Kutnohorite
Kutnohorite, Wessels Mine, Kalahari manganese fields, Northern Cape Province, South Africa. Size 4.4 x 4.2 x 1.9 cm
General
Category	Carbonate mineral
Formula (repeating unit)	CaMn2+(CO3)2
IMA symbol	Kut[1]
Strunz classification	5.AB.10
Dana classification	14.2.1.3
Crystal system	Trigonal
Crystal class	Rhombohedral (3) H-M symbol: (3)
Space group	R3
Unit cell	330.60 ų
Identification
Formula mass	215.0 g/mol (end member)
Color	White, pale pink or light brown
Crystal habit	Aggregates of bundled bladed crystals
Cleavage	Perfect on {1011}
Fracture	Subconchoidal
Tenacity	Brittle
Mohs scale hardness	3.5–4
Luster	Vitreous to dull
Streak	White to pale pink
Diaphaneity	Translucent
Specific gravity	3.12
Density	3.10–3.12
Optical properties	Uniaxial (−)
Refractive index	no = 1.710–1.727, ne = 1.519–1.535
Birefringence	0.191–0.192
Solubility	Soluble in acids
References	[2][3][4][5]

Kutnohorite is a rare calcium manganese carbonate mineral with magnesium and iron that is a member of the dolomite group. It forms a series with dolomite, and with ankerite. The end member formula is CaMn²⁺(CO₃)₂,^[6] but Mg²⁺ and Fe²⁺ commonly substitute for Mn²⁺, with the manganese content varying from 38% to 84%,^[2] so the formula Ca(Mn²⁺,Mg,Fe²⁺)(CO₃)₂ better represents the species. It was named by Professor Bukowsky in 1901 after the type locality of Kutná Hora, Bohemia, in the Czech Republic.^[7] It was originally spelt "kutnahorite" but "kutnohorite" is the current IMA-approved spelling.

Dolomite group

• Dolomite, CaMg(CO₃)₂
• Ankerite, CaFe²⁺(CO₃)₂
• Kutnohorite, CaMn²⁺(CO₃)₂
• Minrecordite, CaZn(CO₃)₂

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:

a = 4.915 Å, c = 16.639 Å^[2]

a = 4.8518(3) Å, c = 16.217(2) Å^[4][5]

a = 4.85 Å, c = 16.34 Å^[3]

Structure

The crystal class is trigonal 3, space group R3, the same as for the other members of the dolomite group. There are layers of (CO₃)(−2 groups perpendicular to the long crystal axis c, and between these layers there are layers of the cations Ca²⁺ and Mn²⁺.^[2] If there were perfect ordering amongst the cations they would separate into different layers, giving rise to the ordered sequence: Ca−(CO₃)−Mn−(CO₃)−Ca−(CO₃)−Mn−(CO₃)− along the c axis;^[2] not all specimens, however, display such ordering.^[8]

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 uniaxial (−) with refractive indices N_o = 1.710 to 1.727 and N_e = 1.519 to 1.535, similar to dolomite. The ordinary refractive index, N_o, 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 rhombs with curved faces, polycrystalline 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 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.

Occurrence

Kutnohorite occurs typically in manganiferous sediments, associated with rhodochrosite, aragonite and calcite.^[5] Notable occurrences include Tuscany, Italy and Kutná Hora, Czech Republic. It probably occurs at the Trepča Mines, Stari Trg, Kosovo, in the Balkans.^[9] At the Eldorado Mine, Ouray County, Colorado, US, it occurs as tiny white crystals partially encrusting quartz and dolomite.^[10] At the Ryujima Mine, Nagano Prefecture in Japan, magnesian kutnohorite occurs with quartz and rhodochrosite.^[11] The type locality is Poličany, Kutná Hora, Central Bohemia Region, Bohemia, Czech Republic, and type material is conserved at Harvard University, Cambridge, Massachusetts, US.^[5]