Crystal structure and crystal chemistry of lithium-bearing muscovite-2M(1)

Crystal-structure refinements were done on Li-bearing muscovite-2M(l) crystals from microgranite and granitic pegmatite rocks in order to characterize their crystal chemistry and their relationships with muscovite and trioctahedral lithium-containing micas. In addition to the substitution mechanism Li-[6](+) Al-[6](-1)3+ Al-[4](-2)3+ Si-[4](2)4+, Li-bearing muscovite shows additional substitutions, such as mechanism Li-[6](+) Fe-[6](2+) Al-[6](-1)3+ ([6])(square -1), indicating that the structure deviates from ideal dioctahedral character. Single-crystal X-ray-diffraction data were collected for five crystals in space group C2/c; the agreement factor, R-obs, varies between 0.033 and 0.042. The mean tetrahedral cation-oxygen atom distances range from 1.637 to 1.646 Angstrom and from 1.629 to 1.647 Angstrom for T1 and T2 sites, respectively. Variation in <T-O > distances is associated with the Li+ / (Li+ + Al3+) ratio, octahedral M2 site expansion (9.30 less than or equal to volume(M2) less than or equal to 9.90 Angstrom (3)) and reduction in size of the M1 site. Moreover, as the Li+ / (Li+ + Al3+) ratio increases, the silicate ring becomes less distorted (5.9 less than or equal to alpha less than or equal to 11.4 degrees), the basal oxygen-atom planes become less corrugated (0.147 less than or equal to Deltaz less than or equal to 0.232 Angstrom), and the interlayer separation narrows (3.337 less than or equal to interlayer separation less than or equal to 3.422 Angstrom).