Tennantite

Tennantite
A sample of tennantite
General
Category	Sulfosalt minerals
Formula (repeating unit)	Cu6[Cu4(Fe,Zn)2]As4S13
Strunz classification	2.GB.05
Crystal system	Cubic
Crystal class	Hextetrahedral (43m) H-M symbol: (4 3m)
Space group	I43m
Identification
Color	Flint-gray to iron-black, cherry-red in transmitted light
Crystal habit	massive to well formed crystals
Twinning	Contact and penetration twins
Cleavage	None
Fracture	Subconchoidal to uneven
Tenacity	Somewhat brittle
Mohs scale hardness	3 - 4.5
Luster	Metallic, commonly splendent
Streak	reddish gray
Diaphaneity	Opaque, except in very thin fragments
Specific gravity	4.65
Polish luster	gray, inclining to black to brown to cherry-red
Optical properties	Isotropic
Refractive index	n greater than 2.72
References

Tennantite is a copper arsenic sulfosalt mineral with an ideal formula Cu₁₂As₄S₁₃. Due to variable substitution of the copper by iron and zinc the formula is Cu₆[Cu₄(Fe,Zn)₂]As₄S₁₃. It is gray-black, steel-gray, iron-gray or black in color. A closely related mineral, tetrahedrite (Cu₁₂Sb₄S₁₃) has antimony substituting for arsenic and the two form a solid solution series. The two have very similar properties and is often difficult to distinguish between tennantite and tetrahedrite. Iron, zinc, and silver substitute up to about 15% for the copper site.

The mineral was first described for an occurrence in Cornwall, England in 1819 and named after the English chemist Smithson Tennant (1761-1815).

It is found in hydrothermal veins and contact metamorphic deposits in association with other Cu–Pb–Zn–Ag sulfides and sulfosalts, pyrite, calcite, dolomite, siderite, barite, fluorite and quartz.