A macroscopic energy balance model for crystalline swelling of 2:1 phyllosilicates is presented. Crystalline swelling for a static system is modeled by a balance among the potential energies of attraction, repulsion and resistance. The potential energy of attraction is due to both the electrostatic interaction between the interlayer cations and the negative surface charge sites and to van der Waals attraction between layers. The potential energy of repulsion is due to the net hydration energy for the interlayer cations, the net hydration energy for the negative surface charge sites and Born repulsion. The potential energy of resistance represents irreversible work needed to overcome the mechanical resistance of the clay water system to both expansion and collapse. The potential energy of resistance is responsible for both hysteresis and the stepwise nature of crystalline swelling.

A numerical solution of the crystalline swelling model is presented and shown to yield reasonable estimates of basal spacings for octahedrally charged clays. Measured and predicted basal spacings are directly compared and are in general agreement (r2 = 0.39). Most of the scatter for the measured vs. predicted basal spacing relationship is attributed to inaccuracies of the assumptions used for the numerical solution. The crystalline swelling model readily accounts for the effects of layer charge and nature of the interlayer cations upon crystalline swelling, but does not account for the effect of charge site location upon crystalline swelling.

Illite polytype quantification allows the differentiation of diagenetic and detrital illite components. In Paleozoic shales from the Illinois Basin, we observe 3 polytypes: 1Md, 1M and 2M1. 1Md and 1M are of diagenetic origin and 2M1 is of detrital origin. In this paper, we compare experimental X-ray diffraction (XRD) traces with traces calculated using WILDFIRE© and quantify mixtures of all 3 polytypes, adjusting the effects of preferred orientation and overlapping peaks. The broad intensity (“illite hump”) around the illite 003, which is very common in illite from shales, is caused by the presence of 1Md illite and mixing of illite polytypes and is not an artifact of sample preparation or other impurities in the sample. Illite polytype quantification provides a tool to extrapolate the K/Ar age and chemistry of the detrital and diagenetic end-members by analysis of different size fractions containing different proportions of diagenetic and detrital illite polytypes.

Members of an alunite-natroalunite solid solution series occur in intimate association with hydrated halloysite in deposits within caves of the Guadalupe Mountains, namely Carlsbad Cavern and Lechuguilla Cave. The alunite and natroalunite crystals consist of cube-like rhombs; crystal diameters range from 0.5 to 8 µm. This mineral association is found in sediments within bedrock pockets, solution cavity fills, floor deposits and wall residues. Sulfur stable isotope values (δ34S, CTD) for cave alunite and natroalunite are negative [+0.1 to −28.9 per mill (‰); n = 12 and mean = 16.8‰] and are comparable to the cave gypsum and native sulfur values reported by other investigators. The association of alunite/natroalunite with hydrated halloysite in these cave deposits suggests that the cave-forming waters contained significant concentrations of sulfuric acid. Formation of these minerals is related to the excavation of the carbonate rocks that formed Carlsbad Cavern, Lechuguilla Cave and other caves of the Guadalupe Mountains. The sulfuric acid-bearing waters, when exposed to clay-rich sediments, converted clay minerals and quartz to alunite/natroalunite and hydrated halloysite.