Fayalite (Fe2SiO4; 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 Pbnm) with cell parameters a 4.82 Å, b 10.48 Å and c 6.09 Å.
Fayalite forms solid solution series with the magnesium olivine endmember forsterite rich olivine endmember tephroite (Mn2SiO4).
Iron rich olivine is a relatively common constituent of acidic and alkaline rocks such as volcanic obsidians and phonolites where it is associated with amphiboles. Its main occurrence is in ultramafic rocks and less commonly in felsic pegmatite in obsidian. It also occurs in medium-grade thermally metamorphosed
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. 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