KUTNAHORITE

CaMn(CO₃)₂
Hexagonal, R3, a = 4.894, c = 16.50 Å, Z = 3

Although originally known from Czechoslovakia, the status of kutnahorite was unsettled until the definitive study of Frondel and Bauer (1955). The name roepperite was used by Kenngott (Brush, 1872a, 1872b) for the manganesian dolomite earlier reported by Roepper (1870), but the name was not accepted inasmuch as roepperite had previously been assigned to what is now fayalite. Additional data were given by Huang and Kerr (1960) and Cook (1973). The crystal structure was studied by Peacor et al. (1987a) who found Sterling Hill kutnahorite to be highly ordered. The free energy of formation of Sterling Hill kutnahorite at 25^o C and one atmosphere total pressure is -1981.8 kJ mol^-1 according to Mucci (1991), who also studied its solubility. See also Boettcher et al. (1992).

Description

Kutnahorite occurs as massive and granular material in veins; the color is almost always some shade of pink, although highly calcian material may be nearly white. The luster is vitreous; the rhombohedral cleavage is well-developed; cleavage surfaces are commonly curved; and large masses of coarsely-crystallized material are known. The density is 3.12-3.14 g/cm³. Indices of refraction, varying substantially, were given by Cook (1973) and Huang and Kerr (1960).

Composition         

Kutnahorite is a calcium manganese carbonate mineral. Several analyses are presented in Table 22; that by Peacor et al. (1987) of Sterling Hill material is very similar to one of the same specimen by Dr. Jun Ito, published by Cook (1973). Numerous unpublished analyses by the writer, of specimens from both Franklin and Sterling Hill, show minimal substitution of Fe, Mg, and Zn in kutnahorite; values for FeO, MgO, and  ZnO are all < 1.2 wt. %. Mucci (1991) obtained slightly higher values for one specimen. Some of the samples of Frondel and Bauer were further examined by J. R. Goldsmith and others who reported them to be two-carbonate assemblages, one of the phases having a near-kutnahorite composition with “several percent of Mg” and the other to be Ca-rich or Mn-rich, suggesting a solvus in the Ca-rich as well as in the Mn-rich area of the phase diagram (Goldsmith, 1959, 1983).

Occurrence and paragenesis

Kutnahorite is, according to Frondel (1972), the most common member of the dolomite group at Franklin and Sterling Hill, but is more abundant at Sterling Hill. This writer suggests dolomite is much more abundant. The Franklin kutnahorite studied by Frondel and Bauer (1955) occurred as anhedral masses up to 3 cm in size in a vein in franklinite ore. Sterling Hill kutnahorite also occurs in seams in franklinite ore, is commonly associated with willemite, and occurs in masses weighing many kilograms.

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