Fayalite (Fe
₂SiO
₄, commonly abbreviated to Fa) 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 6.09 Å.

Fayalite
Fayalite crystals on substrate. Sample collected from Ochtendung, Eifel, Germany
General
Category	Nesosilicate
Formula (repeating unit)	Fe2SiO4
IMA symbol	Fa[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 (no. 62)
Unit cell	a = 4.8211, b = 10.4779 c = 6.0889 [Å]; Z = 4
Identification
Formula mass	203.771 g·mol−1
Color	Greenish yellow, yellow-brown, brown; pale yellow to amber in thin section
Crystal habit	Commonly granular, compact, or massive
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	[2][3][4]

Fayalite forms solid solution series with the magnesium olivine endmember forsterite (Mg₂SiO₄) and also with the manganese rich olivine endmember tephroite (Mn₂SiO₄).

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.^[2]

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.

At high pressure, fayalite undergoes a phase transition to ahrensite, the iron-bearing analogue of ringwoodite, i.e., contrary to forsterite there is no intermediate form analogous to wadsleyite; under the conditions prevailing in the upper mantle of the Earth, the transition would occur at ca. 6–7 GPa, i.e., at substantially lower pressure than the phase transitions of forsterite.^[5] In high-pressure experiments, the transformation may be delayed, so that it may remain stable to pressures of almost 35 GPa (see fig.), at which point it may become amorphous rather than take on a crystalline structure such as ahrensite.

The name fayalite is derived from Faial (Fayal) Island in the Azores where it was first described in 1840.^[3]