Tetradymite

(repeating unit)Bi₂Te₂SIMA symbolTtd^[1]Strunz classification2.DC.05cDana classification02.11.07.01Crystal systemTrigonalCrystal classHexagonal scalenohedral (3m)
H-M symbol: (3 2/m)Space groupR3mIdentificationColorSteel-gray with dull to iridescent tarnish; white in polished sectionCrystal habitPyramidal prisms, commonly granular, massive to foliated, also bladedTwinningTwin planes {0118} and {0115}CleavagePerfect on {0001}FractureUnevenTenacityLaminae flexible, slightly sectile.Mohs scale hardness1.5 – 2LusterMetallic, splendent on fresh surfaces, dull if tarnishedStreakSteel-grayDiaphaneityOpaqueSpecific gravity7.2 – 7.9References^[2]^[3]^[4]^[5]

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.^[6]

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 = 1.5 – 2) and marks paper. The specific gravity is 7.2 to 7.9.^[6]

The type locality is Zupkov (Zsubko; Schubkau), Stredoslovenský Kraj, Slovak Republic where it was reported in 1831.^[5] It was first found, in 1815, at Telemark in Norway.^[6] It often occurs in high temperature hydrothermal quartz veins associated with native gold and in contact metamorphic deposits.^[3]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ Mineralienatlas
^ ^a ^b http://rruff.geo.arizona.edu/doclib/hom/tetradymite.pdf Handbook of Mineralogy
^ http://webmineral.com/data/Tetradymite.shtml Webmineral data
^ ^a ^b http://www.mindat.org/min-3921.html Mindat.org
^ ^a ^b ^c Spencer 1911.

Attribution:

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This article incorporates text from a publication now in the public domain: Spencer, Leonard James (1911). "Tetradymite". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 26 (11th ed.). Cambridge University Press. p. 670.