forsteritefayalite

Forsterite and fayalite are the magnesium-iron end-members of the olivine solid solution series, a major group of orthorhombic nesosilicate minerals with the general formula (Mg,Fe)2SiO4. In olivine, magnesium and iron occupy the same crystallographic site, allowing continuous substitution between the end-members. Pure forsterite corresponds to Fo100 and pure fayalite to Fa100; intermediate compositions are commonly reported as Fo# or Fa# to indicate relative Mg- or Fe-content.

Olivine forms a continuous solid solution between these end-members and is one of the first minerals to crystallize from mafic to ultramafic magmas. It is also a dominant mineral in the Earth’s upper mantle, particularly in peridotite. The stability of forsterite-rich and fayalite-rich compositions depends on temperature, pressure, and oxygen fugacity; magnesian compositions dominate at higher temperatures and lower Fe-activity, while iron-rich compositions become more stable as temperatures fall or Fe content increases.

In geology, Fo–Fa compositions are used to infer magmatic differentiation, source rock characteristics, and thermal history. Olivine occurs in a range of rock types, including basalt, gabbro, dunite, and peridotite, as well as in some meteorites. Optical and chemical analyses of olivine grains yield Fo–Fa ratios that help reconstruct crystallization sequences, mantle processes, and the oxidation state of geological environments.