| Tetradymite | |
|---|---|
 Tetradymite - British Columbia, Canada | |
| General | |
| Category | Sulfide mineral |
| Formula | 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] |
Tetradymite is a mineral consisting of bismuth, tellurium and sulfide, Bi 2 Te 2 S, 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]
