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 [based on O12-(x+y) (OH)xFy]: 3.198 ≤ Si ≤ 3.538, 0.462 ≤ [IV]Al ≤ 0.811, 1.195 ≤ [VI]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 [IV]Al content, α and ψM1 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 Δz 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.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.