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).