Composition of some smectites and diagenetic illitic clays and implications for their origin

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Authors:Nadeau, P. H.; Bain, D. C.
Author Affiliations:Primary:
Macaulay Inst. Soil Res., Dep. Miner. Soils, Aberdeen, United Kingdom
Volume Title:Clays and Clay Minerals
Source:Clays and Clay Minerals, 34(4), p.455-464. Publisher: Clay Minerals Society, Clarkson, NY, United States. ISSN: 0009-8604
Publication Date:1986
Note:In English. 37 refs.John Hower Memorial Issue; illus. incl. 3 tables
Summary:Chemical analysis by X-ray fluorescence (XRF) and calculated structural formulae of clay-size fractions of smectites from Cretaceous bentonites and illitic clays from Cretaceous, Devonian, and Ordovician bentonites and Jurassic and Permian sandstones indicate the nature and extent of various types of ionic substitution. The determination of tetrahedral (Al, Si) and octahedral (Al, Mg, Fe) composition shows the variable chemistry of these materials. Structural formulae of the illitic clays show that they have tetrahedral charges between 0.4 and 0.8 per half unit cell, and can be divided into phengitic types having octahedral charges of 0.2-0.4 and muscovitic types having octahedral charges <0.2. Evaluation of the formulae in the light of X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) data shows that the occupancy of non-exchangeable interlayer sites (predominantly K) varies from 47% to 90% of that of ideal muscovite. In some minerals as much as 20% of these sites is occupied by ammonium ions (determined independently). The amount of surface silicate charge balanced by non-exchangeable cations versus that balanced by exchangeable cations has been examined in conjunction with TEM data and suggests that in most samples the charges are about equal. The octahedral composition of smectites in Cretaceous bentonites precludes their having served as transformation precursors for most of the Cretaceous illitic bentonites. The results suggest that these illitic clays originated by neoformation. [R.A.H.]
Subjects:Bentonite; Cation exchange capacity; Chemical composition; Chemical properties; Clastic rocks; Clay mineralogy; Clay minerals; Cretaceous; Devonian; Diagenesis; Effects; Experimental studies; Illite; Jurassic; Mesozoic; Paleozoic; Permian; Sedimentary rocks; Sheet silicates; Silicates; Smectite; Spectra; TEM data; X-ray data; X-ray fluorescence spectra
Abstract Numbers:87M/0162
Record ID:1987007580
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland
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