fayalite

Fayalite
General
Category	Silicate mineral
Chemical formula	Fe2SiO4
Identification
Color	Greenish yellow, yellow-brown, brown; pale yellow to amber in thin section
Crystal habit	Commonly granular, compact, or massive
Crystal system	Orthorhombic
Twinning	On [100]; also on [031], as trillings
Cleavage	{010} moderate, {100} imperfect
Fracture	Conchoidal
Mohs scale hardness	6.5 - 7.0
Luster	Vitreous to resinous on fractures
Streak	White
Specific gravity	4.392
Optical properties	Biaxial (-)
Refractive index	α = 1.827, β = 1.869, γ = 1.879
Pleochroism	Faint
References	[1]

Wikimedia Commons has media related to: Fayalite

Fayalite (Fe₂SiO₄) is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system (space group Pbnm) with cell parameters a 4.82 Å, b 10.48 Å and c Å 6.09.

Iron rich olivine is a relatively common constituent of acidic and alkaline igneous rocks such as volcanic obsidians, rhyolites, trachytes and phonolites and plutonic quartz syenites where it is associated with amphiboles. It also occurs in medium-grade thermally metamorphosed iron-rich sediments.

Fayalite is stable with quartz at low pressures, whereas more magnesian olivine is not, because of the reaction olivine + quartz = orthopyroxene. Iron stabilizes the olivine + quartz pair. The pressure and compositional dependence of the reaction can be used to calculate constraints on pressures at which assemblages of olivine + quartz formed.

Fayalite can also react with oxygen to produce magnetite + quartz: the three minerals together make up the "FMQ" oxygen buffer. The reaction is used to control the fugacity of oxygen in laboratory experiments. It can also be used to calculate the fugacity of oxygen recorded by mineral assemblages in metamorphic and igneous processes.

See also

• Forsterite, (Mg₂SiO₄), the magnesium-rich end-member of the olivine solid-solution series.

References

• Deer, W. A., Howie, R. A., and Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). Harlow: Longman ISBN 0-582-30094-0