Cohenite
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.[5]
| 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 | [1][2][3][4] |
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.[4]
Associated minerals include native iron, schreibersite, troilite and wustite.[4]
Similar iron carbides occur also in technical iron alloys and are called cementite.
References
- Mineralienatlas
- Mindat.org Cohenite
- Webmineral.com Cohenite
- Handbook of Mineralogy
- Vagn F. Buchwald, Handbook of Iron Meteorites, University of California Press, 1975 ISBN 978-0520029347