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