Cobaltomenite

Chemical formula: Co²⁺Se⁴⁺O₃·2H₂O

Cobaltomenite is a hydrated cobalt selenite, forming small, prismatic crystals of a characteristic, intensely pinkish-red color.

## Characteristics Cobaltomenite is a hydrated cobalt selenite, belonging to the rare secondary minerals. It forms small, tabular or prismatic crystals, often grouped into radial or fan-shaped aggregates, as well as crusts. Its most characteristic feature is its intense, deep color, ranging from pinkish-red to dark red, which distinguishes it from the host rock. Specimens are usually small, and well-formed crystals are rare. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2-2.5, making it susceptible to scratching. It is quite brittle, and its fracture is conchoidal. The density is approximately 3.42 g/cm³. The luster of cobaltomenite is described as vitreous. Crystals can be transparent to translucent. ## Colors and Varieties Cobaltomenite occurs in a characteristic range of colors from pinkish-red, through dark red, to reddish-brown. Some specimens may show yellowish-brown hues. No named varieties are distinguished. ## History and Name The mineral's name, given in 1882 by E. Bertrand and E. Damour, refers to its chemical composition – the presence of cobalt – and the Greek word *men* meaning moon, which is an allusion to selenium (from Greek *selene* - moon). It was discovered in Cerro de Cacheuta in Mendoza Province, Argentina. ## Uses Due to its rarity and small crystal size, cobaltomenite has no industrial applications. However, it is a valued and sought-after collector's mineral.

Properties

Mohs hardness
2-2.5
Luster
Vitreous
Streak
dark pink
Density
3.42
Fracture
Conchoidal
Transparency
Transparent,Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification The key diagnostic feature of cobaltomenite is its intense, pinkish-red color, often combined with characteristic radial aggregates of small crystals. A pink streak is also a good indicator. Occurrence in the oxidation zones of selenium deposits is an important field clue. ## Distinguishing from Similar Minerals Cobaltomenite can be confused with erythrite (cobalt bloom), which has a very similar color and is also associated with cobalt. However, erythrite is an arsenate, not a selenite, and often forms acicular crystals with a more silky or pearly luster on cleavage surfaces. Accurate differentiation without chemical analysis can be difficult. ## Crystal Forms Cobaltomenite crystals are usually small, with a tabular or short-prismatic habit. They most often occur as radial or fan-shaped clusters and aggregates, as well as thin crusts on other minerals.

Geological environment

## Genesis Cobaltomenite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits containing selenium and cobalt. It forms as a result of the reaction of oxygen-rich waters with primary selenium minerals, such as clausthalite or umangite, in the presence of cobalt ions. ## Mineral Associations It often co-occurs with other selenites and selenates, such as chalcomenite, molybdomenite, as well as with ore minerals like clausthalite, umangite, and klockmannite. Baryte and calcite may also accompany it. ## Localities The most important and historical occurrence (type locality) is Cerro de Cacheuta in Mendoza Province, Argentina, from where classic specimens originate. Other known localities include the Tumiñico mine in Argentina and the Shinkolobwe uranium deposit in the Democratic Republic of Congo.

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, transparent crystals of an intense, deep, red color. Rich crusts and radial aggregates that create an aesthetic contrast with the light host rock are highly prized. Crystal size is a key factor – specimens with crystals exceeding a few millimeters are exceptionally rare and sought after. ## Popular Localities Classic, most valued cobaltomenite specimens come from its type locality – Cerro de Cacheuta in Argentina. They serve as the benchmark for this mineral and fetch the highest prices in the collector's market.

Care and storage

## Cleaning Cobaltomenite specimens should be cleaned with the utmost care due to their high brittleness and softness. For dust removal, it is best to use a soft brush or compressed air from a safe distance. Avoid washing with water, and if absolutely necessary, use distilled water and immediately, gently dry the specimen. ## What to Avoid Contact with chemicals, acids, and detergents should be strictly avoided. The mineral is sensitive to high temperatures and sudden changes in temperature. It should not be exposed to prolonged direct sunlight, which can cause it to fade. Protect from moisture. ## Storage Cobaltomenite should be stored in a separate, padded display box to prevent scratches and mechanical damage. It should not be stored with harder minerals. It is best displayed in a closed cabinet, away from heat sources and moisture.

External references

Sources

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