The formation of an interstratified structure in dioctahedral smectite was assumed to be influenced by (1) the overall layer charge density and its distribution in the structure, (2) the solvation energy of the cation, and (3) the nature of the solvation agent. By holding factors (2) and (3) constant it was possible to calculate the average local charge densities $\overline {{\rm{QA}}}$, $\overline {{\rm{QC}}} $, and $\overline {{\rm{QE}}} $ which are necessary for formation of 10-, 14-, and 16.8-Å mixed-layer phases in potassium-treated and ethylene glycol (EG) saturated smectites. The values of $\overline {{\rm{QA}}} $, $\overline {{\rm{QC}}} $, and $\overline {{\rm{QE}}} $ were 1.99, 1.2, and 0.56 esu/unit cell, respectively. Ammonium-treated smectites saturated with EG gave corresponding mean local charge densities of 2.7, 1.6, and 0.72 esu/unit cell. Calculations were made under the limiting condition QA > QC > QE > 0.

For K-smectites saturated with EG, Qtot = 1.99pA + 1.2pC + 0.56pE, where Qtot is the total charge (esu/unit cell), and pA, pC, and pE are probability coefficients for 10-, 14-, and 16.8-Å phases in the interstratified structure. The above equation calculated with the aid of least squares and without the limiting condition yields

$${{\rm{Q}}_{{\rm{tot}}}} = {\rm{2}}{\rm{.05pA + 1}}{\rm{.29pC + 0}}{\rm{.33pE}}{\rm{.}}$$

Cretaceous bentonites were collected in outcrop from the Sweetgrass Arch and the Disturbed Belt in Montana. The mixed-layer illite-smectite (I/S) components of the bentonites from the Sweetgrass Arch have from 0 to 25% illite layers and no detectable structurai Fe2+, whereas the samples from the Disturbed Belt have from about 25 to 90% illite layers, and all contain Fe2+. A positive correlation (r = 0.89) exists between the percentage of structural iron that is Fe2+ and the amount of fixed interlayer K in the I/S.

The higher percentage of illite layers in the samples from the Disturbed Belt is attributed to reactions related to elevated temperatures caused by burial beneath thrust sheets. The increase in Fe2+/Fe3+ with increasing percentages of illite layers is tentatively attributed to a redox reaction involving the oxidation of organic matter. Although there is no statistical evidence for an increase in octahedral charge with an increase in illite layers when all the samples are considered together, iron reduction may have contributed as much as 10 to 30% of the increase in total structural charge that occurred in any given sample during metamorphism. The remaining structural charge increase can be attributed to the substitution of Al3+ for Si4+ in the tetrahedral sites.

Mixed-layer illite/smectite (I/S) was formed by reacting the Chambers or Polkville montmorillonite hydrothermally at 270° and 350°C from several hours to more than 15 weeks. Reactions were conducted in closed vessels containing K or mixed K-Na, K-Ca, or K-Mg solutions of varying concentrations. The reaction rate and the rate of ordering of I/S for the reaction smectite + K+ → mixed-layer I/S + SiO2 was inhibited by the addition of Na+ Ca2+, and Mg2+; the inhibitory strength of Na+, Ca2+ and Mg2+, on an equivalent basis, increased approximately in the ratio 1:10:30. The first order reaction-rate constants for the reactions at 270° and 350°C indicate an activation energy of about 30 kcal/mole.

In the experimental system studied, the reaction smectite → mixed-layer I/S appeared to proceed by solid state transformation, as suggested by: (1) rapid dissolution of large amounts of silica, probably creating an Al-enriched residue; (2) similarity of particle size and morphology of the mixed-layer products to those of the original montmorillonite, implying no extensive dissolution of Al3+; and (3) relatively high activation energy compared to published values for silicate dissolution.

Monoionic K- and NH4-smectites saturated with ethylene glycol form mixed-layer structures which usually consist of three kinds of layers: a 10-Å, non-expanded layer; a 14-Å, partly expanded layer; and a 16.8-Å, completely expanded layer. In some samples, the 14-Å layers formed 60–70% of all layers present. When saturated with water vapor the smectites commonly consisted of three kinds of layers (10-, 12.6-, and 15.5-Å). Generally these samples contained fewer expanded layers than those saturated with ethylene glycol. This result is attributed to the smaller dipole moment of water compared with that of ethylene glycol. The greater solvation energy of NH4+ in comparison with that of K+ causes the expansion of a part of layers which did not expand in the K forms. This result indicates that there is an inhomogeneous distribution of layer charge in the smectite structure. The prevalent type of mixed layering in the studied samples is that of random distribution of layers.

The Tulameen coal field is part of an Eocene nonmarine basin which received extensive volcaniclastic sediments due to its location within an active magmatic arc. Bentonite partings in the coal originally consisted of glassy rhyolitic tephra with phenocrysts of sanidine, biotite, and quartz. During the initial alteration, which took place within the swamp or shortly after burial, glass was transformed to either smectite-cristobalite-clinoptilolite or to smectite-kaolinite. The formation of kaolinite depended on the degree of leaching of silica and alkalies in the swamp environment. Some beds are nearly 100% kaolinite and can be designated as tonsteins. The smectite shows no evidence of interlayering; the kaolinite is well ordered. During alteration, sodium, originally a component of the glass, was lost from the system.

