Tetradymite

    Tetradymite is a mineral consisting of bismuth, tellurium and sulfide, Bi₂Te₂S, also known as telluric bismuth. If sulfur is absent the mineral is tellurobismuthite and the formula is then Bi₂Te₃. Traces of selenium are usually present.

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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-grey, and the lustre 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 where it was reported in 1831. It was first found, in 1815, at Telemarken in Norway; crystals are from Schubkau. It often occurs in quartz associated with native gold.

References

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.