The three-dimensional crystal structures of illite-smectite (I-S) in K-bentonite samples from the Appalachian Basin are characterized by rotational disorder in the stacking sequence of 2:1 illite layers, different proportions of n60° rotations (as opposed to n120°) in the rotated layers, and layers with centrosymmetric trans-vacant (tv) octahedral sites that are randomly interstratified with noncentric cis-vacant (cv) layers. The proportion of cv interstratification in the I-S increases with tetrahedral A1 and decreases with octahedral Mg and Fe content. The I-S minerals in the northern Appalachian basin K-bentonites are characterized by high (79% average) proportions of cv (Pcv) layers. In contrast, I-S from equivalent K- bentonites from the southern Appalachian basin has low Pcv values (38% average). These values do not correlate with expandability or rotational disorder. The geographic distribution of these I-S structural parameters may have resulted from possibly short-term, hot, and advective fluid migrations that differed in Mg concentrations and/or other physical and chemical parameters.

The three-dimensional crystal structure of rotationally disordered illite/smectite (I/S) in K-bentonite samples from the Appalachian basin and neighboring areas is described using the parameters of 1) P0, the proportion of zero-degree layer stacking rotations, such as in the polytype series 1Md-1M; 2) Pcv, the proportion of 2:1 layers with cis-vacant (cv) octahedral sites that are randomly interstratified with trans-vacant (tv) layers; and 3) P60 the proportion of layers with n·60° rotations (as opposed to n·120°) in the rotated layers. These parameters were determined by computer modeling of experimental randomly oriented powder X-ray diffraction patterns.

The proportion of cv interstratification in the I/S increases with A1 and decreases with Mg and Fe content. The proportion of n·60° rotations in the rotated layers increases with Mg and Fe content. The cv 120° disordered structure correlates with tetrahedral A1 for Si substitution and increasing tetrahedral charge. The tv n·60° disordered structures correlate with octahedral Mg for A1 substitution. The data indicate that the type of unit cell and nature of rotational disorder in I/S is controlled by the octahedral Mg content. The three-dimensional structures do not show any systematic correlation with Reichweite and percent expandability as determined from diffraction patterns of oriented sample preparations.