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

Phengitic muscovite-2M(1), crystals [([12])(K0.88-0.99Na0.01-0.09Ca0.Ba-00-0.06(0.00-0.01))([6])(Al1.64-1.88Fe0.06-0.292+Fe0.01-0.163+Mg0.00-0.16Mn0.00-0.07Ti0.00-0.06)([4])(Si2.87-3.30Al0.70-1.13)(OH)(1.56-2.07)F0.00-0.41O9.91-10.25] from peg matites 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-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 <T1-O> less than or equal to 1.647 Angstrom and 1.640 Angstrom less than or equal to <T2-O> 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 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-2M(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*, which is consistent with the high or values and the long A-O11 bond length.