Cation ordering in lepidolite

Three lepidolite-lM 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 lM (Rr = 0.035) and 2M2 Gr : 0.048) crystals from Radkovice, Czechoslovakia are ordered in their respective ideal space groups so that M(l) : Li6.e1(Mn,Mg)s0e for lepidolite-1M and M(l) : Lir o for lepidolite-2M2. In contrast, the lepidolite-lM 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: 1.88A) 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 lM 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. Introduction Three recent refinements of layer silicate structures have shown that additional cation ordering may be present when symmetry constraints are relaxed from an assumed higher order space group to a lower one. Lower order space group refinements have shown that margarite-2Mr (Guggenheim and Bailey, 1975, 1978) has an ordered arrangement of tetrahedral cations while zinnwaldite-lM (Guggenheim and Bailey, 1977) and a dioctahedral lM mica (Sidorenko, et al., 1975) showed both tetrahedral and octahedral ordering. In lM polytypes of ideal space group (CZlm) symmetry, octahedral ordering is possible between the M(1) site on the mirror plane and the two equivalent sites related by the mirror plane (both designated as M(2)). In zinnwaldite, the space group has been shown to be C2 and the two M(2) sites are not symmetricallyrelated by a mirror plane as in space group C2lm. They are, therefore, designated as M(2) and M(3). The M(2) site was shown to be occupied by aluminum whereas lithium, iron and vacancies are distributed randomly over the M(1) and M(3) sites. The lepidolite micas have been shown to have several octahedral ordering schemes. The lepidolite-3T structure (Brown, 1978) crystallizes in space group P3l2 and has two sites that are large and lithium-rich and a small aluminum-rich third site. This type of ordering scheme is analogous to the zinnwaldite structure. In other lepidolite structures, when the ideal symmetry has been used in the refinement procedure, one large site is located in the trans arrangement at M(1) and two smaller equivalent octahedra in cls orientation. This ordering pattern with M(1) larger than the two M(2) octahedra appears to be adopted even in structures where the octahedral composition might suggest alternate ordering models. Fluor-polylithionite-lM (Takeda and Burnham, 1969), lepidolite-lM (Sartoi, 1976), lepidolite-2M2 (Takeda, et al., l97l; Sartori, et al., 1973) and lepidolite-2M1 (Sartori, 1977; Swanson and Bailey, 1981) are all lepidolite micas that appear to have such an ordering scheme where the M(1) site contains the larger lithium ion and the two smaller symmetry-related M(2) sites have an average composition near Lis.5Als.5. The novel ordering pattern for lepidolite-3T is significant in that such an ordering scheme indicates 0003-004x/8 l/l I I 2-t 221 $02.0