Rhodochrosite occurs in small (<1 cm) rhombohedral crystals which may have curved faces. It occurs as massive material, druses, late-stage coatings, and components of vein assemblages. The color is commonly strong pink but grades through light pink to nearly white, and much material may be misidentified as calcite or kutnahorite. Altered material can have a chalky appearance. Rhodochrosite cannot be reliably distinguished from kutnahorite or manganoan calcite by visual examination. Frondel and Bauer reported densities varying from 3.5-3.6 g/cm³ and the w index of refraction varying from 1.791 to 1.806. Rhodochrosite has a perfect rhombohedral cleavage, not always evident in fine-grained material, and a vitreous luster. There is no discernible fluorescence in ultraviolet. Rhodochrosite is best differentiated from kutnahorite using X-ray methods. Some Franklin rhodochrosite and manganoan calcite specimens were included in the thermal studies of Kulp et al. (1949).

Composition

Rhodochrosite is a manganese carbonate mineral. Extant analyses by Browning (1890) and Frondel and Bauer (1955) (Table 22) show that the studied specimens have between 75 and 95 mole % MnCO₃; most are calcian. However, relatively few specimens have been analyzed, and local solid solution limits of Zn, Fe, and Mg are unstudied. Frondel and Bauer (1955) reported a local miscibility gap between calcite and rhodochrosite between 40 and 75 mole % MnCO₃, but Frondel (1972) reported a solid solution series with “only a small central gap” between calcite and rhodochrosite.

Occurrence and paragenesis

At Franklin, rhodochrosite is known as massive material, and fine crystals occur associated with willemite and with fluorite.

	Figure 23-12. Hemispherules of rhodochrosite encrusting serpentinized franklinite-willemite ore; they are in turn coated with other carbonates, a layer of serpentine, and finally massive willemite (white); from Franklin. Specimen is 5 cm in maximum dimension. Smithsonian Institution, #143734. Photo by Vic Krantz.

The preponderance of the preserved specimen material is from veins such as are shown in figures 12-45 and 12-46. Rhodochrosite is a common constituent of the carbonate veins at Franklin, associated with willemite, dolomite, and serpentine and providing very attractive specimens. Frondel (1972) has noted that kutnahorite, rhodochrosite, and calcite may coexist in a single veinlet; the writer has not observed this association. Some rhodochrosite occurs as the pink carbonate mineral commonly associated with pyrochroite; some of this material may be manganoan calcite.

At Sterling Hill, as at Franklin, rhodochrosite is chiefly a vein mineral, occurring commonly with willemite and with serpentine, especially in the north orebody and the lower reaches of the mine, where calcite, sphalerite, and other minerals are associated (Figures 22-72 and 22-73).

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
Link to homepage
This page created: January 13, 2001