Tridymite

Formula: SiO₂

Origin of name: from Greek, for three and twin

Crystal system: triclinic

Crystal class: 1

Twinning: very common on {10-16} or {30-34} contact or penetration

Unit cell: a = 9.88 Å, b = 17.1 Å, c = 16.3 Å, a = 90°, b = 90°, g = 90°

Colour: colourless to white

Diaphaneity: transparent to translucent

Luster: vitreous, sometimes pearly on {001}

Habit: crystals tabular, triclinic form is stable only below 100°C, above that temperature crystals are orthorhombic (and pseudohexagonal)

Hardness: 7

Specific gravity: 2.26 to 2.27

Cleavage: none

Tenacity: conchoidal, brittle

Streak: 

Synonyms/varieties: tridamite

Comments: polymorphous with quartz, coesite, cristobalite and stishovite