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Mineralogical ClassificationTrilithionite and polylithionite structures (abs.)

7th Jun 2005 17:50 UTCMarco E. Ciriotti

• Brigatti, M.F., Caprilli, E., Malferrari, D., Medici, L., Poppi, L. (2005): Crystal structure and chemistry of trilithionite-2M2 and polylithionite-2M2. European Journal of Mineralogy, 17, 475-481.



Abstract:



The crystal chemistry of three Li-bearing mica-2M2 crystals from pegmatites has been studied by chemical analyses and single crystal X-ray diffraction; their belonging to the trilithionite-polylithionite join is highlighted by the following compositional ranges in atoms per formula unit : 3.198 Si 3.538, 0.462 Al 0.811, 1.195 Al 1.390, 0.031 (Fe+Mn) 0.072, 1.522 Li 1.757, 0.872 K 0.906, 0.030 Na 0.073, 0.000 (Cs+Rb) 0.099, 1.541 F 1.722. The correlation between F and Li content is confirmed, as observed in Li-rich micas. Crystal structure refinements were carried out in space group C2/c (R values vary between 0.030 and 0.031). The crystal chemistry is mostly influenced by tetrahedral chemical composition. Increasing Al content, and M1 parameters increase; Si content involves a lowering of the interlayer separation and tetrahedral thickness. Li content affects octahedral thickness. The stability of 2M2 polytype seems to be induced by a relative increase of z tetrahedral parameter, which reduces the repulsion between basal tetrahedral oxygen atoms. Unlike Li-bearing muscovite, trioctahedral Li-bearing mica crystals show an octahedral occupancy not related to octahedral charge.
 
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