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

Yang Hexiong Yang, M. M. Hirschmann

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Abstract

Reports structure refinements of three ferromagnesian cummingtonite crystals with Mg/(Mg + Fe) ≃ 0.63. An unheated crystal with XMgM4 = 0.051 has C2/m symmetry, but crystals heat treated at 600 and 700°C, with XMgM4 = 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

Original languageEnglish (US)
Pages (from-to)916-922
Number of pages7
JournalAmerican Mineralogist
Volume80
Issue number9-10
DOIs
StatePublished - Jan 1 1995

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