ROTATIONALLY DISORDERED ILLITE SMECTITE IN PALEOZOIC K-BENTONITES/

The three-dimensional crystal structure of rotationally disordered ill ite/smectite (I/S) in K-bentonite samples from the Appalachian basin a nd neighboring areas is described using the parameters of 1) P-0, the proportion of zero-degree layer stacking rotations, such as in the pol ytype series 1Md-1M; 2) P-cv the proportion of 2:1 layers with cis-vac ant (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 laye rs. These parameters were determined by computer modeling of experimen tal randomly oriented powder X-ray diffraction patterns. The proportio n 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 degre es disordered structure correlates with tetrahedral Al for Si substitu tion and increasing tetrahedral charge. The tv n . 60 degrees disorder ed structures correlate with octahedral Mg for Al substitution. The da ta indicate that the type of unit cell and nature of rotational disord er in I/S is controlled by the octahedral Mg content. The three-dimens ional structures do not show any systematic correlation with Reichweit e and percent expandability as determined from diffraction patterns of oriented sample preparations.