Crystal structure and chemistry of trilithionite-2M(2) and polylithionite-2M(2)

The crystal chemistry of three Li-bearing mica-2M(2) 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 [based on O12-(x+y) (OH)(x)F-y] : 3.198 <= Si <= 3.538, 0.462 <= [IV]Al <= 0.811, 1.195 <= [IV]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 0.03 1). The crystal chemistry is mostly influenced by tetrahedral chemical composition. Increasing [IV]Al content, cc and psi(MI) parameters increase; Si content involves a lowering of the interlayer separation and tetrahedral thickness. Li content affects octahedral thickness. The stability of 2M(2) polytype seems to be induced by a relative increase of Delta z tetrahedral parameter, which reduces the repulsion between basal tetrahedral oxygen atoms. Unlike Li-bearing muscovite, trioctabedral Li-bearing mica crystals show an octahedral occupancy not related to octahedral charge.