Sulfides: Chalcocite

Composition: Cu₂S

Crystal system: Orthorhombic, typically massive and finegrained

7.3.1
massive chalcocite with pyrite
Butte mine MT USA

Properties:

• Submetallic lustre
• Black color
• Black streak
• Knife scratches very easily; fingernail barely scratches or not at all (Mohs Hardness 2.5-3)
• High density
• Cleavage not obvious
• Conchoidal fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Occurrence:

Chalcocite is the predominant sulfide in secondary deposits formed at the water table as a result of the weathering of copper-bearing primary ore deposits. It also occurs with other sulfides in hydrothermal vein and replacement ore deposits.

7.3.2
massive chalcocite with pyrite 
Butte mine MT USA

Chalcocite is an important source of copper.