Tetradymite

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]

A sample of tetradymite

Tetradymite
Tetradymite - British Columbia, Canada
General
Category	Sulfide mineral
Formula (repeating unit)	Bi2Te2S
IMA symbol	Ttd[1]
Strunz classification	2.DC.05c
Dana classification	02.11.07.01
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3m
Identification
Color	Steel-gray with dull to iridescent tarnish; white in polished section
Crystal 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 scale hardness	1.5 – 2
Luster	Metallic, splendent on fresh surfaces, dull if tarnished
Streak	Steel-gray
Diaphaneity	Opaque
Specific gravity	7.2 – 7.9
References	[2][3][4][5]

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]