Forsterite

Formula:  Mg₂SiO₄

Origin of name:  For A. Forster, English collector

Crystal system:  Orthorhombic

Crystal class:  2/m 2/m 2/m

Twinning:  On {100}, {011}, or {012}

Unit cell: a = 4.7874 Ǻ, b = 10.32 Ǻ, c = 6.027 Ǻ

Colour:  Green, lemon-yellow, white

Diaphaneity:

Luster:  Vitreous to greasy

Habit:  Crystals thick tabular, vertically striated; massive, compact or granular; embedded grains

Hardness:  7

Specific gravity:  3.28

Cleavage:  2; {010} and {100} imperfect

Tenacity:  Conchoidal; brittle

Streak:  Colourless

Synonyms/varieties:  Boltonite, chrysolite, onivine (group name), peridot (gem variety)

Comments:  Series with fayalite; member of olivine series, trimorphous with ringwoodite and wadsleyite