Cohenite

Chemical formula: Fe₃C

Cohenite is an iron carbide with a tin-white color, found mainly in iron meteorites and in some terrestrial environments.

## Characteristics Cohenite is a mineral from the carbide group, chemically classified as iron carbide. It forms elongated, lath-like, or acicular crystals, which rarely reach significant sizes. It most often occurs as inclusions and lamellae within the mass of iron meteorites. Its appearance is typically metallic, with a characteristic tin-white color that quickly tarnishes and darkens on a fresh surface due to oxidation. ## Physical Properties This mineral has a hardness ranging from 5.5-6 on the Mohs scale, placing it on par with some steel tools. It is brittle and opaque. Its density is significant, approximately 7.20 g/cm³, which is typical for iron-rich minerals. It exhibits distinct cleavage in three directions. The luster is strongly metallic. ## Colors and Varieties Cohenite is a mineral with a constant chemical composition and does not form color varieties. Its only observed color is tin-white, which over time may be covered by a darker film of oxides. ## History and Name The mineral was named in honor of Emil Wilhelm Cohen (1842-1905), a German mineralogist and petrographer from the University of Greifswald, who first described this mineral in 1889. The discovery was made on material from the Magura meteorite, found in present-day Slovakia. ## Uses Cohenite has no direct industrial application. Its significance is primarily scientific, serving as an indicator of the conditions prevailing during the formation of asteroid parent bodies. It is also a valued collector's item, especially in the form of well-formed inclusions in cut and etched iron meteorite slices.

Properties

Mohs hardness: 5.5-6
Luster: Metallic
Streak: Black
Density: 7.20
Cleavage: {100}, {010}, and {001}.
Fracture: Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Cohenite is identified by its metallic luster, tin-white color (on a fresh fracture), and geological context – its occurrence in iron meteorites. Its hardness (higher than kamacite and taenite) is also a helpful characteristic. In acid-etched meteorite slices, cohenite remains bright and does not show Widmanstätten figures, which distinguishes it from the surrounding iron-nickel alloys. ## Differentiation from Similar Minerals From kamacite and taenite, the main components of iron meteorites, cohenite differs by its greater hardness and brittleness. Schreibersite, another common accessory mineral in meteorites, is also very similar but often has a more yellowish hue and is even harder. Accurate differentiation of these phases often requires laboratory analysis. ## Crystal Forms Cohenite forms tabular, lath-like, or acicular crystals. It usually occurs as inclusions oriented along the crystal structure of the surrounding kamacite. It is rarely found as isolated, well-formed crystals.

Geological environment

## Genesis Cohenite is a mineral formed under strongly reducing conditions, at high pressure, and in the presence of carbon. Its primary formation environment is the interiors of asteroids, from which iron meteorites originate. It crystallizes during the very slow cooling of iron-nickel alloy. On Earth, its occurrence is extremely rare. It has been found in some rocks formed under high pressure and temperature conditions in the Earth's mantle, as well as a product of human activity in industrial processes (as a component of steel). ## Mineral Associations This mineral primarily co-occurs with kamacite and taenite (the main components of iron meteorites). It is also accompanied by other accessory minerals typical of this environment, such as schreibersite, troilite, and graphite. ## Localities The most important cohenite localities are associated with known iron meteorites. Classic localities include the Magura meteorite (Slovakia), Canyon Diablo (USA), Campo del Cielo (Argentina), and Sikhote-Alin (Russia). Terrestrial occurrences are very rare and include rocks from Disko Island (Greenland), among others.

Rarity

Rare

For collectors

## Quality Criteria The collector's appeal of cohenite is inextricably linked to the quality of the meteorite specimen in which it occurs. Most valued are cut and etched meteorite slices that display large, well-formed, silvery cohenite crystals, contrasting with the darker background of kamacite and taenite. Specimens where cohenite forms distinct, sharp structures are rated higher than those with fine, dispersed inclusions. The state of preservation, meaning the absence of rust and signs of oxidation, is crucial for the specimen's value. ## Popular Localities Among the most sought-after specimens by collectors are those from classic iron meteorites, such as Canyon Diablo (USA), where cohenite can be abundant, and Mundrabilla (Australia), known for its large crystals of this mineral.

Care and storage

## Cleaning Cohenite specimens, especially those in a meteorite matrix, are very susceptible to oxidation. Cleaning should be limited to an absolute minimum. If necessary, compressed air can be used to remove dust. Contact with water and any chemical agents should be avoided. ## What to Avoid The greatest threat to cohenite is moisture and oxygen, which cause its rapid rusting and decomposition. Specimens should absolutely be stored in dry places. Touching with bare hands is also inadvisable, as acids and moisture from hands accelerate corrosion. ## Storage The best storage method is to keep specimens in sealed, acid-free containers with a desiccant (e.g., silica gel). In the case of meteorite slices, they are often coated with a thin layer of specialized oil or lacquer to protect them from air exposure.