Fayalite

Fayalite (chemical formula Fe₂SiO₄, often shortened to Fa) sits at the iron-rich end of the olivine solid-solution series. Like all olivine-group minerals, it forms crystals in the orthorhombic system, with unit-cell dimensions of a = 4.82 Å, b = 10.48 Å, and c = 6.09 Å. It can mix in a solid solution with forsterite (Mg₂SiO₄), the magnesium end-member, as well as with tephroite (Mn₂SiO₄), the manganese-rich end-member. Iron-rich olivine turns up fairly often in acidic and alkaline igneous rocks—things like volcanic obsidians, rhyolites, trachytes, phonolites, and plutonic quartz syenites, where it hangs out with amphiboles. Its main home, though, is in ultramafic volcanic and plutonic rocks; it shows up less in felsic plutonic rocks and only rarely in granite pegmatite. You can also find it in lithophysae inside obsidian, as well as in medium-grade thermally metamorphosed iron-rich sediments and impure carbonate rocks. At low pressures, fayalite can coexist with quartz, something more magnesium-rich olivine cannot do. That’s because magnesian olivine reacts with quartz to form orthopyroxene, but iron stabilizes the olivine-plus-quartz pair. The way this reaction depends on pressure and composition lets geologists figure out the pressures under which olivine and quartz assemblages formed. Fayalite also reacts with oxygen to produce magnetite and quartz; together, these three minerals make up the “FMQ” oxygen buffer. Scientists use this reaction to control oxygen fugacity in lab experiments, and it can also help calculate the oxygen fugacity recorded by mineral assemblages from metamorphic and igneous settings. Under high pressure, fayalite transforms directly into ahrensite, the iron-bearing equivalent of ringwoodite. Unlike forsterite, there’s no intermediate phase like wadsleyite. In Earth’s upper mantle, this transition happens at about 6–7 GPa—significantly lower pressure than forsterite’s phase changes. In high-pressure experiments, the transformation can be sluggish, so fayalite may stay stable up to nearly 35 GPa, at which point it might turn amorphous instead of forming a crystalline structure like ahrensite.