1911 Encyclopædia Britannica/Tetrahedrite

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TETRAHEDRITE, a mineral consisting typically of copper sulph-autimonite, Cu3SbS3, 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′6X2S6+xR″6X2S9, where R′=Cu, Ag; X= Sb, As, Bi; R″=Fe, Zn, and x is a small fraction, often 1/10 or 1/5 . Numerous special names have been applied to varieties differing in chemical composition; the arsenic compound, Cu3AsS3, , 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. Tefrahcdrite is an important ore of copper, the formula Cu3SbS3, corresponding 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 forthis reason was named tetrahedrite. Fig. 1

Fig. 1. Fig. 2.

Crystals of Tetrahedrite.

shows a combination of a tetrahedron and a triakis-tetrahedron {211}, 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 41/2, 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 intermixture 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 chalcop rite, 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, Switzerland, and under the name binnite was long considered as a distinct species.  (L. J. S.)