Forsterite

Forsterite
Forsterite (big tabular and colorless) on sanidine (little colorless crystals) with hematite (reddish)
General
Category	Nesosilicates
Formula (repeating unit)	Magnesium silicate (Mg2SiO4)
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
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 (Mg₂SiO₄; commonly abbreviated as Fo) 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 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). Both are mainly known from meteorites.

Peridot is the gemstone variety of forsterite olivine.

Forsterite reacts with quartz to form the orthopyroxene mineral enstatite in the following reaction:

Pure forsterite is composed of magnesium, oxygen and silicon. The chemical formula is Mg₂SiO₄. Forsterite, fayalite (Fe₂SiO₄) and tephroite (Mn₂SiO₄) are the end-members of the olivine solid solution series; other elements such as Ni and Ca substitute for Fe and Mg in olivine, but only in minor proportions in natural occurrences. Other minerals such as monticellite (CaMgSiO₄), an uncommon calcium-rich mineral, share the olivine structure, but solid solution between olivine and these other minerals is limited. Monticellite is found in contact metamorphosed dolomites.