Fayalite

Formula:  Fe₂SiO₄

Origin of name:   From locality; Fayal Island, Azores

Crystal system:   Orthorhombic

Crystal class:   2/m 2/m 2/m

Twinning:   On {100}; on {031} as trillings

Unit cell: a = 4.799 Ǻ, b = 10.39 Ǻ, c = 6.063 Ǻ

Colour:  Greenish yellow, yellowish brown, brown

Diaphaneity:

Luster:   Vitreous to greasy

Habit:   Crystals thick tabular, often with wedge-shaped terminations, small; massive, compact or granular

Hardness:   7

Specific gravity:   4.32

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

Tenacity:   Conchoidal; brittle

Streak:   Colourless

Synonyms/varieties:   Hortonolite (Mg, Mn -rich var.), knebelite (Mn -rich var.), olivine (group name), roepperite (Mn, Zn -rich var.)

Comments:   Series with forsterite and with tephroite, member of the olivine series