Rotationally Disordered Illite/Smectite in Paleozoic K-Bentonites

Douglas K. McCarty and R. C. Reynolds Jr.
Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755

Abstract: 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 Al 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 Al for Si substitution and increasing tetrahedral charge. The tv n·60° 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.

Key Words: Bentonite • Cis-vacant octahedra • Crystal structure • Illite/Smectite • Polytype • Rotational disorder • X-ray diffraction

Clays and Clay Minerals; June 1995 v. 43; no. 3; p. 271-284; DOI: 10.1346/CCMN.1995.0430302
© 1995, The Clay Minerals Society
Clay Minerals Society (www.clays.org)