Forsterite

Forsterite
Forsterite (big tabular and colorless) on sanidine (little colorless crystals) with hematite (reddish)
General
Category	Nesosilicates
Formula (repeating unit)	Magnesium silicate (Mg2SiO4)
IMA symbol	Fo[1]
Strunz classification	9.AC.05
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M Symbol: (2/m 2/m 2/m)
Space group	Pbnm
Unit cell	a = 4.7540 Å, b = 10.1971 Å c = 5.9806 Å; Z = 4
Identification
Formula mass	140.691 g·mol−1
Color	Colorless, green, yellow, yellow green, white
Crystal habit	Dipyramidal prisms often tabular, commonly granular or compact massive
Twinning	On {100}, {011} and {012}
Cleavage	Perfect on {010} imperfect on {100}
Fracture	Conchoidal
Mohs scale hardness	7
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.21 – 3.33
Optical properties	Biaxial (+)
Refractive index	nα = 1.636 – 1.730 nβ = 1.650 – 1.739 nγ = 1.669 – 1.772
Birefringence	δ = 0.033 – 0.042
2V angle	82°
Melting point	1890 °C[2]
References	[3][4][5]

Forsterite (Mg₂SiO₄; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthorhombic system (space group Pbnm) with cell parameters a 4.75 Å (0.475 nm), b 10.20 Å (1.020 nm) and c 5.98 Å (0.598 nm).^[2]

Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites. In 2005 it was also found in cometary dust returned by the Stardust probe.^[6] In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star.^[7]

Two polymorphs of forsterite are known: wadsleyite (also orthorhombic) and ringwoodite (isometric, cubic crystal system). Both are mainly known from meteorites.

Peridot is the gemstone variety of forsterite olivine.