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 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 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.