Mixed-layer illite/smectite (I/S) clays in lower Eocene sediments of the Texas Gulf Coast decrease in expandability with depth as found in previous studies of clay diagenesis, but at the same depth the 1/S clays from sand laminae tend to be more expandable than clays from adjacent shales. The I/S clays with greater expandability from sands collapse to phases with very low expandabilities after saturation with K. This non-ideal smectite behavior indicates that many of the expandable layers in I/S clays from sand laminae possess an illite-level negative layer charge on the structure. The Greene-Kelley Li-saturation test of the I/S clays also reveals that a considerable portion of the increased layer charge deficiency is created by tetrahedral substitution. The illitization of individual smectite layers in an I/S clay is envisioned to be a two step process instead of the single step rapid transformation implied in many earlier studies. The first rate-limiting step is the creation of sufficiently large negative layer charge in the lattice, primarily by AI for Si substitution in the tetrahedral sheet. The second rate-limiting step is the supply of K to the high-charged expandable layers. The abundance of high-charged expandable layers in I/S clays from sand laminae suggests that the K supply is influenced more by competing ions in the interstitial waters than by the absolute activity of K.

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.

Samples of mixed-layer illite/smectite were investigated from a single bentonite bed zoned with respect to expandability from 90 to 30%. Chips of natural rocks were embedded in a resin, using a procedure designed to preserve the original fabric, cut with an ultramicrotome, and observed by high-resolution transmission electron microscopy (HRTEM). These observations confirmed the X-ray powder diffraction (XRD) model of mixed-layer clays, i.e., that illite/smectite grains in natural rocks are built of mixed-layer crystals, from 1 to as many as 15 silicate layers thick (4–6 interlayers per crystal on average). These crystals are present either as individual particles (loose crystals) or, typically, they form nearly parallel face-to-face groupings called here quasi-crystals. Free fundamental smectite and illite particles as defined by Nadeau and coworkers were essentially absent.

Illite and smectite interlayer spacings were 10 and 13.5 Å, respectively. Crystal thickness and number of interlayers were measured for 35–100 mixed-layer crystals per sample. Illite/smectite expandabilities were calculated from these data in two ways: either neglecting the crystal edges or accounting for them. The former determinations agree well with XRD estimates of expandability and the latter, with expandabilities calculated from the distributions of fundamental particle thickness measured by a shadowing technique in the TEM. This result explains the systematic discrepancy between XRD and TEM measurements of illite/smectite expandability.

For a series of mixed-layer illite-smectite (I-S) minerals from a drillhole near the Kakkonda geothermal field, one-dimensional structure analysis by X-ray diffraction (XRD) was performed using Casaturated specimens in both air-dried and ethylene glycol-solvated states. The expandability characteristics of component layers were also examined by means of alkylammonium exchange and Li saturation. The K content in the illite layers was 1.5–1.7/O20(OH)4 in the I-S series from 3 to 85% of I-layer content (% I). The layer charge of the smectite layer varied slightly within the range of 0.3–0.5/O10(OH)2 by alkylammonium exchange experiments and the expandability was independent of the beidellite content within a range of 0–0.5 by the Li-saturation test. The degree of long-range ordering represented by Reichweite (R) parameters varied from R0 to R3 via R1 and R2 with increase in % I. The I-S samples contained <10% vermiculite as the third component and the vermiculite content tended to decrease with progressive illitization.

In contrast to the smectitic R0 samples (<10% I), more illitic R0 (e.g. 35% I) and >R1 I-S samples showed complicated expandability with alkylammonium exchange. The XRD patterns of dodecylammonium-exchanged I-S samples can be interpreted by random interstratification of several types of sub-units such as layer-doublets, layer-triplets and layer-quartets present in the crystallites. This interpretation is consistent with the variation in the occurrence probabilities of layer-multiplets calculated from the junction probabilities and the proportions of layers. Because the interpretation indicates that I-S is a stack of various types of the sub-units, the smectite illitization can be described by a systematic change in the type and proportion of the sub-units constituting crystallites.

Interstratified illite-smectite (I-S) occurring authigenically in diverse earth crust environments reacts toward more illite-rich phases as temperature increases. For that reason, I-S is used for geothermometry when prospecting for hydrocarbons or ore mineral deposits. This study develops the mathematical relations for characterizing the coherent stacking potential of fundamental particles (FP) using the expandability ratio K, where K is defined as (%SMAX –; %SXRD)/%SMAX. The ratio can be applied to differentiating I-S samples from shales, bentonites, and hydrothermal alterations. In particular, patterns on a K vs. T diagram, where T is the average thickness of fundamental particles (FPs), appear to be indicative of the geological conditions related to I-S formation. Shale samples plot in the negative K domain of the diagram, possibly due to the intimate mixing of detrital particles. Both bentonitic and hydrothermal samples display trends of increasing K with T, which suggests the coherent stacking potential progressively decreases as FPs increase in thickness. Hydrothermal samples are more extensively distributed on the diagram than samples from bentonites. This result may reflect differences in particle growth conditions (nutrients and space) between bentonites (short supply) and hydrothermal alterations (good supply).

The reaction of a Wyoming-type bentonite with pH 13.5 solutions was investigated experimentally at 35 and 60°C for periods of 1 to 730 days. Some crystal properties of the starting montmorillonitic clay remain unchanged, i.e. stability of the octahedral sheet, total cation exchange capacity (CEC) and CEC after neutralization of the octahedral charge, full expandability in the Casaturated state, and size distribution. Other properties are changed, e.g. there is an increase in the expandability after octahedral charge neutralization; a slight increase in the average layer charge; a decrease of the total surface area; and a particle morphological change from flakes to hexagonal shape.

The composition and the structure of the smectite layers did not change significantly during the reaction. The increasing number of expandable layers after octahedral charge neutralization is attributed to modifications in the stacking sequence. The number of interlayers surrounded by two charged tetrahedral sheets increases with reaction time.