Tennantite

(mineralogy) (Cu,Fe)₁₂As₄S₁₃ A lead-gray mineral crystallizing in the isometric system; it is isomorphous with tetrahedrite; an important ore of copper.

Environment

Fairly common, usually with gangue, sphalerite, and galena, in medium- and low-temperature ore veins.

Tetrahedrite commonly well crystallized in sharp, distinct, well-formed tetrahedrons; tennantite usually less well developed, and often richer in forms and more cubic in habit. Also massive.

Gray to iron black. Luster metallic; hardness 3-4Ɖ (tennantite harder); specific gravity 4.6-5.1 (tetrahedrite denser); streak brown to black; cleavage none; fracture subconchoidal to uneven. Brittle; some specimens of tennantite translucent deep red in thin splinters.

Copper, iron antimony, and arsenic sulfide; really a whole series of related minerals (isomorphous series) with antimony and arsenic as end-members. Copper is the predominant metal, but zinc, mercury, silver, and lead can also enter into the composition. Tetrahedrite (the antimony-rich member) is far more common than tennantite.

Easily fused, releasing arsenic or antimony fumes, or both, plus sulfur. Fused globule dissolved in nitric acid gives good red-brown precipitate with ammonia (iron), and blue (copper) color to solution. Lead and silver can be shown to be present by adding hydrochloric acid to the nitric acid solution. The blue-green solution turns greener and a white precipitate forms. These tests should suffice; normally the tetrahedral crystals are enough, when visible.

When crystals are present they are the best guide. Failing these, the problem becomes more difficult, since there are many gray metallic sulfides with which these two can be confused. Detection of copper, iron, lead, and silver should permit running down tetrahedrite-tennantite by elimination. The subconchoidal, brilliant fracture, lack of cleavage, and comparative hardness are all significant.

Tetrahedrite is one of the most common and economically the most important of the sulfosalts. Often an important ore of copper, it is usually found in typical copper vein associations. Fine glossy crystals, probably the best in the U.S., have come from Bingham Canyon, Utah. It is found in many European localities (in Germany, England, and Czechoslovakia) and in some of the famous South American copper mines (Cerro de Pasco, Casapalca, and Morococha, all in Peru).

Tennantite is much rarer. The best-formed and multifaced crystals have come from the isolated sulfide pocket occurrence at Binnatal, Switzerland. It is also found in many U.S. mines, where it is difficult to distinguish from the tetrahedrite found in other sections of the same veins. Tsumeb, Namibia, has become an important locality with sharp 1-in. (2-3 cm) tennantite tetrahedrons. An occurrence in Zacatecas, Mexico, of like-sized crystals first called tennantite turned out to be a mixture of the two (tennantite and tetrahedrite) grown over crystals of chalcopyrite that are scattered over a crystallized quartz crust.

Tennantite
A sample of tennantite
General
Category	Mineral: sulfosalt
Chemical formula	Cu12As4S13
Identification
Molar mass	1,471.40 gm
Color	Steel gray to iron-gray
Crystal habit	massive to well formed crystals
Crystal system	Cubic
Cleavage	None
Fracture	Hackly
Mohs Scale hardness	3½ - 4
Luster	Metallic
Streak	reddish gray
Diaphaneity	Opaque
Density	4.65

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% for the copper site.

The mineral was named after the English chemist Smithson Tennant (1761-1815).

Tennantite from Ireland

See also

• Bornite
• Cuprite
• Tetrahedrite
• List of minerals
• List of minerals named after people

References

• Min. galleries
• Mindat.org

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