Rhodonite with unusually high magnesium and low iron contents, Mn3.73 Mg0.73Ca0.51Fe0.03Si5O15, occurs in a metamorphosed sedimentary evaporile sequence at Balmat, New York. It coexists with an unusual pyroxene having average composition close to MnMgSi2O6, talc, calcite, and a number of other minerals. Refinement of the crystal structure using single–crystal X–ray diffraction data shows that Mg has a marked preference for the M4 site (Mg0.47Mn0.53), and that M1, M2, and M3 have nearly equal Mg occupancies of slightly more than 0.1. Relations with coexisting phases indicate that both the Mg content and degree of ordering of Mg in M4 are near the maximum to be expected for naturally occurring rhodonite.

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