Crystal structure of a new representative of Cr micas

The structure of the Cr-containing 2M1 mica is determined [Enraf-Nonius automated diffractometer, MoKα radiation, 937 reflections with |F|>3σ(F), Rhkl=0.048]. The parameters of the monoclinic unit cell are a=5.240(3), b=9.103(2), and c=19.93(4) A; β=95.59(3) deg.; V=946(3) A3; sp. gr. C2/c; and Z=4. The structural motif is identical to those observed in other dioctahedral 2M1 micas. Included into the M(2) ocahedron, the chromium atom increases its dimensions and increases the b parameter of the unit cell. It is shown that the structure-modeling formulas are of limited application to the unusual chemical composition of the mica under study (higher Cr content and presence of Ba and F)