Chalcocite
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Chalcocite, copper(I) sulfide, is an important copper ore mineral. It is opaque and dark gray to black, with a metallic luster. It has a hardness of 2.5–3 on the Mohs scale. It is a sulfide with a monoclinic crystal system.
The term chalcocite... Wikipedia
Chemical formula: Cu2S
Cleavage: Indistinct on
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Tabular to prismatic crystals also massive to granular, (pseudo-orthorhombic)
Crystal system: Monoclinic
Fracture: Conchoidal
IMA symbol: Cc