TETRAHEDRITE, a mineral consisting typically of copper sulphantimonite, but often of complex composition. The copper is usually isomorphously replaced by variable amounts of silver, iron, zinc, mercury, lead or cobalt, and the antimony by arsenic or bismuth. Numerous special names have been ap plied to varieties differing in chemical composition ; the arsenic compound, is known as tennantite (after Smithson Ten nant). The old German name Fahlerz includes both tetrahedrite and tennantite. Tetrahedrite is an important ore of copper, the formula corresponding with 57.5% of this metal; it is also largely worked as an ore of silver, of which it sometimes contains as much as 3o%. Well-developed crystals are of fre quent occurrence; they belong to the tetrahedral class of the cubic system, and their tetrahedral form is a very characteristic fea ture of the mineral, which for this reason was named tetrahedrite. Fig. I shows a combination of a tetrahedron and a triakis-tetra hedron { 2111, and fig. 2 a tetrahedron with the rhombic dodeca hedron. The colour is steel-grey to iron-black, and the lustre
metallic and brilliant. The streak is usually black; sometimes, however, it is dark cherry-red, and very thin splinters of the min eral then transmit a small amount of blood-red light. The hard ness is 4.5, and the specific gravity varies with the composition from 4.4 to 5.1. There is no cleavage, and the fracture is con choidal.
Tetrahedrite occurs in metalliferous veins, usually in the zone of secondary enrichment. Fine groups of crystals, coated on their surface with brassy or brilliantly tarnished chalcopyrite, were formerly found at Herodsfoot mine, near Liskeard in Cornwall. Tennantite occurred as small crystals of cubic or dodecahedral habit in many Cornish copper mines, especially in the neighbour hood of Redruth : it is also found as small, brilliant crystals very rich in faces in the white crystalline dolomite of the Binnenthal in the Valais, Switzerland, and under the name binnite was long considered as a distinct species. (L. J. S.)