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Chemical variation in micas from the Cairngorm pluton, Scotland

Published online by Cambridge University Press:  05 July 2018

T. N. Harrison
T. N. Harrison*
Affiliation:
Department of Geology and Mineralogy, Marischal College, University of Aberdeen, Aberdeen AB9 1AS, Scotland
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Abstract

Electron microprobe analyses of micas from the Cairngorm pluton in the Eastern Grampian Highlands of Scotland show extensive compositional variation in biotite, despite a lack of chemical variation in the host granite. Biotite has high Fe/(Fe + Mg) (0.6–0.85) and Alvi (0.6–2.1 a.f.u.), and enrichment trends in these two parameters are attributable to the Al-Tschermak and dioctahedral-trioctahedral substitutions, the latter becoming dominant with increasing Alvi content. Ti content is low (0.2–0.4 a.f.u.), and is largely controlled by a Tschermak-type substitution. Biotite is also unusually rich in Mn (up to 2.57 wt. % MnO), which increases with both Alvi and Fe/(Fe+Mg). F contents generally range between 0.55 and 2.05 wt.% All compositional variation in biotite can be attributed to the extensive development of a fluid phase during the late-magmatic and subsolidus evolution of the pluton. The presence of an abundant fluid phase has resulted in the alteration of biotite to muscovite, which has occurred in response to de-stabilization of the biotite as octahedral R2+ cations are lost in favour of Al. Extreme build-up of this fluid phase has resulted in the crystallization of muscovite as a late, interstitial primary phase. Both primary and replacive muscovite have Fe/(Fe+Mg) > 0.50, 15–36 mol. % celadonite and <1 mol. %paragonite.

Keywords

biotitemuscovitechemical variationCairngorm plutonScotland
Type
Silicate Mineralogy
Information
Mineralogical Magazine , Volume 54 , Issue 376 , September 1990 , pp. 355 - 366
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1990

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Footnotes

*

Present address: 53½ Powis Place, Aberdeen AB2 3TT.

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