Tetrahedrite

Tetrahedrite is a copper antimony sulfosalt mineral with formula: (Cu,Fe)
₁₂Sb
₄S
₁₃. It is the antimony endmember of the continuous solid solution series with arsenic-bearing tennantite. Pure endmembers of the series are seldom if ever seen in nature. Of the two, the antimony rich phase is more common. Other elements also substitute in the structure, most notably iron and zinc, along with less common silver, mercury and lead. Bismuth also substitutes for the antimony site and bismuthian tetrahedrite or annivite is a recognized variety. The related, silver dominant, mineral species freibergite, although rare, is notable in that it can contain up to 18% silver.

Tetrahedrite
Tetrahedrite, hübnerite and quartz from Mundo Nuevo Mine, Huamachuco, Sánchez Carrión, La Libertad, Peru (size: 61 x 57 mm, 110 g)
General
Category	Sulfosalt mineral
Formula (repeating unit)	(Cu,Fe) 12Sb 4S 13
Strunz classification	2.GB.05
Crystal system	Cubic
Crystal class	Hextetrahedral (43m) H-M symbol: (4 3m)
Space group	I43m
Unit cell	a = 10.39(16) Å; Z = 2
Identification
Color	Steel gray to iron-gray
Crystal habit	Groups of tetrahedral crystals; massive, coarse to fine compact granular
Twinning	Contact and penetration twins on {111}
Cleavage	None
Fracture	Uneven to subconchoidal
Tenacity	Somewhat brittle
Mohs scale hardness	​3 1⁄2 - 4
Luster	Metallic, commonly splendent
Streak	Black, brown to dark red
Diaphaneity	Opaque, except in very thin fragments
Specific gravity	4.97
Optical properties	Isotropic
Refractive index	n greater than 2.72
References	[1][2]

Tetrahedrite gets its name from the distinctive tetrahedron shaped cubic crystals. The mineral usually occurs in massive form, it is a steel gray to black metallic mineral with Mohs hardness of 3.5 to 4 and specific gravity of 4.6 to 5.2.

It occurs in low to moderate temperature hydrothermal veins and in some contact metamorphic deposits. It is a minor ore of copper and associated metals. It was first described in 1845 for occurrences in Freiberg, Saxony, Germany.