Crystal structure of P2₁/m ferromagnesian amphibole and the role of cation ordering and composition in the P2₁/m-C2/m transition in cummingtonite

Reports structure refinements of three ferromagnesian cummingtonite crystals with Mg/(Mg + Fe) ≃ 0.63. An unheated crystal with X_Mg^M4 = 0.051 has C2/m symmetry, but crystals heat treated at 600 and 700°C, with X_Mg^M4 = 0.167 and 0.237, respectively, have P2₁/m, symmetry. In P2₁/m cummingtonite from this study and from Hirschmann et al. (1994), the A silicate chain is S rotated and the B chain is O rotated. This differs from the structure of P2₁/m manganoan cummingtonite, in which both A and B chains are O rotated (Papike et al., 1969). Documented in nature for the first time, the S-rotated silicate chains in amphibole suggest that, like the P2₁/c to C2/c phase transition in clinopyroxene, the change of the A-chain configuration from S to O rotation is intrinsic to the P2₁/m to C2/m transformation in ferromagnesian cummingtonite. -from Authors