Forsterite | |
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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.
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