Fayalite
Fayalite crystal group from Ochtendung, Eifel, Germany
General
Category	Nesosilicate
Formula (repeating unit)	Fe2SiO4
Strunz classification	09.AC.05
Crystal symmetry	Orthorhombic 2/m 2/m 2/m
Unit cell	a = 4.8211 Å, b = 10.4779 Å, c = 6.0889 Å; Z=4
Identification
Color	Greenish yellow, yellow-brown, brown; pale yellow to amber in thin section
Crystal habit	Commonly granular, compact, or massive
Crystal system	Orthorhombic Dipyramidal
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
Diaphaneity	Transparent
Specific gravity	4.392
Optical properties	Biaxial (-)
Refractive index	nα = 1.731 – 1.824 nβ = 1.760 – 1.864 nγ = 1.773 – 1.875
Birefringence	δ = 0.042 – 0.051
Pleochroism	Faint
2V angle	Measured: 74° to 47°, Calculated: 54° to 66°
References	[1][2]

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.

The atomic scale structure of fayalite looking along the a axis. Oxygen is shown in red, silicon in pink, and iron in blue. A projection of the unit cell is shown by the black rectangle.

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. Its main occurrence is in ultramafic volcanic and plutonic rocks and less commonly in felsic plutonic rocks and rarely in granite pegmatite. It also occurs in lithophysae in obsidian. It also occurs in medium-grade thermally metamorphosed iron-rich sediments and in impure carbonate rocks.^[1]

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.

The name fayalite is derived from Faial (Fayal) Island in the Azores where it was first described in 1840.^[2]

See also [edit]

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

References [edit]

• 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

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