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The crystal structures and site-populations of 12 richterites have been refined to R indices of 1-2% using single-crystal MoKa X-ray diffraction data. Potassium-fluor-richterites from lamproites are characterized by [41 (Si+AI) sums less than 8.0 apfu (atoms per fo rmula unit), suggesting the presence of other isomorphous tetrahe dral substituents; richterites from other environments do not show this feature. The results of the structure re fine ments (mean bond lengths and mean atomic numbers) and crystal-chemical arguments show [41 Ti 4+ to occur at the T(2) site in lamproitic richterites. Some richterites also show significant amounts ofTi 4+ in octahedral coordination; combination of site occupancy refinement and stereochemical data show [61 Ti 4 + to occur almost entirely at the M(I) site in these amphiboles, locally associated with the occurrence of 0 2 -at the adjacent 0(3) site(s).

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