|Crystal class||Hextetrahedral (43m)
H-M symbol: (4 3m)
|Color||Flint-gray to iron-black, cherry-red in transmitted light|
|Crystal habit||massive to well formed crystals|
|Twinning||Contact and penetration twins|
|Fracture||Subconchoidal to uneven|
|Mohs scale hardness||3 - 4.5|
|Luster||Metallic, commonly splendent|
|Diaphaneity||Opaque, except in very thin fragments|
|Polish luster||gray, inclining to black to brown to cherry-red|
|Refractive index||n greater than 2.72|
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 arsenic component of tennantite causes the metal smelted from the ore to be harder than that of pure copper, because it is a copper-arsenic alloy. In the later 20th century it was found that arsenical coppers had been more widely used in antiquity than had been previously realised, and it has been proposed that discoveries made by smelting ores like tennantite were significant steps in the progress towards the Bronze Age.
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