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Composition variation of illite-vermiculitesmectite mixed-layer minerals in a bentonite bed from Charente (France)

Published online by Cambridge University Press:  09 July 2018

A. Meunier ,
B. Lanson  and
B. Velde
A. Meunier*
Affiliation:
HYDRASAUMR 6532, CNRS-University of Poitiers, 40 av. Recteur Pineau, 86022 Poitiers Cedex
B. Lanson
Affiliation:
LGIT–Maison des GéoSciences, CNRS - University Joseph Fourier, PO Box 53, 38041 Grenoble Cedex 9
B. Velde
Affiliation:
Laboratoire de Géologie, E.N.S., 24 rue Lhomond, 75231 Paris Cedex 05, France
*
*E-mail: alain.meunier@hydrasa.univ-poitiers.fr
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Abstract

Mineralogical and chemical variations were studied in the upper half of a 1 m thick discontinuous bentonite bed interlaminated in the Lower Cenomanian sedimentary formations of the northern Aquitaine basin (France). X-ray diffraction patterns obtained from the <2 mm fraction in the Ca and K-saturated states were decomposed and compared to those calculated from decomposition parameters. They revealed the presence of two highly expandable illite-expandable (I-Exp) mixedlayer minerals (MLMs). The relative proportions of the two MLMs evolve steadily with depth leading to the decrease of the cation exchange capacity and of the (Na + Ca) content towards the centre of the bentonite bed. However, the system is essentially isochemical and Mg, Al, Si, K and Fe are roughly constant in the bulk samples. It is thought that the mineralogical zonation results from the initial stages of the smectite formation in an ash layer.

In the Ca-saturated state, the expandable component of these MLMs was for the most part homogeneous with the presence of 2 sheets of ethylene glycol molecules in the interlayer. However, the heterogeneous hydration behaviour of these expandable layers was enhanced by the K-saturation test. From this test, the presence of three layer types with contrasting layer charge was evidenced from their contrasting swelling abilities. The C12-alkylammonium saturation test applied to samples in which the octahedral charge had previously been neutralized (Hofmann-Klemen treatment) showed that the tetrahedral charge is located on specific layers. These layers are responsible for the heterogeneous hydration behaviour. Low-charge smectite layers are mostly octahedrally substituted, whereas for intermediate- and high-charge layers this montmorillonitic charge is complemented by additional tetrahedral substitutions (0.30 and 0.35–0.40 charge per O10(OH)2, respectively).

Keywords

bentoniteCharenteFrancedecompositionhigh-charge smectitehydration propertiesmixedlayer mineralsXRD
Type
Research Article
Information
Clay Minerals , Volume 39 , Issue 3 , September 2004 , pp. 317 - 332
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2004

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