Tetradymite is a mineral , tellurium 2S is absent the mineral is tellurobismuthite and the formula is then Bi2Te3. Traces of selenium
Crystals are rhombohedral , but are rarely distinctly developed; they are twinned together in groups of four; hence the name of the mineral, from the Greek for fourfold. There is a perfect cleavage parallel to the basal plane and the mineral usually occurs in foliated masses of irregular outline. The color is steel-gray, and the luster metallic and brilliant. The mineral is very soft (H = 1.5 – 2) and marks paper. The specific gravity is 7.2 to 7.9.
The type locality is Zupkov (Zsubko; Schubkau), Stredoslovenský Kraj, Slovak Republic It was first found, in 1815, at Telemark It often occurs in high temperature hydrothermal veins and in contact metamorphic