X-ray Diffraction Identification of the Polytypes of Mica, Serpentine, and Chlorite

S. W. Bailey
Department of Geology & Geophysics, University of Wisconsin Madison, Wisconsin 53706
1987 GEORGE W. BRINDLEY LECTURE The third George W. Brindley lecture was delivered at the 24th annual meeting of The Clay Minerals Society in Socorro, New Mexico, on October 19, 1987, by S. W. Bailey, Department of Geology & Geophysics, University of Wisconsin, Madison, Wisconsin. The following article is based on that lecture.—Editor

Abstract: Six standard polytypes of micas exist that have c-periodicities between 1 and 6 layers; these can be subdivided into groups A and B according to whether the octahedral cations are in the same set of positions in every layer or alternate regularly between I and II possible sets of positions (alternate slant directions), respectively. Correspondingly, 12 standard polytypes exist for 1:1 layers of the serpentine-type structures, which are divisible into four groups A–D. Groups A and B differ as in the micas, as do groups C and D. Groups A and B have ±a/3 interlayer shifts, whereas groups C and D have zero or ±b/3 interlayer shifts. Six structural types of semi-randomly or regularly stacked 1-layer chlorites exist that differ in the slant of the interlayer sheet (I or II) and its position (a or b) relative to 6-member rings in adjacent 2:1 layers.

Criteria have been established that permit the X-ray diffraction identification of the above-mentioned groups as well as the individual polytypes within each group. These criteria involve inspection of the strong X-ray diffraction reflections of index k = 3n for identification of the groups and of the generally weaker k ≠ 3n reflections for identification of the polytypes (assuming indexing on orthohexagonal axes throughout for convenience). In all hydrous phyllosilicates the octahedral cations and anions repeat at intervals of b/3 and thus contribute strongly to X-ray diffraction reflections of index k = 3n. The intensities of these reflections identify the two mica groups, the four serpentine-type groups, and the six chlorite groups. The periodicities of these reflections along Z* is that between identical octahedral sheets. The basal oxygen atoms do not repeat at intervals of b/3 and are a primary contributor to the intensities of k ≠ 3n reflections. The period along Z* for these reflections is that between identical basal oxygen planes, and the periodicity plus the symmetry identify the individual trioctahedral polytypes. For dioctahedral polytypes of the kaolin-group minerals and chlorites, the position of the vacant octahedral site must be considered also.

These general principles can be illustrated especially well by single-crystal precession photographs and extrapolated to powder photographs.

Key Words: Chlorite • Kaolin • Mica • Polytype • Serpentine • X-ray diffraction

Clays and Clay Minerals; June 1988 v. 36; no. 3; p. 193-213; DOI: 10.1346/CCMN.1988.0360301
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)