Crystal chemistry of Mg-, Fe-bearing muscovites-2M(sub 1)

Phengitic muscovite-2[M.sub.1] crystals [Mathematical Expression Omitted] from pegmatites and peraluminous granites were refined to investigate the influence of phengitic substitution on the mica structure. Single-crystal X-ray diffraction data were collected for eleven crystals in the C2/c space-group (agreement factor 2.1% [less than or equal to] [R.sub.obs] [less than or equal to] 3.9%). Tetrahedral Si and Al cation disorder was found for each sample, with the mean tetrahedral cation-oxygen distances ranging from 1.639 [Angstrom] [less than or equal to] [less than or equal to] 1.647 [Angstrom] and 1.640 [Angstrom] [less than or equal to] [less than or equal to] 1.646 [Angstrom]. As phengitic substitution increases, the octahedral sheet expands and requires a less distorted (more hexagonal) tetrahedral ring (7.70 [degrees] [less than or equal to] [Alpha] [less than or equal to] 11.38 [degrees]) and low and low corrugation of the basal O plane (0.1796 [Angstrom] [less than or equal to] [Delta]z [less than or equal to] 0.2296 [Angstrom]). The electron density at the M2 site is greater than that required for the ideal muscovite-2[M.sub.1] structure, and a small excess of electron density is found in the M1 site. The inner sixfold coordination of the interlayer (A) cation is elongated along [c.sup.*], which is consistent with the high [Alpha] values and the long A-O11 bond length.