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Kutnohorite is a rare calcium manganese carbonate mineral with the formula in the dolomite group of minerals. It forms a solid solution with the other group members dolomite and ankerite. The mineral was originally spelt "kutnahorite" but "kutnohorite" is the current IMA-approved spelling.
Kutnohorite occurs typically in Manganese sediments, associated with rhodochrosite, aragonite and calcite. Notable occurrences include Tuscany, Italy and Kutná Hora, Czech Republic.
It probably occurs at the Trepča Mines, Stari Trg, Kosovo, in the Balkans.The Mineralogical Record (2007) 38-4:284 At the Eldorado Mine, Ouray County, Colorado, US, it occurs as tiny white crystals partially encrusting quartz and dolomite.Rocks & Minerals (2009) 84-5:423
At the Ryujima Mine, Nagano Prefecture in Japan, magnesian kutnohorite occurs with quartz and rhodochrosite.Akio Tsusue (1967) Magnesian Kutnahorite from Ryujima Mine, Japan. American Mineralogist 52:1751
Composition
Specimens of Kutnohorite typically differ from the ideal formula , the manganese content varying from 38% to 84%. Manganese is commonly subsituted by and , so the formula better represents the species.
Unit cell
There are three formula units per unit cell (Z = 3) and the lengths of the sides are a close to 4.9 Å and c between 16 Å and 17 Å, although different sources give slightly different values, as follows:
Structure
The crystal class is trigonal , space group R, the same as for the other members of the dolomite group. There are layers of groups perpendicular to the long crystal axis c, and between these layers there are layers of the cations and . If there were perfect ordering amongst the cations they would separate into different layers, giving rise to the ordered sequence: along the c axis; not all specimens, however, display such ordering.Peacor, D R, Essene, E J and Gaines, A M (1987) Petrologic and crystal-chemical implications of cation order-disorder in kutnahorite. American Mineralogist 72:319
Properties
Optical properties
Kutnohorite may be white, pale pink or light brown. The pink shades are due to increased manganese and the brown colours are due to increased iron content. The mineral is translucent with a white to pale pink streak and vitreous to dull luster. It is Birefringence with Refractive index No = 1.710 to 1.727 and Ne = 1.519 to 1.535, similar to dolomite. The Birefringence, No, is high, comparable with spinel (1.719).
Physical properties
Kutnohorite occurs as aggregates of bundled blades of white through rose pink to light brown crystals. Also as simple Rhombus with curved faces, spherules and in massive and granular habits. It has perfect rhombohedral cleavage, typical of carbonates. It is brittle with a subconchoidal fracture and it is quite soft, with Mohs hardness 3.5 to 4, between calcite and fluorite. Specific gravity is 3.12, denser than both dolomite and calcite. It is soluble in acids, as are all carbonates.
Dolomite group
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