Cohenite

Cohenite
Structure of cohenite (or cementite)
General
Category	Native element mineral, carbide
Formula (repeating unit)	(Fe,Ni,Co)3C
Strunz classification	1.BA.05
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Unit cell	a = 5.09 Å, b = 6.74 Å, c = 4.52 Å; Z = 4
Identification
Color	Tin-white; oxidizes to light bronze then golden yellow
Crystal habit	Platy to needlelike crystals; also as rims on or in dendritic intergrowths with iron
Cleavage	Good on {100}, {010}, and {001}
Tenacity	Brittle
Mohs scale hardness	5.5–6
Luster	Metallic
Diaphaneity	Opaque
Specific gravity	7.2 – 7.65
Other characteristics	Strongly magnetic
References

Cohenite is a naturally occurring iron carbide mineral with the chemical structure (Fe, Ni, Co)₃C. This forms a hard, shiny, silver mineral which was named by E. Weinschenk in 1889 after the German mineralogist Emil Cohen, who first described and analysed material from the Magura meteorite found near Slanica, Žilina Region, Slovakia. Cohenite is found in rod-like crystals in iron meteorites.

On Earth cohenite is stable only in rocks which formed in a strongly reducing environment and contain native iron deposits. Such conditions existed in some places where molten magmas invaded coal deposits, e.g. on Disco Island in Greenland, or at the Bühl near Kassel in Germany.

Associated minerals include native iron, schreibersite, troilite and wustite.

Similar iron carbides occur also in technical iron alloys and are called cementite.