Alloclasite

Chemical formula: Co<sup>3+</sup>(AsS)<sup>3-</sup>

Alloclasite is a rare cobalt sulfarsenide, forming radial aggregates with a metallic luster, closely related to arsenopyrite.

## Characteristics Alloclasite is a rare mineral from the sulfide group, specifically a cobalt sulfarsenide. Its name comes from Greek words meaning "other" and "to break," referring to its distinct cleavage compared to similar minerals. It typically occurs in compact, granular, or radial aggregates. Well-formed crystals are extremely rare and have a prismatic or acicular habit, often forming stellate groupings. ## Physical Properties This mineral is characterized by a metallic luster and is opaque. It has a relatively high density, approximately 5.9 g/cm³. It is brittle, and its fracture is uneven. Its Mohs hardness is 5, placing it in the same category as apatite. ## Colors and Varieties Alloclasite is steel-gray to silver-white in color. On weathered surfaces, a pink erythrite coating or iridescent, colorful tarnishes may appear, resulting from oxidation. No commercial or color varieties are distinguished. ## History and Name The mineral was first described in 1866 by Theodor Kjerulf and Christian Peter Mauritz Berg. The name, given by Felix Kengott, comes from the Greek words *állos* (ἄλλος) - "other" and *klásis* (κλάσις) - "to break," referring to its distinct physical properties (mainly cleavage) from the visually similar marcasite. ## Uses Due to its rarity, alloclasite has no industrial application. It is solely an object of interest for collectors and scientists studying ore deposits.

Properties

Mohs hardness
5
Luster
Metallic
Streak
Black
Density
5.9
Cleavage
Indistinct on {101}
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Key diagnostic features of alloclasite include its steel-gray color, metallic luster, high density, and characteristic radial or stellate aggregates. It often co-occurs with other cobalt and arsenic minerals, which is an important clue. ## Distinguishing from Similar Minerals Alloclasite is difficult to distinguish from arsenopyrite and glaucodot without advanced studies (e.g., chemical analysis). Arsenopyrite typically has a brighter, more silvery hue and often forms better-developed, pseudo-orthorhombic crystals. Glaucodot is even rarer and visually almost identical. It is distinguished from cobaltite by the absence of regular crystal forms (cubes, octahedra). ## Crystal Forms Crystals are extremely rare, usually poorly formed, with a prismatic habit. The most common forms are radial, stellate, or fibrous aggregates, as well as granular masses and compact forms embedded in the host rock.

Geological environment

## Genesis Alloclasite is a hydrothermal mineral formed at moderate to low temperatures. It forms in ore veins rich in cobalt, nickel, and arsenic, often in association with carbonate rocks (dolomite, calcite). ## Mineral Associations It most commonly co-occurs with cobaltite, skutterudite, safflorite, arsenopyrite, glaucodot, as well as silver minerals, native bismuth, calcite, and quartz. Erythrite (cobalt bloom) forms as a result of its weathering. ## Localities The most important and historical localities, from which the best-formed specimens originate, are Oravița in Romania (type locality) and Bou Azzer in Morocco. It also occurs in small quantities in mines in the Cobalt region of Ontario (Canada), near Schneeberg in Germany, and in some deposits in China.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued by collectors are specimens with well-formed, radial or stellate aggregates, clearly contrasting with the host rock (matrix). Rare, macroscopic crystals are also highly prized. Luster, lack of weathering, and mechanical damage are crucial. Specimens from classic, historical localities, such as Oravița, command higher prices. ## Popular Localities The most known and desired specimens come from the Bou Azzer region in Morocco, which currently supplies the best specimens to the collector market. Historical specimens from Oravița in Romania are rare and of great importance to collectors specializing in minerals from type localities.

Care and storage

## Cleaning Alloclasite specimens should only be dry-cleaned, using a soft brush to remove dust. All contact with water and chemical agents should be avoided, as the mineral is unstable and easily oxidizes. ## What to Avoid Moisture must be absolutely avoided, as it accelerates the weathering process and the formation of secondary minerals (e.g., erythrite). It should not be cleaned in ultrasonic cleaners or exposed to acids or other chemicals. Prolonged exposure to air can cause dull tarnishes to form. ## Storage It is recommended to store specimens in dry, airtight containers (e.g., boxes with a membrane or desiccant), away from light and sudden temperature changes. It is best to store them under stable room conditions.

External references

Sources

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