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We report structure refinements of three ferromagnesian cummingtonite crystals with Mg/(Mg + Fe) ≈ 0.63. An unheated crystal with X M4  Mg = 0.051 has C2/m symmetry,but crystals heat treated at 600 and 700°C, with X M4  Mg  = 0.167 and 0.237, respectively, have P2 1 /m symmetry. In P2 1 /m cummingtonite from this study and from Hirschmann et ai. (1994), the A silicate chain is S rotated and the B chain is 0 rotated. This differs from the structure of P2, /m manganoan cummingtonite, in which both A and Bchains are 0 rotated (Papike et aI., 1969). Documented in nature for the first time, the S-rotated silicate chains in amphibole suggest that, like the P2 1 /c to C2/c phase transition in clinopyroxene, the change of the A-chain configuration from S to 0 rotation is intrinsic to the P2 1 /m to C2/m transformation in ferromagnesian cummingtonite.

Crystal structure of P2 1 /m ferromagnesian amphibole and the role of cation ordering and composition in the P2 1 /m-C2/m transition in cummingtonite