Cobaltarthurite

Chemical formula: Co₂+Fe₃+₂(As₅+O₄)₂(OH)₂·₄H₂O

Cobaltarthurite is a rare, hydrated cobalt and iron arsenate, forming aggregates of small, acicular crystals with an intense green color.

## Characteristics Cobaltarthurite is a secondary mineral from the arthurite group, chemically classified as a hydrated cobalt and iron arsenate. It occurs as aggregates of small, acicular or capillary crystals, which can form crusts, coatings, or small, radial aggregates. Individual crystals are usually microscopic in size, and their forms are rarely visible to the naked eye. The mineral is characterized by a vivid, emerald-green or yellowish-green color. ## Physical Properties The mineral's hardness on the Mohs scale is approximately 3-4, making it relatively soft. It has a vitreous luster and is translucent. Its calculated density is 3.35 g/cm³. Due to its microcrystalline nature, features such as cleavage or fracture are difficult to observe in macroscopic specimens. ## Colors and Varieties This mineral occurs in shades of green, from emerald-green to yellowish-green. No distinct color varieties or commercial varieties have been identified. ## History and Name The name "cobaltarthurite" refers to its chemical composition – the dominance of cobalt – and its structural relationship to the mineral arthurite. It was described as a new mineral in 1996 by J.A. Mandarino, J.D. Grice, and P.J. Dunn. Type specimens come from the nickel-cobalt mine in La Reconquistada, Spain. ## Uses Cobaltarthurite has no industrial application. Its significance is purely scientific and as a collector's item, being valued for its rarity and attractive appearance.

Properties

Mohs hardness: 3-4
Luster: Vitreous
Streak: Pale green
Density: 3.35
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Cobaltarthurite is primarily recognized by its characteristic appearance: small, acicular crystals forming radial aggregates or crusts of a vivid green color. Key to identification is the locality (oxidation zones of ore deposits rich in cobalt and arsenic) and co-occurrence with other arsenates. ## Distinguishing from Similar Minerals It can be confused with other secondary green-colored copper, nickel, or cobalt minerals, such as annabergite (usually lighter, nickel-green), olivenite (usually darker, olive-green), or other minerals from the arthurite group. Definitive differentiation often requires advanced chemical analyses (EDS) to confirm the dominance of cobalt and iron. ## Crystal Forms Crystals are elongated, acicular, or capillary. They usually form radial or chaotic aggregates, as well as thin crusts and coatings on the surface of the host rock.

Geological environment

## Genesis Cobaltarthurite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits that are rich in arsenic and cobalt. It forms as a result of low-temperature hydrothermal processes occurring in the presence of oxygen-rich waters. ## Mineral Associations This mineral co-occurs with other arsenates and sulfates formed under similar conditions. It is most often accompanied by: pharmacosiderite, scorodite, erythrite, annabergite, goethite, and also minerals from the jarosite group. ## Localities The most important and classic locality, from which type specimens originate, is the La Reconquistada mine in Pastrana, Murcia region, Spain. Other confirmed localities include the Clara mine in Oberwolfach (Black Forest, Germany) and several places in the Bou Azzer district in Morocco, known for its rich cobalt and arsenic deposits.

Rarity

Very rare

For collectors

## Quality Criteria Most valued by collectors are specimens with well-formed, rich aggregates of acicular crystals with an intense, emerald-green color. Samples where cobaltarthurite forms an aesthetic contrast with associated minerals or the host rock are also highly regarded. Due to the microscopic size of the crystals, the quality of a specimen is often assessed under magnification. ## Popular Localities Specimens from the type locality in Spain (La Reconquistada) and from the Clara mine in Germany are most sought after by collectors specializing in rare minerals or the systematics of secondary minerals.

Care and storage

## Cleaning Cobaltarthurite specimens are extremely delicate and fragile. Cleaning should be kept to an absolute minimum. If necessary, compressed air can be used from a safe distance to remove loose dust particles. Avoid contact with water and any chemical agents. ## What to Avoid Ultrasonic cleaners, steam cleaning, acids, solvents, and detergents should be absolutely avoided. The mineral is sensitive to high temperatures, which can lead to dehydration and destruction. It should be protected from direct sunlight, which can cause fading. Due to the acicular form of its crystals, it is very susceptible to mechanical damage. ## Storage It is recommended to store specimens in closed, stable containers (e.g., "micromount" type) to protect them from dust, moisture, and physical damage. Display should be in conditions of controlled humidity and away from sources of vibration and direct light.

Sources

IMA Mineral List (CNMNC)