New data on rhodochrosite are presented, including unpublished analyses. Seventy-nine analyses are calculated to MnCO3, FeCO3, MgCO3 and CaCO3 and are plotted in three ternary diagrams. The frequency of appearance of Ca, Fe, and Mg in significant quantities as isomorphous constituents of the 79 rhodochrosite specimens is found to be in the ratio 21:17:12. Complete isomorphous miscibility with calcite and siderite is demonstrated. The series to magnesite is incomplete.

The refractive indices and specific gravities of rhodochrosite are shown to vary directly with the composition, and to be calculable from the proportions of the four end members of the system. The composition of a specimen, however, cannot be deduced from the specific gravity and optical properties alone. The no of pure MnCO3 is shown to be 1.816, and the specific gravity is 3.70. The theoretical specific gravity calculated from the molecular weight and the volume of a unit cell is considerably larger.

Rhodochrosite from Butte, Montana, is described and analyses of rhodochrosite and ankerite from Butte are given.

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