Cobalttsumcorite

Chemical formula: Pb₂+Co₂+₂(As₅+O₄)₂·₂H₂O

Cobaltotsumcorite is a rare lead cobalt arsenate, forming characteristic reddish-purple crystals and aggregates.

## Characteristics Cobaltotsumcorite is a hydrated lead cobalt arsenate belonging to the tsumcorite group. It forms small but well-formed tabular or prismatic crystals, often grouped into radial, spherical, or rosette aggregates. Its most characteristic feature is an intense reddish-purple to purplish-red color. It is a secondary mineral, forming in the oxidation zones of ore deposits. ## Physical Properties Cobaltotsumcorite crystals exhibit a luster from vitreous to slightly adamantine. The mineral is translucent to opaque. It is relatively soft, with a Mohs hardness of 4.5. It is a rather heavy mineral due to its lead content, and its density is approximately 5.2 g/cm³. ## Colors and Varieties The dominant and essentially sole color of this mineral is reddish-purple in various shades, from purplish-red to violet-red. No color or commercial varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominance of cobalt – and its relation to tsumcorite, another mineral from the same group. It was described as a new mineral in 1999 based on specimens found in the Tsumeb mine in Namibia, which is its type locality. ## Uses Due to its rarity, small crystal size, and toxic arsenic content, cobaltotsumcorite has no industrial applications. It is solely a mineral of scientific and collector's interest.

Properties

Mohs hardness: 4.5
Luster: Vitreous to adamantine
Streak: Light red
Density: 5.2
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of cobaltotsumcorite is its intense reddish-purple color and characteristic forms of occurrence – radial aggregates of small, tabular crystals. Co-occurrence with other arsenates in oxidation zones is also an important clue. ## Distinguishing from Similar Minerals Cobaltotsumcorite can be confused with erythrite, which also has a similar color and forms radial aggregates. However, erythrite is much lighter and softer (hardness 1.5-2.5). It can also be confused with cobaltian calcite or dolomite, which, however, react strongly with acids, unlike cobaltotsumcorite. ## Crystal Forms Crystals are typically small, with a tabular or short-prismatic habit. They most often occur as radial, spherical, or fan-shaped aggregates, as well as coatings and crusts.

Geological environment

## Genesis Cobaltotsumcorite is a secondary mineral, forming in the oxidation zones (so-called iron caps) of polymetallic deposits rich in arsenic, lead, and cobalt. It forms as a result of the weathering of primary sulfides and arsenides, such as skutterudite, cobaltite, or galena. ## Mineral Associations This mineral often co-occurs with other secondary arsenates and minerals of the oxidation zone. Typical associated minerals include tsumcorite, skutterudite, germanite, galena, tennantite, anglesite, quartz, and dolomite. ## Localities The most important and classic locality for cobaltotsumcorite is the Tsumeb mine in Namibia, from which the best specimens originate. It is also known from several other localities worldwide, including Bou Azzer in Morocco and a mine in Saxony, Germany.

Rarity

Very rare

For collectors

## Quality Criteria The most prized cobaltotsumcorite specimens are those that exhibit an intense, saturated reddish-purple color. Well-formed, radial aggregates and rosettes, contrasting with a light matrix rock (e.g., white dolomite or quartz), are highly valued. The size of the aggregates and the absence of mechanical damage also significantly increase the specimen's collector value. ## Popular Localities By far the most sought-after specimens by collectors come from the type locality – the Tsumeb mine in Namibia. Specimens from Bou Azzer in Morocco are also valued, although they typically form smaller aggregates.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its arsenic content, contact with water should be minimized. If water is necessary, it should be distilled water, and the specimen should be thoroughly dried immediately. Always wash hands after handling the mineral. ## What to Avoid Avoid contact with acids and other chemicals that can damage the mineral. Do not heat it or clean it with ultrasonics. Prolonged exposure to moisture can lead to its slow decomposition. ## Storage Cobaltotsumcorite specimens should be stored in a dry place, in closed boxes or display cases, to protect them from dust and mechanical damage. Due to arsenic toxicity, it should be kept away from children and pets, and separate from minerals that might react with it.

Sources

IMA Mineral List (CNMNC)