Kutnohorite

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
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
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	3½ to 4
Luster	Vitreous to dull
Streak	White to pale pink
Diaphaneity	Translucent
Specific gravity	3.12
Optical properties	Uniaxial (–)
Refractive index	No = 1.727, Ne = 1.535 No = 1.710 to 1.727, Ne = 1.519 to 1.535
Diagnostic features	Pink colour, crystal habit and density.
Solubility	Soluble in acids
References

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, but Mg²⁺ and Fe²⁺ commonly substitute for Mn²⁺, with the Mn content varying from 38% to 84%, 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. It was originally spelt “kutnahorite” but “kutnohorite” is the current IMA-approved spelling.

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:

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⁺. 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, not all specimens, however, display such ordering.

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).