Forsterite
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).
| Forsterite | |
|---|---|
| General | |
| Category | Nesosilicates |
| Formula (repeating unit) | Magnesium silicate (Mg2SiO4) |
| IMA symbol | Fo |
| 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 |
| References | |
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. In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star.
Two polymorphs of forsterite are known: wadsleyite (also orthorhombic) and ringwoodite (isometric, cubic crystal system). Both are mainly known from meteorites.