Chalcocite

Chalcocite (/ˈkælkəˌsaɪt/),^[6][7] 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.

Chalcocite
Chalcocite from Cornwall
General
Category	Sulfide minerals
Formula	copper(I) sulfide: Cu2S
IMA symbol	Cc[1]
Strunz classification	2.BA.05a
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	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
Identification
Color	Dark gray to black
Crystal 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 scale hardness	2+1⁄2–3
Luster	Metallic
Streak	Shiny black to lead gray
Diaphaneity	Opaque
Specific gravity	5.5–5.8
Fusibility	2–2.5
References	[2][3][4][5]

The term chalcocite from the Greek khalkos, meaning "copper". It is also known as redruthite, vitreous copper, or copper-glance.^[8]

Occurrence

Chalcocite is sometimes found as a primary vein mineral in hydrothermal veins. However, most chalcocite occurs in the supergene enriched environment below the oxidation zone of copper deposits as a result of the leaching of copper from the oxidized minerals. It is also often found in sedimentary rocks.

It has been mined for centuries and is one of the most profitable copper ores. The reasons for this is its high copper content (66.7% atomic ratio and nearly 80% by weight) and the ease at which copper can be separated from sulfur.

Since chalcocite is a secondary mineral that forms from the alteration of other minerals, it has been known to form pseudomorphs of many different minerals. A pseudomorph is a mineral that has replaced another mineral atom by atom, but it leaves the original mineral's crystal shape intact. Chalcocite has been known to form pseudomorphs of the minerals bornite, covellite, chalcopyrite, pyrite, enargite, millerite, galena and sphalerite.

Chalcocite crystals from the Mammoth Mine, Mount Isa – Cloncurry area, Queensland, Australia (size: 3.0 x 2.9 x 2.4 cm)

See also

• Copper(I) sulfide
• Copper monosulfide