Compositional Variation in Component Layers in Natural Illite/smectite

-Published chemical data for suites of mixed-layer minerals from diagenetic sedimentary rocks, hydrothermally altered tufts, and a metasomatic bentonite bed indicate that the layer charge and composition of the different components of illite/smectite (I/S) differ from one geological environment to another. It appears that the composition of the elemental smectite and illite layers in the I/S is more or less constant for samples within each geologic setting. In the examples considered, the smectite layers are predominantly montmorillonitic in character (i.e., the charge is in the octahedral site), whereas the illite layers show different types of charge sites, depending upon the suite studied. Illite layers appear to have about the same charge in all three suites studied, slightly more than 0.7 per O~o(OH)2 unit, whereas the smectite layers in the different suites range in charge from about 0.3 to 0.7 per Oao(OH)2 unit. Cationexchange capacities reflect these differences in charge, although not ideally. The differences in the composition of the component layers in each geologic suite of mixed-layer clays are probably due either to differences in the bulk chemistry of the rocks in the different suites or to differences in intensive variables, such as temperature and pressure, of the regime under which they have formed. Key Words--Bentonite, Cation-exchange capacity, Diagenesis, Hydrothermal, Illite, Interstratification, Smectite. I N T R O D U C T I O N Mixed-layer clay minerals have been reported in many low-temperature geological environments. Their distinctive X-ray powder diffraction diagrams have led to the rather widespread use of illite/smectite (I/S) to determine mineral facies at low temperatures (see Velde, 1985, for a survey of this problem). I/S, however, does not occur in sedimentary rocks of all bulk compositions; the host material must be dominated by silica and alumina. The geological environments in which the I/S is formed include burial diagenesis in sedimentary rocks, hydrothermal alteration of acidic rocks, high geothermal activity where acidic or pelitic rocks are present, and metasomatic alteration of glassy acidic materials (bentonites and metabentonites). The most striking characteristic of the I/S is the gradual change in composition brought about by a variation in the proportion of the mixed-layer components. The almost universal occurrence of this continuous variation allows I/S to be described in terms of the solid solution of two components; in fact, it is useful to treat these minerals as a solid solution series when considering their thermodynamic properties (see, e.g., Aagaard and Helgeson, 1983). If such use is to be made of the chemical properties of mixed-layer minerals, however, the component layers should have the same compositions in all occurrences; i.e., each layer of Copyright 9 1986, The Clay Minerals Society smectite should have the same composit ion as each other smectite layer in the mineral, and each smectite layer in a given mixed-layer mineral should be of much the same composit ion as the smectite layers in the I / S found in a different geologic setting. In other words, the component layers in the I/S formed during burial diagenesis should have the same compositions as those formed by hydrothermal alteration. I f the compositions of mixed-layer components are different in different environmental settings, even though the bulk composit ion of the systems are similar, the thermodynamic information cannot be used interchangeably from one series to another. I f the compositions of the mixed-layer components are the same for different series, however, intensive and extensive variables will act on the minerals in the same way, and these minerals can be used as indicators of paleoconditions. Inoue and Utada (1983) suggested that differences between the charge on the illite layers ofhydrothermal I/S and the iUite layers in the I/S in the shale suite of Hower and Mowatt (1966) could be due to differences in the geological environment. The present paper attempts to define these differences to a greater extent and to demonstrate the importance of variations in layer composit ion of the smectite and illite components of I/S.