Crystal structure and chemistry of trilithionite-2M2 and polylithionite-2M2
Brigatti, Maria Franca; Caprilli, Enrico; Malferrari, Daniele; Medici, Luca; Poppi, Luciano
European Journal of Mineralogy Volume 17 Number 3 (2005), p. 475 - 481
published: Jun 14, 2005
ArtNo. ESP147051703008, Price: 29.00 €
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 â¤ Si â¤ 3.538, 0.462 â¤ [IV]Al â¤ 0.811, 1.195 â¤ [VI]Al â¤ 1.390, 0.031 â¤ (Fe+Mn) â¤ 0.072, 1.522 â¤ Li â¤ 1.757, 0.872 â¤ K â¤ 0.906, 0.030 â¤ Na â¤ 0.073, 0.000 â¤ (Cs+Rb) â¤ 0.099, 1.541 â¤ F â¤ 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, Î± 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 Î´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.