Tennantite

Tennantite is a copper arsenic sulfosalt mineral. Its chemical formula is Cu12As4S13. It is found in hydrothermal veins and contact metamorphic deposits. It is grey-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 or the copper site.
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(n.)
A blackish lead-gray mineral, closely related to tetrahedrite. It is essentially a sulphide of arsenic and copper.
  

General Information:
 Chemical Formula:
(Cu,Fe)12As4S13
 Composition:
(Molecular Weight = 1,479.10 gm)
Copper 51.56 % Cu
Arsenic 20.26 % As
Sulfur 28.18 % S 
 Empirical Formula:
Cu12As4S13
 Name Origin:
Named after the English chemist, S. Tennant (1761-1815).
Physical Properties:
 Cleavage:
None
 Color:
steel gray or black.
 Density:
4.6 - 4.7, Average = 4.65
 Diaphaniety:
Opaque
 Habits:
Massive - Granular - Common texture observed in granite and other igneous rock., Massive - Uniformly indistinguishable crystals forming large masses., Disseminated - Occurs in small, distinct particles dispersed in matrix.
 Hardness:
3.5-4 - Copper Penny-Flourite
 Luminescence:
None.
 Luster:
Metallic
 Streak:
reddish gray
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