Arsenpolybasite

Chemical formula: (Ag,Cu)₁₆As₂S₁₁

Arsenpolybasite is a rare sulfosalt mineral, an arsenic analogue of polybasite, prized by collectors for its well-formed, pseudohexagonal crystals.

## Characteristics Arsenpolybasite is a silver, copper, and arsenic sulfosalt belonging to the polybasite group. It forms tabular, pseudohexagonal crystals, often with a triangular outline, which can be single or form fan-shaped aggregates. Crystals are typically thin and brittle, with characteristic horizontal striations on the side faces. Its appearance is very similar to the closely related polybasite. ## Physical Properties This mineral is characterized by a metallic to submetallic luster. It is opaque. Its Mohs hardness is 3, making it a relatively soft mineral. The density ranges from 6.15 to 6.23 g/cm³. ## Colors and Varieties Arsenpolybasite ranges in color from iron-black to steel-gray. In thin splinters, it may be translucent red. No distinct color or commercial varieties are recognized; it is classified primarily based on its chemical composition as the arsenic-dominant member of the polybasite group. ## History and Name The mineral's name, given in 1981 by P.J. Dunn, R.C. Rouse, B.D. Sturm, and J.A. Nelen, refers to its chemical composition (dominance of arsenic) and structural relationship to polybasite. It was recognized by the International Mineralogical Association (IMA) as a distinct mineral species. It replaced the previously used name "eugenite". ## Uses Due to its rarity and small accumulations, arsenpolybasite has no significance as a silver ore. However, it is a valuable and sought-after specimen for advanced mineral collectors, especially those with well-formed crystals.

Properties

Mohs hardness: 3
Luster: Metallic
Streak: Black
Density: 6.15-6.23
Cleavage: Imperfect on {001}
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of arsenpolybasite is its crystal habit – thin, tabular, pseudohexagonal plates, often with a triangular outline and striations on the side faces. Metallic luster and black or steel-gray color are also characteristic. Red internal reflections visible in strong light can be a helpful clue. ## Distinguishing from Similar Minerals Arsenpolybasite is visually indistinguishable from polybasite without advanced chemical analysis (e.g., EDS) to confirm the dominance of arsenic over antimony. It can also be confused with stephanite, which, however, forms thicker, prismatic or tabular crystals and does not exhibit red internal reflections. Other similar sulfosalts, such as pearceite, also require chemical analysis for certain identification. ## Crystal Forms Crystals are typically thin, tabular, with a pseudohexagonal or triangular outline. They often occur in fan-shaped or rosette-like aggregates. Characteristic horizontal striations are visible on the side prism faces.

Geological environment

## Genesis Arsenpolybasite is a low- to medium-temperature hydrothermal mineral. It forms in ore veins, usually rich in silver, arsenic, and sulfur. It is one of the later minerals to crystallize in the deposit formation sequence. ## Mineral Associations It most commonly co-occurs with other silver sulfosalts and sulfides, such as polybasite, pearceite, stephanite, acanthite, proustite, as well as with quartz, calcite, barite, and various base metal sulfides (galena, sphalerite, chalcopyrite). ## Localities Key global localities known for well-formed specimens include: - Imiter Mine in Morocco, known for excellent crystals. - Freiberg and St Andreasberg in Germany (historic localities). - Husky Mine in Yukon Territory, Canada. - Guanajuato in Mexico. - Potosí in Bolivia. - Uchucchacua Mine in Peru.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued arsenpolybasite specimens are those with sharp, well-defined, undamaged crystals with a distinct pseudohexagonal outline. Large, single crystals or aesthetic groups on a contrasting matrix (e.g., on quartz or calcite) command the highest prices. Luster, absence of mechanical damage, and distinct striations on the side faces also significantly increase the collectible value of a specimen. ## Popular Localities Collectors particularly seek specimens from the Imiter Mine in Morocco, which are considered among the best in the world in terms of crystal size and form. Historic specimens from classic European localities, such as Freiberg in Germany, are also highly prized.

Care and storage

## Cleaning Arsenpolybasite specimens should be cleaned with utmost care due to their brittleness. It is recommended to use a soft brush to remove dust. If wet cleaning is necessary, use only distilled water and immediately dry the specimen thoroughly, for example, with compressed air. ## What to Avoid Contact with chemicals, acids, and detergents should be strictly avoided. The mineral is sensitive to moisture, which can cause its slow decomposition. It should not be heated or exposed to prolonged intense light, which can accelerate oxidation processes. ## Storage Arsenpolybasite should be stored under stable, dry conditions. It is best placed in a closed, padded display box or cabinet to protect it from mechanical damage, dust, and humidity fluctuations.

Sources

IMA Mineral List (CNMNC)