Tetrahedrite
a name given to a group of minerals having considerable diversity in composition, but presenting the same general general formula. Named from the prevailing tetrahedral habit of its crystals. Crystallization isometric, frequently twinned; luster, metallic; color and streak, steel-gray to iron-black; opaque; fracture, sub-conchoidal, uneven; brittle. Composition, essentially a sulphantimonite of copper, with the formula 4CuS-+Sb2S3; but in consequence of part of the copper being frequently replaced by iron, zinc, silver, mercury, and occasionally cobalt, and part of the antimony by arsenic or bismuth, the general formula is usually written as (Cu, Fe, Zn, Ag, Hg)+(Sb, As, Bi) 2S3. Dana divides tetrahedrites as follows: (1) An antimonial series. (2) An arsenio-antimonial series. (3) A bismuthic-arsenioantimonial and an arsenical series, in which the antimony is entirely replaced by arsenic. The varieties are: (1) Ordi nary, containing very little or no silver. (2) Argentiferous = freibergite. (3) Mercuriferous = schwatzite, spaniolite, schwatzite, spaniolite , and hermesite. (4) Platiniferous. Fieldite, aphthonite and polytelite are subspecies. An abundant ore in many parts of the world, sometimes, where rich in silver, mined for that metal only.