tetradymite

{{infobox mineral |name = Tetradymite | category = Sulfide mineral | boxwidth = | boxbgcolor = | image = Tetradymite.jpg | caption = Tetradymite - British Columbia, Canada | imagesize = 260px | formula = Bi2Te2S | IMAsymbol = Ttd | molweight = | strunz = 2.DC.05c | dana = 02.11.07.01 | system = Trigonal | class = Hexagonal scalenohedral (m) H-M symbol: ( 2/m) | symmetry = Rm | color = Steel-gray with dull to iridescent tarnish; white in polished section | colour = | habit = Pyramidal prisms, commonly granular, massive to foliated, also bladed | twinning = Twin planes {0118} and {0115} | cleavage = Perfect on {0001} | fracture = Uneven | tenacity = Laminae flexible, slightly sectile. | mohs = 1.52 | luster = Metallic, splendent on fresh surfaces, dull if tarnished | streak = Steel-gray | diaphaneity = Opaque | gravity = 7.27.9 | density = | polish = | opticalprop = | refractive = | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }}

Tetradymite is a mineral consisting of bismuth, tellurium and sulfide, Bi2Te2S, also known as telluric bismuth. If sulfur is absent the mineral is tellurobismuthite and the formula is then Bi2Te3. Traces of selenium are usually present. left|thumb|180px|A sample of tetradymite 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 for 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-gray, and the luster metallic and brilliant. The mineral is very soft (H = 1.52) and marks paper. The specific gravity is 7.2 to 7.9.