Fayalite (, 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 forms solid solution series with the magnesium olivine endmember forsterite (Mg<sub>2</sub>SiO<sub>4</sub>) and also with the manganese rich olivine endmember tephroite (Mn<sub>2</sub>SiO<sub>4</sub>).
thumb|left|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. 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.