1910–1911
Britannica 11th
Encyclopaedia Britannica · 11th edition
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.…
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- Vol. 26, pp. 670-670