TETRAHEDRITE

This article appears in Volume V26, Page 671 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: TAV-THE

TETRAHEDRITE , a

mineral

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

COBALT (symbol Co, atomic
weight
- WEIGHT
59)

, 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

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

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

ELEMENT (Lat. elementum)

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

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

CORNWALL

. Good crystals are also met with at Kapnik-Banya in

Hungary

HUNGARY
HUNGARY (Hungarian Magyarorszdg)

, 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

VALAIS (Ger. Wallis, Ital. Vallese)

, Switzer-land, and under the name binnite was long considered as a distinct species. (L. J. S.)

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