TETRADYMITE  , a

mineral consisting of bismuth telluride and sulphide, Bi2Te2S, also known as " telluric bismuth." Sometimes sulphur is absent and the formula is then Bi2Tea; 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, rerpaSupos, 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 = 1) 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; grunlingite (Bi,SaTe), from Caldbeck Fells in Cumber-land; and wehrlite, from Hungary . (L . J .