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Comparison of sedimentary chlorite compositions by X-ray diffraction and analytical TEM

Published online by Cambridge University Press:  09 July 2018

C. K. Whittle
C. K. Whittle*
Affiliation:
Department of Geology, University of Sheffield, Mappin Street, Sheffield, S1 3JD
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Abstract

Authigenic chlorites occurring as pore linings in reservoir sandstones have been investigated by XRD and TEM/EDAX. Both methods identified the Ib (β = 90°) polytype. TEM/EDAX analyses suggest that the chlorites are Al-rich, Mg-chamosites with Fe/(Fe + Mg) > 0·47. Octahedral Al exceeds tetrahedral Al, resulting in octahedral totals of less than the theoretical 12·0. Compositions were estimated from XRD data using the equations after Brindley (1961) and Shirozu (1958). XRD-determined compositions are relatively enriched in tetrahedral Al and octahedral Mg, giving Fe/(Fe + Mg) > 0·4 and suggesting that the chlorites are Al-rich clinochlores. XRD estimates plot in the same compositional area as authigenic Ib (β = 90°) chlorites of Hayes (1970), but also within the compositional area of metamorphic chlorites. Caution should be exercised when estimating the composition of sedimentary chlorites by XRD as results appear to overestimate Mg content because of the excess of octahedral R3+ over tetrahedral R3+.

Type
Research Article
Information
Clay Minerals , Volume 21 , Issue 5 , December 1986 , pp. 937 - 947
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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