Composition of Some Smectites and Diagenetic Illitic Clays and Implications for Their Origin

-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 (A1, Si) and octahedral (A1, 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 muqh as 20% of these sites is occupied by ammonium ions (determined independently). The amount of surface silicate charge balanced by nonexchangeable 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 ilfitic clays originated by neoformation. Key Words--Bentonite, Chemical composition, Illite, Interstratified, Layer charge, Potassium, Smectite, X-ray fluorescence. I N T R O D U C T I O N Clay mica, sericite, hydrous mica, and illite are terms that have been used in the literature to refer to claysize phyllosilicate minerals which are common in soils, sediments, and hydrothermal alteration products. The chemistry and mineralogy of these clays have been widely examined and discussed (Hower and Mowatt, 1966; Weaver and Pollard, 1973; Brown et al., 1978; Srodofl and Eberl, 1984). Although these materials have mica-like characteristics, they differ from muscovite in chemical composition (higher Si and Mg, lower A1 and K) and in geologic occurrence, which led Grim et al. (1937) to propose the name illite as " . . . a general term for clay cons t i tuen t s . . , belonging to the mica group." Many materials formerly identified as illites have since been shown to contain expandable layers by X-ray powder diffraction (XRD) and are now referred to as interstratified (or mixed-layered) clays, termed illite/ smectite (I/S), i l l i te/montmorillonite (I/M), or mica/ smectite (M/S). These clays have usually been examined by XRD as oriented, sedimented aggregates that have been solvated with ethylene glycol, thereby expanding the interlayer space of the smectite layers (Figure 1). Calculations based on one-dimensional models of the intensities of the 001 reflections have enabled the layer and type of layer sequence, whether random or regular, to be specified (Reynolds and Hower, 1970; Reynolds, 1980; Srodofi, 1980). The chemistry of illitic clays is no less complicated than their structure. Thus, ionic substitution of AP + Copyright 9 1986, The Clay Minerals Society for Si 4+ in the tetrahedral sheet and of Mg 2+ for AP + in the octahedral sheet may vary signifcantly, as may the resulting net negative layer charge. Illitic clays are usually dioctahedral and have a half-unit-cell formula