Chemistry:Cohenite

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Cohenite
Kristallstruktur Zementit.png
Structure of cohenite (or cementite)
General
CategoryNative element mineral, carbide
Formula
(repeating unit)
(Fe,Ni,Co)3C
Strunz classification1.BA.05
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPnma
Unit cella = 5.09 Å, b = 6.74 Å,
c = 4.52 Å; Z = 4
Identification
ColorTin-white; oxidizes to light bronze then golden yellow
Crystal habitPlaty to needlelike crystals; also as rims on or in dendritic intergrowths with iron
CleavageGood on {100}, {010}, and {001}
TenacityBrittle
Mohs scale hardness5.5–6
|re|er}}Metallic
DiaphaneityOpaque
Specific gravity7.2 – 7.65
Other characteristicsStrongly magnetic
References[1][2][3][4]

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.[2] Cohenite is found in rod-like crystals in iron meteorites.[6]

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 Disko Island in Greenland , or at the Bühl near Kassel in Germany .[4]

Associated minerals include native iron, schreibersite, troilite and wustite.[4]

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

See also

References