Chalcocite
Image: Robert M. Lavinsky (CC BY-SA 3.0)
Mineral NameChalcocite
Chemical FormulaCu₂S
Mohs Hardness2.5
Crystal Systemorthorhombic
Lustermetallic
Streak Colorgrayish black
Cleavageindistinct on {110}
Fractureconchoidal
Specific Gravity5.7
Colorsdark gray, lead-gray, black, often with blue or green tarnish
Transparencyopaque
Type LocalitySt Ives Consols (incl. Wellesley Mine; Wheal Mary), St Ives, St Ives District, Cornwall, England, UK
Mineral GroupSulfides
Usesimportant copper ore
Associated Mineralsbornite, covellite, chalcopyrite, pyrite, native copper, quartz, calcite
Raritycommon
Description
This specimen shows a dark, lead-gray to black mass of chalcocite, often exhibiting a dull metallic luster and sometimes a bluish or greenish tarnish. It forms dense, fine-grained aggregates or pseudohexagonal crystals.
Geological Context
Chalcocite is a secondary copper sulfide mineral, commonly found in the supergene enrichment zones of copper deposits where primary copper sulfides are altered by weathering. It can also occur in hydrothermal veins.
