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Practical Notes Concerning the Indexing of X-Ray Powder Diffraction Patterns of Clay Minerals

Published online by Cambridge University Press:  02 April 2024

S. W. Bailey
S. W. Bailey*
Affiliation:
Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706
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Abstract

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Hydrous phyllosilicate minerals exhibit many variations in unit-cell shape and symmetry, distortion due to cation ordering, polytypic repeats along Z, and lateral modulations that can affect the ease of indexing of X-ray powder diffraction patterns. In indexing the patterns of monoclinic and triclinic hydrous phyllosilicates it is important that the indices of all overlapping but nonequivalent lines be listed and used in the indexing program, with weights assigned according to their multiplicities. Deviations from orthogonal unit-cell geometry show up by a broadening or a splitting of nearly superimposed lines, as occurs also if the observed ß angle differs from that produced by a layer shift of ideal magnitude (e.g., a/3). Observed powder intensities should be correlated with those expected from the structure, and an index should not be assigned solely on the basis of the calculated d-value that is closest to that observed. Assigned indices should not violate space group extinctions. Multilayer structures show up in reflections of type k ≠ 3n for C-centered unit cells, not in 00/reflections.

Keywords

ChloriteCrystal structureIndexingKaoliniteMicaModulated structuresOrderingX-ray powder diffraction
Type
Research Article
Information
Clays and Clay Minerals , Volume 39 , Issue 2 , April 1991 , pp. 184 - 190
Copyright
Copyright © 1991, The Clay Minerals Society

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