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The conversion of smectite to illite during diagenesis: evidence from some illitic clays from bentonites and sandstones

Published online by Cambridge University Press:  05 July 2018

P. H. Nadeau ,
M. J. Wilson ,
W. J. McHardy  and
J. M. Tait
P. H. Nadeau
Affiliation:
The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, Scotland
M. J. Wilson
Affiliation:
The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, Scotland
W. J. McHardy
Affiliation:
The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, Scotland
J. M. Tait
Affiliation:
The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, Scotland
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Abstract

Diagenetic illitic clays from seven North American bentonites of Ordovician, Devonian, and Cretaceous ages and from three subsurface North Sea sandstones of Permian and Jurassic ages have been examined by X-ray diffraction (XRD) and transmission and scanning electron microscopy (TEM and SEM). XRD indicates that the clays from the bentonites are randomly and regularly interstratified illite/smectites (I/S) with 30–90% illite layers, whereas the clays from the Jurassic and Permian sandstones are regularly interstratified I/S, with 80–90% illite layers, and illite respectively. TEM of shadowed materials shows that randomly interstratified I/S consists primarily of mixtures of elementary smectite and ‘illite’ particles (10 and 20Å thick respectively) and that regularly interstratified I/S and illite consist mainly of ‘illite’ particles 20–50 Å thick and > 50 Å thick respectively. Regularly interstratified I/S from bentonites and sandstones are similar with regard to XRD character and particle thickness distribution. These observations can be rationalized if the interstratified XRD character arises from an interparticle diffraction effect, where the smectite interlayers perceived by XRD, result from adsorption of exchangeable cations and water or organic molecules at the interfaces of particles generally < 50Å thick. A neoformation mechanism is proposed by which smectite is converted to illite with increasing depth of burial in sedimentary rocks, based on dissolution of smectite particles and the precipitation/growth of ‘illite’ particles occurring within a population of thin phyllosilicate crystals.

Keywords

smectiteillitediagenesisclaysbentonitesandstone
Type
Research Article
Information
Mineralogical Magazine , Volume 49 , Issue 352 , June 1985 , pp. 393 - 400
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1985

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