Polytypism of Micas. III. X-Ray Diffraction Identification

Z. Weiss and A. Wiewióra
Coal Research Institute, 716 07 Ostrava-Radvanice Czechoslovakia
Institute of Geological Sciences, Polish Academy of Sciences 02 089 Warszawa, Poland

Abstract: Classification and identification criteria of maximum-degree-of-order (MDO) polytypes of homo- and meso-octahedral micas based on the distribution of the intensities of 20l (13l) and 02l reflections are proposed. Calculated |F(20l)|2 and |F(02l)|2 values for single crystals of micas with different compositions are given for one-, two-, and three-layer polytypes. Transmission powder diffractometry is proposed as a suitable method for the identification of the different groups of mica polytypes from polycrystalline specimens. Calculated powder patterns and the characteristic properties of the diffraction patterns of random and highly oriented aggregates are employed for identification purposes. The individual MDO polytypes are designated by generalized Ramsdell symbols which also contain information about their position in the classification system.

Key Words: Crystal structure • Mica • Order-disorder • Polytype • Single crystal X-ray diffraction • X-ray powder diffraction

Clays and Clay Minerals; February 1986 v. 34; no. 1; p. 53-68; DOI: 10.1346/CCMN.1986.0340107
© 1986, The Clay Minerals Society
Clay Minerals Society (www.clays.org)