Tetradymite

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Tetradymite
Tetradymite.jpg
Tetradymite - British Columbia, Canada
General
Category Sulfide mineral
Formula
(repeating unit)
Bi2Te2S
Strunz classification 02.DC.05c
Dana classification 02.11.07.01
Identification
Color Steel-gray with dull to iridescent tarnish; white in polished section
Crystal habit Pyramidal prisms, commonly granular, massive to foliated, also bladed
Crystal system Trigonal - hexagonal scalenohedral (32/m)
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 [1][2][3]

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.

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-grey, and the lustre metallic and brilliant. The mineral is very soft (H = 1.5 - 2) and marks paper. The specific gravity is 7.2 to 7.9.

The type locality is Zupkov (Zsubko; Schubkau), Stredoslovenský Kraj, Slovak Republic where it was reported in 1831.[3] It was first found, in 1815, at Telemark in Norway. It often occurs in high temperature hydrothermal quartz veins associated with native gold and in contact metamorphic deposits.[1]

References[edit]

Attribution

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