TETRAHEDRITE  , a

mineral consisting typically of copper sulph-antimonite, CuaSbSa, 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 . In general, the formula is R'6X2Se-}-xR"6X2S9, where R' = Cu, Ag; X= Sb, As, Bi; R" = Fe, Zn, and x is a small fraction, often 31-0 or . Numerous special names have been applied to varieties differing in chemical composition; the arsenic compound, CuaAsS3, is known as tennantite (after Smithson Tennant) . The old German name Fahlerz includes both tetrahedrite and tennantite, and so does the term " grey copper ore of miners . Tetrahedrite is an important ore of copper, the formula CuaSbSa, cot responding with 57'5 per cent. of this metal; it is also largely worked as an ore of silver, of which element it sometimes contains as much as 30 per cent . Well-developed crystals are of frequent occurrence; they belong to the tetrahedral class of the cubic system, and their tetrahedral form is a very characteristic feature of the mineral, which for this reason was named tetrahedrite . Fig. r Crystals of Tetrahedrite . shows a combination of a tetrahedron and a triakis-tetrahedron 12111, and fig . 2 a tetrahedron with the rhombic dodecahedron . Interpenetrating twinned crystals sometimes occur . 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 mineral then transmit a small amount of blood-red light . The hardness is 4, and the specific gravity varies with the composition from 4.4 to 5.1 . There is no cleavage, and the fracture is conchoidal . The material is often very impure owing to intimate inter-mixture with chalcopyrite . Tetrahedrite occurs in metalliferous veins associated with chalcopyrite, pyrites, blende, galena, &c . Fine groups of crystals, coated on their surface with brassy or brilliantly tarnished chalcopyrite, were formerly found at Herodsfoot mine, near Liskeard in Cornwall . Good crystals are also met with at Kapnik-Banya in Hungary, in the Harz, Peru, and other places . Tennantite occurs as small crystals of cubic or dodecahedral habit in many Cornish copper mines, especially in the neighbourhood 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, Switzer-land, and under the name binnite was long considered as a distinct species . (L . J .