STRUCTURES OF INTERGROWN TRICLINIC AND

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 a d in other places by embayed boundaries. Electron microprobe analyses show no compositional differences between the two polytypes. Structural refinements ofeach 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 ofAI and Mg over interlayer sites M3 and M4, but not in the distribution ofFe 3+, Fe 2+, 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 A indicates ignificantly less of the smaller AI present in the M4 site of IIb-2 than for the mean M-O,OH value of 1.957 A for the M4 site of IIb-4. The textural relations and the structural refinements suggest that the monoclinic IIb-2 structure is inherently ess 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