1911 Encyclopædia Britannica/Tetradymite

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TETRADYMITE, a mineral consisting of bismuth telluride and sulphite, Bi2Te2S, also known as "telluric bismuth." Sometimes sulphur is absent and the formula is then Bi2Te3; traces of selenium are usually present. 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, τετραδῦμος, fourfold. There is a perfect cleavage parallel to the basal plane; and the mineral usually occurs in foliated masses of irregular outline. The colour is steel-grey, and the lustre metallic and brilliant. The mineral is very soft (H=11/2) and marks paper; the specific gravity is 7.2 to 7.6. It was first found, in 1815, at Telemarken in Norway; crystals are from Schubkau near Schemnitz in Hungary. It often occurs in quartz associated with native gold. Other species very similar to tetradymite, but with different formulae, are: joseite, from San José near Marianna in Brazil; grünlingite (Bi4S3Te), from Caldbeck Fells in Cumberland; and wehrlite, from Hungary.  (L. J. S.)