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Compositional variation within some sedimentary chlorites and some comments on their origin

Published online by Cambridge University Press:  05 July 2018

C. D. Curtis ,
C. R. Hughes ,
J. A. Whiteman  and
C. K. Whittle
C. D. Curtis
Affiliation:
Departments of Geology and Metallurgy
C. R. Hughes
Affiliation:
Departments of Geology and Metallurgy
J. A. Whiteman
Affiliation:
Departments of Geology and Metallurgy University of Sheffield, Mappin Street, Sheffield S1 3JD
C. K. Whittle
Affiliation:
Departments of Geology and Metallurgy
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Abstract

A range of authigenic sedimentary chlorites from sandstones has been studied by analytical transmission electron microscopy. Selected area (single crystal) electron diffraction patterns are of the Ib (β = 90°) polytype confirming the earlier observations of Hayes (1970).

TEM analyses show all samples to be relatively rich in both Al and Fe. In the general formula (Mg,Fe,Al)n [Si8−xAlxO20](OH)16, x varies between 1.5 and 2.6; Fe/(Fe + Mg) between 0.47 and 0.83 and n between 10.80 and 11.54. Octahedral Al is close to 3 in this formulation and Fe2+ predominates over Fe3+. Swelling chlorites have significantly different compositions which are consistent with smectite/chlorite interstratifications.

The Ib (β = 90°) polytype appears to be stable under conditions of moderate to deep burial. It replaces berthierine and swelling chlorites formed at lower temperatures. As commonly seen in grain coatings, however, it precipitates from porewater; solutes probably being contributed from several mineral decomposition reactions.

Keywords

chloriteelectron microscopysandstonesedimentary rocks
Type
Research Article
Information
Mineralogical Magazine , Volume 49 , Issue 352 , June 1985 , pp. 375 - 386
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1985

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