Tetradymite
Tetradymite is a mineral consisting of bismuth telluride and sulphide, Bi2Te2S, also known as telluric bismuth. Sometimes sulphur is absent and the formula is then BiTe; 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, ??, 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 = 11/8) and marks paper; the specific gravity is 7~2 to 7. 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 Jos near Marianna in Brazil; grunlingite (BiaS~Te), from Caldbeck Fells in Cumberland; and wehrlite, from Hungary.
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