Tetradymite

Formula: Bi₂Te₂S

Origin of name: from Greek for four and twin

Crystal system: Hexagonal

Crystal class: 3 2/m

Twinning: Fourlings, with twin plane {0118}, also {0115}

Unit cell: a = 4.2381, c = 29.589

Colour: Pale steel-gray, yellow gray, tarnishes iridescent and dull

Diaphaneity: Opaque

Luster: Metallic

Habit: Crystals indistinct, steep pyramidal, horizontal striations, massive, granular to foliated or bladed, lamellar

Hardness: 1.5 to 2

Specific gravity: 7.1 to 7.5

Cleavage: 1; {001} perfect

Tenacity: Laminae flexible but not elastic, slightly sectile, uneven fracture

Streak: Pale steel-gray

Synonyms/varieties: Csiklovaite

Comments:  Found in hydrotherma quartz-gold veins o f moderate to high temperature and contact metamorphic deposits.