CRYSTAL-STRUCTURE OF A NEW REPRESENTATIVE OF CR MICAS

The structure of the Cr-containing 2M(1) mica is determined [Enraf-Non ius automated diffractometer, MoKalpha radiation, 937 reflections with \F\ > 3 sigma(F), R-hkl = 0.048]. The parameters of the monoclinic un it cell are a = 5.240(3), b = 9.103(2), and c = 19.93(4) Angstrom; bet a = 95.59(3)degrees; V = 946(3) Angstrom(3): sp. gr. C2/c; and Z = 4. The structural motif is identical to those observed in other dioctahed ral 2M(1) micas. Included into the M(2) octahedron, the chromium atom increases its dimensions and increases the b parameter of the unit cel l. It is shown that the structure-modeling formulas are of limited app lication to the unusual chemical composition of the mica under study ( high Cr content and presence of Ba and F).