Cupropolybasite
Chemical formula: [Cu¹⁺₆Sb³⁺₂S²⁻₇][Ag¹⁺₉Cu¹⁺S²⁻₄]
A brittle, black sulfosalt mineral, a copper analog of polybasite, found in tabular crystals with a hexagonal outline.
Properties
- Mohs hardness
- 3-3.5
- Color
- Black, dark gray
- Luster
- Metallic
- Streak
- Black
- Density
- 6.310
- Cleavage
- Imperfect on {001}
- Fracture
- None observed
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Cupropolybasite can be identified by its characteristic habit – hexagonal, tabular crystals. Key features include its black color, black streak, metallic luster, and relatively low hardness. It often occurs in association with other silver and copper sulfosalts. ## Distinguishing from Similar Minerals It can be confused with polybasite, from which it is visually indistinguishable – definitive differentiation requires chemical analysis (EDS, WDS) to determine the dominance of copper over silver in a specific structural position. Stephanite can also be similar, but it crystallizes in the orthorhombic system and forms different crystal habits, more often prismatic or pseudohexagonal twins. Hematite is much harder (5.5-6.5) and has a reddish-brown streak. ## Crystal Forms Crystals are most often thin- or thick-tabular, with a distinct hexagonal outline. The basal faces (pinacoid) may be striated with characteristic triangular patterns. It also occurs in granular aggregates and irregular masses.
Geological environment
## Genesis Cupropolybasite is a hydrothermal mineral. It forms in low- to medium-temperature ore veins rich in silver, copper, and antimony. ## Mineral Associations It most commonly co-occurs with other sulfosalts, such as polybasite, pearceite, tetrahedrite, and tennantite. It is also accompanied by sulfides, e.g., chalcopyrite, acanthite, sphalerite, galena, as well as native silver, quartz, calcite, and dolomite. ## Localities Important worldwide localities include: Silvana Mine in British Columbia (Canada); Fresnillo Mine in Zacatecas (Mexico); Harrys, Fefan, and Himmelsfurst in Saxony (Germany); Příbram and Jáchymov (Czech Republic); Comstock Lode Mine in Nevada and Mollie Gibson Mine in Aspen, Colorado (USA).
Rarity
Rare
For collectors
## Quality Criteria Specimens with well-formed, sharp crystals with a hexagonal outline and a strong, metallic luster are most highly valued by collectors. Groups of crystals, as well as aesthetic compositions with associated minerals, such as quartz or other sulfosalts, are highly prized. Crystal size and lack of damage significantly increase a specimen's value. ## Popular Localities The best quality specimens come from historical localities in Germany (e.g., Freiberg), the Czech Republic (Příbram), and from mines in Mexico (Fresnillo) and the USA (Aspen, Comstock Lode).
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Compressed air can be used from a safe distance. Due to its brittleness and softness, ultrasonic cleaners and mechanical cleaning should be avoided. ## What to Avoid Avoid contact with acids and other chemicals that can damage the mineral's surface. Cupropolybasite is sensitive to impacts and scratches. Prolonged exposure to humid air can cause patination and dulling of the luster. ## Storage Cupropolybasite specimens are best stored in separate, padded boxes or display cases to protect them from mechanical damage and dust. It is recommended to store them in stable conditions, away from sources of moisture and chemical contaminants.