Fayalite (Fe2SiO4, commonly abbreviated to Fa) is the iron-rich end-member of the olivine solid-solution series.

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 and in impure carbonate rocks.

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.