Auropolybasite

Chemical formula: [Ag₁+₉Au₁+S₂-₄][Ag₁+₆Sb₃+₂S₂-₇]

Auropolybasite is a rare sulfosalt of silver, gold, and antimony, forming tabular crystals with a metallic luster.

## Characteristics Auropolybasite is a complex sulfosalt from the pearceite-polybasite group, containing silver, gold, and antimony. It occurs as hexagonal, tabular crystals, often forming fan-shaped or rosette-like aggregates. Crystals are usually very small, rarely exceeding a few millimeters. The mineral is opaque and has a black color with a metallic luster. ## Physical Properties Auropolybasite has a hardness of approximately 3 on the Mohs scale. Its luster is metallic, and its streak is black. It is a brittle mineral with an uneven fracture. The calculated density is approximately 6.45 g/cm³. ## Colors and Varieties This mineral is black, sometimes with reddish internal reflections visible in very thin fragments under strong light. No color or commercial varieties are distinguished. ## History and Name Auropolybasite was first described in 2005 by E.A.J. Burke, C.J. Stanley, A.C. Roberts, P.C. Piilonen, M.A. Cooper, L. Horváth, A. Pring, F.C. Hawthorne, and J.D. Grice. Its name refers to its chemical composition – it is a gold-dominant (Latin *aurum* - gold) analog of polybasite, with which it forms a series. ## Uses Due to its extreme rarity and microscopic crystal sizes, auropolybasite has no industrial application. It is solely of scientific importance and is an object of interest for specialized collectors of rare minerals.

Properties

Mohs hardness: 3
Luster: Metallic
Streak: Black
Density: 6.45
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Identifying auropolybasite without advanced analytical methods is practically impossible. Preliminary identification is based on the characteristic habit of hexagonal, tabular crystals, black color, metallic luster, and co-occurrence with other gold and silver minerals in hydrothermal veins. Definitive identification requires chemical analysis (e.g., EDS) to confirm the presence of gold as the dominant element over copper. ## Distinguishing from Similar Minerals Auropolybasite is visually indistinguishable from other minerals in its group, especially polybasite and pearceite. Polybasite has a very similar habit and properties, but antimony dominates over arsenic, and silver over gold. Pearceite is the arsenic analog of polybasite. Distinguishing these minerals requires specialized research equipment. ## Crystal Forms Crystals are thin, tabular, with a hexagonal outline. They often form fan-shaped, rosette-like, or irregular aggregates. They also occur as single, isolated crystals grown on other minerals.

Geological environment

## Genesis Auropolybasite is a mineral of hydrothermal origin. It forms in low-temperature ore veins rich in precious metals (gold and silver) and sulfosalts. ## Mineral Associations This mineral co-occurs with other sulfides and sulfosalts. In its type locality (Krásná Hora mine, Czech Republic), it was found in association with quartz, calcite, pyrite, chalcopyrite, sphalerite, galena, tetrahedrite, bournonite, polybasite, pyrargyrite, and native gold. ## Localities Confirmed occurrences of auropolybasite are extremely rare. The type locality is the Staročeská vein in the Krásná Hora-Milešov mine in central Czech Republic. Another documented locality is the Comstock mine in the Virginia City district, Nevada, USA.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of an auropolybasite specimen primarily depends on the presence of well-formed, sharp crystals with a distinct hexagonal outline. Specimens where crystals form aesthetic, rosette-like or fan-shaped aggregates are exceptionally valued. Contrast with a light matrix rock (e.g., quartz) significantly enhances visual appeal. Due to the microscopic nature of most specimens, their size is crucial – any crystal exceeding 1-2 mm is considered exceptional. ## Popular Localities The only sources of specimens available on the collector's market are two known localities: the Krásná Hora mine in the Czech Republic and historical finds from the Comstock mine in Nevada, USA. Specimens from both locations are extremely rare and sought after by specialized collectors of systematic minerals and "micromount" type specimens.

Care and storage

## Cleaning Auropolybasite specimens are extremely delicate and rare. Cleaning should be kept to an absolute minimum. If necessary, compressed air can be used to remove loose dust particles. Avoid contact with water and any chemical agents. ## What to Avoid Avoid ultrasonic cleaners, chemical agents (acids, bases, solvents), high temperatures, and sudden temperature changes. The mineral is brittle, so it should be protected from impacts and scratching. Prolonged exposure to moisture can lead to slow oxidation. ## Storage Specimens should be stored in stable conditions, in a closed, padded display box, to protect them from dust, mechanical damage, and humidity changes. Due to the small size of the crystals, storage in "micromount" type boxes is ideal.

Sources

IMA Mineral List (CNMNC)