CRYSTAL-STRUCTURE OF P2(1) M FERROMAGNESIAN AMPHIBOLE AND THE ROLE OFCATION ORDERING AND COMPOSITION IN THE P2(1)/M-C2/M TRANSITION IN CUMMINGTONITE/

We report structure refinements of three ferromagnesian cummingtonite crystals with Mg/(Mg + Fe) approximate to 0.63. An unheated crystal wi th X(Mg)(M4) = 0.051 has C2/m symmetry, but crystals heat treated at 6 00 and 700 degrees C, with X(Mg)(M4) = 0.167 and 0.237, respectively, have P2(1)/m symmetry. In P2(1)/m cummingtonite from this study and fr om Hirschmann et al. (1994), the A silicate chain is S rotated and the B chain is O rotated. This differs from the structure of P2(1)/m mang anoan cummingtonite, in which both A and B chains are O rotated (Papik e et al., 1969). Documented in nature for the first time, the S-rotate d silicate chains in amphibole suggest that, like the P2(1)/c to C2/c phase transition in clinopyroxene, the change of the A-chain configura tion from S to O rotation is intrinsic to the P2(1)/m to C2/m transfor mation in ferromagnesian cummingtonite. Documentation of different spa ce groups for magnesio-cummingtonite crystals that differ only in site occupancies confirms that the M4-site population controls the relativ e stabilities of the C2/m and P2(1)/m phases and places limits on the tolerance of the C2/m structure for Mg on the M4 site. At room tempera ture, ferromagnesian cummingtonite with X(Mg)(M4) > 0.15 +/- 0.02 has P2(1)/m symmetry. In the P2(1)/m structure, the separation between M4- O6A and M4-O6B and between M4-O5A and M4-O5B bond distances increases with increasing Mg content, as does the kinking angle (O5-O6-O5) of th e A and B chains. Structural distortions away from C2/m symmetry, part icularly in the configuration of the A chain, are greater for Fe-beari ng cummingtonite than for Mn-bearing varieties at similar Mg concentra tion, indicating that for a given Mg content, ferromagnesian amphibole is more stable in the P2(1)/m structure than manganoan amphibole.