A later thermal event, which affected only the southern part of the basin, metamorphosed the smectite to a regularly interstratified illite/smectite with 55% illite layers and rectorite-type superlattice (IS-type). The source of potassium was dissolution of sanidine. Vitrinite reflectance measurements of the coal suggest that the smectite was stable to 145–160°C, at which temperature it transformed to K-rectorite.

The absence of randomly interstratified intermediates, even in beds rich in potassium, suggests that the transformation of smectite to K-rectorite was controlled by a steep thermal gradient possibly resulting from local magmatism or circulating geothermal fluids.

Mixed-layer kaolinite/smectites containing 40–90% kaolinite layers have been synthesized from Wyoming smectite. Run times were 4 months at 156°C in solutions of A1C13∙6H2O and KCl or CaCl2. Similar KCl runs devoid of Al3+ yielded mixed-layer illite/smectites. The supply of Al3+, rather than pK or pH, seems to control the alteration of smectite toward kaolinite or illite.

Statistical analyses of chemical data from the literature of corrensite minerals suggest a large compositional variability, more evident in octahedral than in tetrahedral coordination. Mg occupies 40–80% of the octahedral sites, with Al and Fe2+ making up the remainder. Approximately 15–30% of the tetrahedral sites are filled by Al. Despite this compositional variability, distinct fields for the several types of mixed-layer trioctahedral chlorite/trioctahedral swelling layer are not apparent. Statistical analyses of the composition of corrensite compared with saponite, vermiculite, and chlorite suggest that corrensite is an intermediate between trioctahedral chlorite and trioctahedral smectite. If Fe/(Fe + Mg) > 50%, chlorite alone is favored, but with increasing Mg, chlorite appears to transform into corrensite and then, by iron oxidation, into trioctahedral smectite. Despite the chemical variability between corrensite, chlorite, and saponite, corrensite appears chemically to be a well-defined species. On the other hand, corrensite cannot be characterized chemically on the basis of its swelling component. Thus, the current definition of corrensite as a regular 1:1 interstratification of trioctahedral chlorite and either trioctahedral smectite or vermiculite is appropriate.

Mixed-layer clays composed of randomly interstratified kerolite/stevensite occur as lake and/or spring deposits of probable Pliocene and Pleistocene age in the Amargosa Desert of southern Nevada, U.S.A. The percentage of expandable layers of these clays, determined from computer-simulated X-ray diffractograms, ranges from almost 0 to about 80%. This range in expandabilities most likely results from differences in solution chemistry and/or temperature at the time of formation. An average structural formula for the purest clay (sample P-7), a clay with about 70% expandable layers, is:

$${\left[ {\left( {M{g_{2,72}}A{l_{0,07}}F{e_{0.03}}L{i_{0.09}}} \right)\left( {S{i_{3.96}}A{l_{0.04}}} \right){O_{10}}{{\left( {OH} \right)}_2}} \right]^{ - 0.21}}{\left[ {X_{0.21}^ + } \right]^{ + 0.21}}.$$

The data suggest that talc, kerolite, and stevensite form a continuous structural series based on layer charge.

Regular vertical variations in mineralogy and chemistry indicate that underclay beneath the Herrin (No. 6) coal in southwestern Illinois has undergone in situ alteration. Alteration resulted from the downward movement of hydrogen ions, as indicated by the progressive leaching of acid-sensitive minerals adjacent to the coal. Mineralogical trends observed in the underclay with increasing depth below the coal include: (1) a decrease in the expandability of mixed-layer illite/smectite (I/S); (2) an increase in the amount of ordered I/S with respect to randomly interstratified I/S; (3) an increase in the amount of discrete illite with respect to expandable clays; and (4) an increase in chlorite and calcite. Ordered I/S is the dominant mixed-layer clay where calcite is present, but randomly interstratified I/S dominates where calcite is absent. The pH of the underclay also increases with depth. These trends are consistent with an origin by acid leaching of a preexisting mineral assemblage that included illite, chlorite, and calcite. Other acid-alteration trends may be expected for different precursor minerals and for different leaching intensities and durations.

Chemical activity diagrarns, prepared to illustrate the properties expected if mixed-layer mont-morillonite/illite is regarded as a solid solution, are compared to those derived from a treatment of these materials as a mixture of two phases. If the system is a solid solution, the coexisting aqueous solution should range from higher dissolved silica contents in the presence of kaolinite and a montmorillonite end member to lower dissolved silica in the presence of kaolinite and an illitic end member. Silica concentration in the aqueous solution might vary by a factor of as much as six. If the system is two phase, the silica content of a solution in equilibrium with kaolinite and both phases would be fixed at a given T and P, as would a solution equilibrated with both phases and K-feldspar. In the absence of a third phase, silica in equilibrium with both phases should be nearly constant, but increase with increasing ratio of K+/H+ in solution. Available data on coexisting aqueous solutions apparently are more nearly consistent with two phases than with a solid solution.

