Rotationally disordered illite-smectite in Paleozoic K-bentonites

Abstract- The three-dimensional crystal structure of rotationally disordered illite/smectite (I/S) in K-ben-tonite samples from the Appalachian basin and neighboring areas is described using the parameters of 1) Po, the proportion of zero-degree layer stacking rotations, uch as in the polytype series IMd- IM; 2) Pcv, the proportion of 2:1 layers with cis-vacant (cv) octahedral sites that are randomly interstratified with trans-vacant ( v) layers; and 3) Pro, the proportion of layers with n- 60 ~ rotations (as opposed to n. 120 ~ in the rotated layers. These parameters were determined bycomputer 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 A 1 for Si substitution a d 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 asdetermined from diffraction patterns of oriented sample preparations