Structures of Intergrown Triclinic and Monoclinic IIb Chlorites from Kenya

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

Abstract: 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 differ only in the distribution of Al 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 Al 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.

Key Words: Bond length • Cation distribution • Chlorite • Crystal structure • Polytype • Single crystal X-ray diffraction

Clays and Clay Minerals; August 1989 v. 37; no. 4; p. 308-316; DOI: 10.1346/CCMN.1989.0370403
© 1989, The Clay Minerals Society
Clay Minerals Society (www.clays.org)