tetradymite

Tetradymite is a metallic sulfide mineral in the bismuth–tellurium–sulfur system. Its ideal chemical formula is Bi2Te3S, and it is a member of the tetradymite group, which features layered structures that can accommodate some variation in tellurium, sulfur, and sometimes selenium. The mineral tends to form in hydrothermal veins and contact zones associated with intrusive rocks.

In hand specimen, tetradymite typically appears as lead-gray to black, highly metallic crystals or crusts with a bright luster. The streak is gray to gray-black. It has a relatively low Mohs hardness, commonly about 2, and a high specific gravity, usually around 7.5 to 8.0, reflecting its bismuth content. Crystals are commonly tabular or prismatic and may occur in aggregates.

Tetradymite crystallizes in the rhombohedral (trigonal) system and exhibits a layered, anisotropic structure, which can give rise to good basal cleavage in some samples. It is commonly found with minerals such as quartz, sphalerite, pyrite, chalcopyrite, and native bismuth, and it is among the minerals that bear economically important amounts of tellurium as a byproduct.

Economically, tetradymite is an ore of tellurium and, to a lesser extent, bismuth. Tellurium obtained from tetradymite and related minerals is used in alloying and in some thermoelectric and photovoltaic applications. The mineral occurs worldwide in various hydrothermal ore deposits, with notable localities reported in accord with tellurium-rich environments.

See also: tetradymite group, tellurium minerals, bismuth minerals.