Three lepidolite-1M and two lepidolite-2M2 mica structures from three localities have been refined using single crystal X-ray data to determine cation ordering schemes in both ideal and subgroup symmetries. The 1M (R1 = 0.035) and 2M2 (R1 = 0.048) crystals from Radkovice, Czechoslovakia are ordered in their respective ideal space groups so that M(1) = Li0.91(Mn,Mg)0.09 for lepidolite-1M and M(1) = Li1.0 for lepidolite-2M2. In contrast, the lepidolite-1M from Tanakamiyama, Japan (R1 = 0.062) is similar topologically to zinnwaldite in subgroup symmetry C2. The octahedra related by the pseudo-mirror plane are significantly different in size (mean M–O,F,OH = 1.88Å) and electron count (11.5 and 6.0). The difference in electron count between these two octahedral sites is more substantial than in zinnwaldite. The previous 2M2 refinement of Sartori et al. (1973) is confirmed but the 1M structure (Sartori, 1976) is better described as similar to that of the Tanakamiyama lepidolite, although due to systematic errors in the data an ordered model is not unequivocally established. Cation ordering similar to that found in the Tanakamiyama lepidolite is promoted by a high fluorine content, but parameters of crystallization other than fluorine content are important.

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