Crystal Structure and Crystal Chemistry of Lithium-Bearing Muscovite-2 M 1

Crystal-structure refinements were done on Li-bearing muscovite-2M1 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 [6]Li+ [6]Al3+−1[4]Al3+−2[4]Si4+2, Li-bearing muscovite shows additional substitutions, such as mechanism [6]Li+ [6]Fe2+ [6]Al3+−1[6]□ −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, Robs, varies between 0.033 and 0.042. The mean tetrahedral cation–oxygen atom distances range from 1.637 to 1.646 Å and from 1.629 to 1.647 Å for T1 and T2 sites, respectively. Variation in distances is associated with the Li+/(Li+ + Al3+) ratio, octahedral M2 site expansion (9.30 ≤ volumeM2 ≤ 9.90 Å3) and reduction in size of the M1 site. Moreover, as the Li+/(Li+ + Al3+) ratio increases, the silicate ring becomes less distorted (5.9 ≤ α ≤ 11.4°), the basal oxygen-atom planes become less corrugated (0.147 ≤ Δz ≤ 0.232 Å), and the interlayer separation narrows (3.337 ≤ interlayer separation ≤ 3.422 Å).