a mineral consisting of bismuth telluride and sulphide, Bi 2 Te 2 S, also known as "telluric bismuth." Sometimes sulphur is absent and the formula is then B12Te3; 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, TETpali ios, 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 (II= i 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 Jose near Marianna in Brazil; griinlingite (Bi 4 S 3 Te), from Caldbeck Fells in Cumberland; and wehrlite, from Hungary. (L. J. S.)