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

Crystal-structure refinements were done on Li-bearing muscovite-2M(l) cryst als from microgranite and granitic pegmatite rocks in order to characterize their crystal chemistry and their relationships with muscovite and triocta hedral 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 dio ctahedral character. Single-crystal X-ray-diffraction data were collected f or five crystals in space group C2/c; the agreement factor, R-obs, varies b etween 0.033 and 0.042. The mean tetrahedral cation-oxygen atom distances r ange from 1.637 to 1.646 Angstrom and from 1.629 to 1.647 Angstrom for T1 a nd 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 reduct ion in size of the M1 site. Moreover, as the Li+ / (Li+ + Al3+) ratio incre ases, the silicate ring becomes less distorted (5.9 less than or equal to a lpha less than or equal to 11.4 degrees), the basal oxygen-atom planes beco me less corrugated (0.147 less than or equal to Deltaz less than or equal t o 0.232 Angstrom), and the interlayer separation narrows (3.337 less than o r equal to interlayer separation less than or equal to 3.422 Angstrom).