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Tschermak substitution as an indicator of palaeotemperature in Silurian K-bentonites from the Southern Uplands of Scotland and Northern Ireland

Published online by Cambridge University Press:  09 July 2018

R. B. Pearce  and
T. Clayton
R. B. Pearce
Affiliation:
Department of Geology, University of Southampton, Highfield, Southampton SO 171BJ, UK
T. Clayton
Affiliation:
Department of Geology, University of Southampton, Highfield, Southampton SO 171BJ, UK
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Abstract

Chemical analyses of the <0.2 μm illite-smectite (I-S) fraction from K-bentonites in Silurian age sediments sampled from three different localities in the Southern Uplands of Scotland and Northern Ireland indicate that the I-S from the three localities shows differences in both K+ content and the degree of Tschermak substitution of octahedral Al3+ for Mg2+. The K+ content correlates with percentage expandability (%E) of I-S as determined by X-ray diffraction (XRD), but both these parameters show poor correlation with palaeotemperature as estimated by organic reflectance methods. Octahedral Mg2+ content shows an inverse correlation with Al3+, and both these parameters show a good correlation with estimated palaeotemperature. Although K-substitution has been retarded at Dob's Linn, the region subjected to the highest temperature, Tschermak substitution of Al3+ for Mg2+ has apparently continued. In these K-bentonites, Tschermak substitution appears to be a better indicator of palaeotemperature than K-substitution.

Type
Research Article
Information
Clay Minerals , Volume 30 , Issue 1 , March 1995 , pp. 15 - 25
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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