ROTATIONALLY DISORDERED ILLITE/SMECTITE IN PALEOZOIC K-BENTONITES

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) P-0, the proportion of zero-degree layer stacking rotations, such as in the polytype series 1Md-1M; 2) P-cv the proportion of 2:1 layers with cis-vacant (cv) octahedral sites that are randomly interstratified with trans-vacant (tv) layers; and 3) P-60, the proportion of layers with n . 60 degrees rotations (as opposed to n . 120 degrees) 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 Al and decreases with Mg and Fe content. The proportion of n . 60 degrees rotations in the rotated layers increases with Mg and Fe content. The cv 120 degrees disordered structure correlates with tetrahedral Al for Si substitution and increasing tetrahedral charge. The tv n . 60 degrees disordered structures correlate with octahedral Mg for Al 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.