High-resolution transmission electron microscopy (HRTEM) has been used to examine illite/smectite from the Mancos Shale; rectorite from Garland County, Arkansas; illite from Silver Hill, Montana; Na-smectite from Crook County, Wyoming; corrensite from Packwood, Washington; and diagenetic chlorite from the Tuscaloosa Formation. Thin specimens were prepared by ion milling, ultramicrotome sectioning, and/or grain dispersal on a holey carbon substrate. Some smectite-bearing clays were also examined after intercalation with dodecylamine hydrochloride (DH). Intercalation of smectite with DH proved to be a reliable method for HRTEM imaging of expanded smectite (d (001) = 16 Å) which could then be distinguished from unexpanded illite (d (001) = 10 Å). Lattice fringes of basal spacings of DH-intercalated rectorite and illite/smectite showed a 26-Å periodicity. These data support X-ray powder diffraction (XRD) studies which suggest that these samples are ordered, interstratified varieties of illite and smectite. The ion-thinned, unexpanded corrensite sample showed discrete crystallites containing 10-Å and 14-Å basal spacings corresponding to collapsed smectite and chlorite, respectively. Regions containing disordered layers of chlorite and smectite were also noted. Crystallites containing regular alternations of smectite and chlorite layers were not common. These HRTEM observations of corrensite did not corroborate XRD data. Particle sizes parallel to the c axis ranged widely for each sample studied, and many particles showed basal dimensions equivalent to more than five layers. For all illite, smectite, and illite/ smectite particles examined, crystallite sizes of about 20 Å in the basal dimension were not observed.

The southern San Joaquin Valley contains more than 7 km of sedimentary fill, largely Miocene and younger in age. Ancient depositional environments ranged from alluvial fans at the basin margins to turbidite fans toward the basin center. Mixed-layer illite/smectite (I/S) dominates the <2-μm fraction of Miocene shales, and kaolinite is abundant in Miocene sandstones. I/S from carbonate-cemented sandstones contains 5–20% more smectite layers than I/S from uncemented sandstones. The timing of cementation correlates with the proportion of smectite layers in the I/S, suggesting that cementation slowed the illitization process. Smectite and I/S with > 80% expandable layers occur at present burial temperatures of 120°-l 40°C in Miocene sandstones and shales. This highly expandable I/S is restricted to areas covered by thick deposits (1000-2500 m) of Pleistocene sediments. Rocks of similar age and at equivalent temperatures, but covered by <900 m of Pleistocene sediments, contain I/S having low expandabilities (<30%).

Microprobe analyses of 16 discrete smectite and smectite-rich I/S clays indicate an average montmorillonite composition of:

$$\left( {C{a_{0.2}}{K_{0.15}}N{a_{0.1}}} \right)\left( {A{l_{3.0}}F{e^{3 + }}_{0.35}M{g_{0.75}}T{i_{0.03}}} \right)\left( {S{i_{7.7}}A{l_{0.3}}} \right){O_{20}}{\left( {OH} \right)_4} \cdot n{H_2}O.$$

Residence time at different temperatures appears to be an important influence on the percentage of smectite layers in I/S from the San Joaquin basin. Areas containing I/S with high expandabilities (e.g., 95% smectite layers) have a time-temperature index (TTI) of 4.0-4.5 at 120°C, whereas areas containing I/S with low expandabilities (e.g., 30% smectite layers) have a TTI of 5.0. Present data suggest that highly expandable I/S changed to slightly expandable I/S over a narrow temperature interval (10°-20°C). Differences in the potassium availability from detrital components and in the K+/H+ activity ratios of pore water do not appear to be related to the differences in the percentage of smectite layers of these I/S clays.

High-resolution transmission electron microscope images of dioctahedral mixed-layer clay structures (illite/smectite) having various substitutional and polytypic schemes were modeled using computer simulation methods. Both one- and two-dimensional calculations were performed using parameters characteristic of a typical range of imaging conditions. One-dimensional images formed by imaging only 00/ diffractions show three important results: (1) The 20-Å periodicity resulting from rigorously ordered R1 illite/smectite can be imaged, but unconventional focus conditions may be necessary. (2) For crystals oriented with the electron beam perfectly parallel to the layers, the brightest fringes in the image correspond to either the octahedral sheets or the interlayer sites, depending on focus conditions. Misorientation of the crystal, however, by only 1° or 2° shifts the positions of the fringes by 1 to 3 Å. Furthermore, in tilted specimens, some defocus values produce images suggesting that smectite layers have a 11–13-Å periodicity, despite the uniform 10-Å periodicity present in the model structure. Thus, direct correlations between image and structure generally should not be made. (3) Two-layer polytypes of pure illite or pure smectite can also produce images with a 20-Å periodicity.

Two-dimensional images additionally showed that the cross fringes produced by some hkl diffractions can be imaged. The simulations showed that these cross fringes ideally might permit the determination of both layer stacking and compositional periodicity, but the fringes are lost by misorientations of a few degrees. These image simulations demonstrated, therefore, that mixed layering of illite and smectite theoretically can be directly imaged by transmission electron microscopy of chemically untreated specimens, but ambiguities may exist in the detailed intepretation of the images.