Selenopolybasite

Chemical formula: Cu₁+(Ag₁+,Cu₁+)₆Ag₁+₉Sb₃+₂(S₂-,Se₂-)₉Se₂-₂

Selenopolybasite is a rare sulfosalt mineral, a selenium analog of polybasite, forming tabular crystals with a metallic luster.

## Characteristics Selenopolybasite is a rare sulfosalt mineral, classified as a member of the polybasite group. It is a selenium analog of polybasite, meaning that selenium partially replaces sulfur in its chemical structure. It forms characteristic, thin, tabular crystals with a hexagonal outline, which often occur as isolated plates or small groups. Crystals are typically small, rarely exceeding a few millimeters. ## Physical Properties This mineral is opaque and characterized by a metallic luster. Its Mohs hardness ranges from 3 to 4, making it relatively soft. It is a brittle mineral, with an uneven to conchoidal fracture. The calculated density is approximately 6.36 g/cm³. ## Colors and Varieties Selenopolybasite is black to iron-black in color. In thin fragments, it may appear reddish translucent. No distinct color varieties or commercial varieties are recognized. ## History and Name The mineral's name, given in 1979 by Zdeněk Johan and Paul Picot, refers to its chemical composition – the dominance of selenium – and its structural similarity to polybasite. It was discovered and described based on material from the uranium mine in Předbořice, Czech Republic. ## Applications Selenopolybasite has no industrial applications. Its significance is purely scientific and collectible, as a rare and well-defined representative of selenium sulfosalts.

Properties

Mohs hardness: 3-4
Luster: Metallic
Streak: Black
Density: 6.36
Cleavage: Imperfect/poor on {001}
Fracture: Uneven to conchoidal
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Selenopolybasite is most easily recognized by its characteristic crystal habit – thin, hexagonal tablets. Its black color, metallic luster, and association with other silver and copper sulfosalts are key diagnostic features. Red internal reflections, visible in strong light at the edges of thin crystals, are also helpful in identification. ## Distinguishing from Similar Minerals Selenopolybasite is visually almost identical to polybasite. A definitive distinction between these two minerals is possible almost exclusively through advanced analytical methods, such as chemical analysis (EDS/WDS) to determine the selenium to sulfur ratio. It can also be confused with other black, tabular minerals with a metallic luster, such as hematite or molybdenite, but differs from them in hardness, crystal form, and occurrence environment. ## Crystal Forms Selenopolybasite crystals are typically thin to thick, tabular, with a hexagonal outline. They often show striations on the basal faces. They occur as single crystals or in small, loose aggregates.

Geological environment

## Genesis Selenopolybasite is a hydrothermal mineral. It forms in ore veins, developing under low and medium temperature conditions. Its crystallization is associated with the presence of solutions rich in selenium, silver, copper, antimony, and sulfur. ## Mineral Associations This mineral co-occurs with other sulfosalts and sulfides. In its type locality (Předbořice, Czech Republic), it was found in association with acanthite, argentopolybasite, chalcopyrite, pearceite, pyrite, sphalerite, native silver, stephanite, and uraninite. It also occurs in paragenesis with quartz and calcite. ## Localities The most important and well-documented selenopolybasite localities worldwide include: - **Czech Republic**: Předbořice, near Kutná Hora – the type locality where the mineral was discovered. - **Germany**: Clara Mine in Oberwolfach (Black Forest) and in veins in the Sankt Andreasberg area in the Harz Mountains. - **USA**: DeLamar Mine in Owyhee County, Idaho.

Rarity

Very rare

For collectors

## Quality Criteria The most prized selenopolybasite specimens by collectors are those with sharply defined, well-formed, hexagonal crystals with a strong, metallic luster. The attractiveness of a specimen is enhanced by the presence of multiple crystals forming an aesthetic group, as well as their size – although even millimeter-sized crystals are considered valuable. Contrasting placement on a light matrix, for example on quartz, is also desirable. Surface purity, absence of mechanical damage, and signs of oxidation are crucial for collectible value. ## Popular Localities Specimens from the type locality in Předbořice, Czech Republic, hold particular historical value. However, some of the world's best, large, and well-formed selenopolybasite crystals, highly valued in the collector's market, come from the DeLamar Mine in Idaho (USA).

Care and storage

## Cleaning Selenopolybasite specimens should be cleaned very carefully, preferably using compressed air to remove dust. Due to its brittleness and softness, contact with hard tools and brushes should be avoided. If wet cleaning is necessary, only distilled water and a soft brush should be used, and the specimen should then be immediately and thoroughly dried. ## What to Avoid Contact with all chemicals, including acids and detergents, which can damage the mineral's surface, should be avoided. Selenopolybasite is sensitive to atmospheric agents and can oxidize, so it should be protected from moisture and prolonged exposure to air. Sudden temperature changes and exposure to direct sunlight should also be avoided. ## Storage It is recommended to store selenopolybasite specimens in stable conditions, preferably in closed, padded boxes or mineralogical display cases, to protect them from mechanical damage, dust, and moisture. Storage in a dry environment, possibly with the use of desiccants, will help preserve its original appearance.

Sources

IMA Mineral List (CNMNC)