Compositional Variation within Some Sedimentary Chlorites and Some Comments on Their Origin

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

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: chlorite • electron microscopy • sandstone • sedimentary rocks

Mineralogical Magazine; June 1985 v. 49; no. 352; p. 375-386; DOI: 10.1180/minmag.1985.049.352.08
© 1985, The Mineralogical Society
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