Chalcocite

Formula:  Cu₂S

Origin of name:  From Greek for copper

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:  Very common on {230}, stellate

Unit cell: a = 15.24 Ǻ, b = 11.89 Ǻ, c = 13.50 Ǻ, β = 116.3°

Colour:  Blackish gray to black

Diapaneity: Opaque

Luster:  Metallic

Habit:  Crystals pseudo-hexagonal prisms formed by twinning; short prismatic or thick tabular; massive

Hardness:  2.5 to 3

Specific gravity:  5.5 to 5.8

Cleavage:  1; {110} indistinct

Tenacity:  Conchoidal; brittle, somewhat sectile

Streak:  Blackish gray

Synonyms/varieties:  Chalcosine, redruthite

Comments: Uncommon as primary hydrothermal mineral but important as secondary mineral. Commonly found in or below the oxidation zone in hydrothermal veins and large low-grade porphyry copper orebodies.