Tetrahedrites

(fahlerz), a group of minerals (complex copper sulfides) that form an isomorphic tennantite-tetrahedrite series with the chemical formula nCu₁₂As₄S₁₃ + mCu₁₂Sb₄S₁₃. The pure members of the series are rarely encountered. Usually the composition is complicated by impurities of Ag, Zn, Fe, and Hg, which replace Cu; and Bi, which replaces As and Sb. Depending on their composition, tetrahedrites are divided into the following varieties: argen-totennantite and freibergite (which contain Ag), sandberger-ite and binnite (which contain Zn and Ag), ferrotennantite and ferrotetrahedrite (which contain Fe), schwatzite (which contains Hg), and annivite (which contains Bi). The structure is complex and of the skeleton type. They crystallize in a cubic system, forming tetrahedral—less frequently cubic and octahedral—crystal faces. Individual grains, granular irregular aggregates, and ore masses are more often encountered. The color is steel gray to iron black. The luster on a fresh fracture is metallic, and the ores tarnish or fade rapidly in air (hence the German name, Fahlerz, from fahl, “faded,” and the Russian bleklye rudy, from bleknut’, “to fade”). Their hardness on the mineralogical scale is 3–4; their density is 4,400–5,100 kg/m³. Tetrahedrites are formed in hydrothermal ore bodies of various genetic types. The major accessory minerals of tetrahedrites are galenite, sphalerite, chalcopyrite, pyrite, quartz, barite, and calcite. They erode readily on the surface and turn into malachite, azurite, cuprite, and so on. The largest deposits of tetrahedrites known are in the USSR (in the Urals, the Caucasus, the Altai, and Kazakhstan); the largest abroad are in Hungary, Great Britain, and the USA.