Stacking Faults in the Kaolin-Group Minerals: Defect Structures of Kaolinite

A. Plançon1, R. F. Giese Jr.2, R. Snyder3, V. A. Drits4 and A.S. Bookin4
1 Centre de Recherche sur les Solides à Organisation Cristalline Imparfaite and Université d'Orléans, 45100 Orléans, France
2 Department of Geology, State University of New York at Buffalo 4240 Ridge Lea Road, Amherst, New York 14226
3 New York State College of Ceramics, Alfred University, Alfred, New York 14802
4 Geological Institute, Academy of Sciences of the U.S.S.R., Pyzevsky 7, Moscow, U.S.S.R.

Abstract: Previous studies of the defect structure of kaolinite have examined samples having a restricted level of defects. This study examined nine kaolinite samples having a large diversity of defect contents, as indicated by Hinckley indexes ranging from 1.44 to 0.18. The samples were chosen so as to cover this range in as regular a manner as possible. The types and abundances of the defects were determined by examining the X-ray powder diffraction profiles for the 02,11 and 20,13 bands. The diffraction intensities were measured by counting for a fixed time in steps of 0.01°2θ. Analysis of these diffraction profiles indicated that (1) the major defect is the existence of a translation between adjacent layers, which is not the usual t  1 (approximately a/3), but is related to that translation by the pseudo-mirror plane coincident with the long diagonal of the unit cell; (2) the existence of a few C layers among the B layer stacking is a minor defect; (3) many of the samples could be accurately modeled only by assuming the existence of two kaolinite phases; (4) the existence of only a few C layers in some samples does not support the idea of a continuous series from kaolinite to dickite through disordered intermediates; and (5) the Hinckley indexes of several samples depend on the relative proportions of the two types of kaolinite in the mixture.

The nine kaolinite samples fall into three groups: those having a low to moderate abundance of defects (Hinckley index > 0.43) are mixtures of two types of kaolinite (one almost free of defects, the other richer in defects); those having low Hinckley indexes (0.43 to 0.18) are single phases with different proportions of defects; and those which contain a single type of kaolinite, unlike the others in the nature of the interlayer translations and the greater abundance of C layers. The agreement between calculated and observed X-ray diffraction profiles is excellent for all specimens, except one sample (from Charentes) for which the fit is acceptable but not perfect.

Key Words: Crystallinity • Defect structure • Hinckley index • Kaolinite • Stacking faults • X-ray powder diffraction

Clays and Clay Minerals; June 1989 v. 37; no. 3; p. 203-210; DOI: 10.1346/CCMN.1989.0370302
© 1989, The Clay Minerals Society
Clay Minerals Society (www.clays.org)