Cochromite

Chemical formula: Co₂+Cr₃+₂O₄

Cochromite is a very rare cobalt and chromium oxide from the spinel group, forming microscopic, black, metallic grains.

## Characteristics Cochromite is an extremely rare mineral from the spinel group, being a cobalt and chromium oxide. It occurs as very fine, individual, octahedral crystals or rounded grains, usually not exceeding 0.1 mm in size. Due to its microscopic size, its visual features are difficult to observe without specialized equipment. It has a black color and a metallic luster. ## Physical Properties This mineral is characterized by high hardness, estimated at 7 on the Mohs scale. Its density is significant, approximately 5.14 g/cm³. The crystals exhibit a metallic luster and are opaque. The streak is gray. ## History and Name The name cochromite directly refers to its chemical composition – the presence of cobalt (Co) and chromium (Cr). The mineral was first described in 1978 by A. M. Asklund, B. L. Gruden, and S. A. H. Grundström based on material found in Outokumpu, Finland.

Properties

Mohs hardness: 7
Luster: Metallic
Streak: Gray
Density: 5.14
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Identification of cochromite is impossible without advanced laboratory techniques, such as chemical analysis (electron microprobe, EDS) and X-ray diffraction (XRD). Preliminary identification in polished sections under a reflected light microscope is based on its black color, metallic luster, high hardness, and co-occurrence with other chromium and cobalt minerals. ## Distinguishing from Similar Minerals Cochromite is visually indistinguishable from other black spinels, such as chromite, magnetite, or hercynite. Definitive differentiation requires confirmation of the presence of both cobalt and chromium as dominant elements in the appropriate positions within the crystal structure. ## Crystal Forms It forms very small, most often perfectly developed (euhedral) octahedral crystals. It also occurs as rounded, irregular grains embedded in the rock.

Geological environment

## Genesis Cochromite forms under specific metamorphic conditions. At the type locality (Outokumpu, Finland), it was found in quartzitic rocks and skarns associated with copper, zinc, and cobalt sulfide deposits that underwent metamorphism under amphibolite facies conditions. ## Mineral Associations This mineral co-occurs with many other metamorphic and ore minerals. In Outokumpu, it was found in association with tremolite, uvarovite, talc, pentlandite, pyrite, chalcopyrite, sphalerite, karelianite, eskolaite, and chromite. ## Localities Cochromite is an extremely rare mineral, confirmed in only a few places worldwide. Besides the type locality in Outokumpu, Finland, its presence has also been reported in Spain (Aguablanca nickel-copper deposit) and in the Bushveld Igneous Complex in South Africa.

Rarity

Extremely rare

For collectors

## Quality Criteria Due to its microscopic nature, cochromite is not a mineral collected for aesthetic purposes. The collector's value of a specimen lies solely in its rarity and scientific significance. The most valuable samples are those from the type locality, with analytically confirmed presence of cochromite, preferably in a rich association with other rare minerals. The specimen is de facto a fragment of the host rock, and its value is enhanced by a clear label and analytical documentation.

Care and storage

## Cleaning Due to its microscopic size and occurrence as inclusions in other minerals, isolated cleaning of individual cochromite crystals is practically impossible and unnecessary. Specimens of the host rock can be cleaned conventionally, e.g., with distilled water and a soft brush. ## What to Avoid Avoid contact with strong acids, which could damage both cochromite and the surrounding minerals. Despite its high hardness, specimens should be protected from impacts and abrasion, which could damage the delicate host rock. ## Storage Specimens containing cochromite should be stored in stable conditions, away from dust and moisture. It is best to keep them in closed collector's boxes or display cases, with a label containing the exact locality.

Sources

IMA Mineral List (CNMNC)