Tetradymite

TETRADYMITE, a mineral consisting of bismuth telluride and sulphide, Bi2Te2S, also known as " telluric bismuth." Sometimes sulphur is absent and the formula is then Bi2Tes; 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, T«Tpa50/^js, 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=i£) 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 Jose near Marianna in Brazil; grunlingile (Bi4S3Te), from Caldbeck Fells in Cumberland; and wehrlite, from Hungary. (L. J. S.)