Enargite

Enargite is a copper arsenic sulfosalt mineral with formula Cu₃AsS₄. It takes its name from the Greek word enarge, "distinct". Enargite is a steel gray, blackish gray, to violet black mineral with metallic luster. It forms slender orthorhombic prisms as well as massive aggregates. It has a hardness of 3 and a specific gravity of 4.45.

Enargite
General
Category	Sulfosalt mineral
Formula	Cu3AsS4
IMA symbol	Eng[1]
Strunz classification	2.KA.05
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) H-M symbol: (mm2)
Space group	Pmn21
Unit cell	a = 7.407(1), b = 6.436(1) c = 6.154(1) [Å]; Z = 2
Identification
Color	Grayish black to iron black; gray to pale pink-brown, deep red internal reflections in polished section
Crystal habit	Tabular to prismatic crystals, striated parallel to {001}; massive
Twinning	Twin plane {320} common, rarely as interpenetrating pseudohexagonal trillings
Cleavage	Perfect on {110}, distinct {100} and {010}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	3
Luster	Metallic to dull
Streak	Black
Diaphaneity	Opaque
Specific gravity	4.4 to 4.5
References	[2][3][4]

Enargite is dimorph of the tetragonal luzonite.^[2]

Occurrence

It is a medium to low temperature hydrothermal mineral occurring with quartz, pyrite, sphalerite, galena, bornite, tetrahedrite–tennantite, chalcocite, covellite and baryte.^[4] It occurs in the mineral deposits at Butte, Montana, San Juan Mountains, Colorado and at both Bingham Canyon and Tintic, Utah. It is also found in the copper mines of Canada, Mexico, Argentina, Chile, Peru, and the Philippines.

Enargite was originally described as a new species from the copper mines of the San Francisco vein, Junin Department, Peru. The name is from Greek έναργής ("distinct"), in reference to its distinct cleavage.^[2][4]

Enargite is related to lazarevicite (named after M. Lazarevic), which has the same chemical formula, but cubic crystalline structure.^[5]