chalcocite

{{Infobox mineral | name = Chalcocite | category = Sulfide minerals | boxwidth = | boxbgcolor = | image = Chalcocite-253980.jpg | caption = Chalcocite from Cornwall | formula = copper(I) sulfide: Cu2S | IMAsymbol = Cc | molweight = | strunz = 2.BA.05a | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/c | unit cell = a = 15.246(4) Å, b = 11.884(2) Å, c = 13.494(3) Å; β = 116.35(1)°; Z = 48 | color = Dark gray to black | habit = Tabular to prismatic crystals also massive to granular, (pseudo-orthorhombic) | twinning = Common on {110} yielding pseudohexagonal stellate forms | cleavage = Indistinct on {110} | fracture = Conchoidal | tenacity = Brittle to sectile | mohs = –3 | luster = Metallic | refractive = | opticalprop = | birefringence = | pleochroism = | streak = Shiny black to lead gray | gravity = 5.5–5.8 | melt = | fusibility = 2–2.5 | diagnostic = | solubility = | diaphaneity = Opaque | other = | references = }}

Chalcocite (), copper(I) sulfide (Cu2S), 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.