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Structures of Intergrown Triclinic and Monoclinic IIb Chlorites from Kenya

Published online by Cambridge University Press:  02 April 2024

Hong Zheng  and
Sturges W. Bailey
Hong Zheng
Affiliation:
Department of Geology & Geophysics, University of Wisconsin, Madison, Wisconsin 53706
Sturges W. Bailey
Affiliation:
Department of Geology & Geophysics, University of Wisconsin, Madison, Wisconsin 53706
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Abstract

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Triclinic IIb-4 and monoclinic IIb-2 clinochlore polytypes from Kenya are intergrown laterally in (001) platelets and are separated optically in some places by sharp lines of demarcation and in other places by embayed boundaries. Electron microprobe analyses show no compositional differences between the two polytypes. Structural refinements of each show that the 2:1 layer is identical in the two structures with no tetrahedral or 2:1 octahedral cation ordering in either ideal or subgroup symmetries. The two interlayer sheets dilfer only in the distribution of A1 and Mg over interlayer sites M3 and M4, but not in the distribution of Fe3+, Fe2+, or vacancies. The monoclinic IIb-2 polytype is less ordered than the triclinic IIb-4 polytype. The mean M-O,OH bond length of 1.990 Å indicates significantly less of the smaller A1 present in the M4 site of IIb-2 than for the mean M-O,OH value of 1.957 Å for the M4 site of IIb-4. The textural relations and the structural refinements suggest that the monoclinic IIb-2 structure is inherently less stable than the triclinic form. This is in accord with the observed lesser abundance in nature of the monoclinic form.

Keywords

Bond lengthCation distributionChloriteCrystal structurePolytypeSingle crystal X-ray diffraction
Type
Research Article
Information
Clays and Clay Minerals , Volume 37 , Issue 4 , August 1989 , pp. 308 - 316
Copyright
Copyright © 1989, The Clay Minerals Society

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