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.

Time-of-flight neutron powder diffraction has been employed to determine precise occupancies of the M1 and M2 metal cation sites in synthetic olivines of compositions (Fe0.3Mn0.7)2SiO4, (Fe0.5Mn0.5)2−SiO4, and (Fe0.7Mn0.3)2SiO4. The distribution coefficient for Fe-Mn exchange in these samples has values of 4.864, 3.976, and 4.078, reflecting the preference of Mn2+ for the M2 site, over Fe2+. These results, and the behaviour of the difference in mean bond lengths of the two sites, indicate that, while showing a tendency towards ordering, the composition-dependence of the solid solution is near ideal.