forsterite-fayalite series, the most important minerals in the olivine family and possibly the most important constituents of the Earth’s mantle. Included in the series are the following varieties: forsterite magnesium silicate (Mg₂SiO₄) and fayalite iron silicate (Fe₂SiO₄).

Compositions intermediate to the end-members of the series (forsterite and fayalite) are also written as Fo_xFa_y, an expression of the ratio of forsterite to fayalite. For example, a composition made up of 80 percent forsterite and 20 percent fayalite is written Fo₈₀ Fa₂₀ and further abbreviated to Fo₈₀.

These minerals are common as green to yellow, glassy crystals in many mafic and ultramafic rocks and are also abundant in chondrite meteorites. Forsterite is common in dunite, gabbros, diabase, basalts, and trachytes. Small amounts of fayalite are present in many volcanic rocks in which sodium is more common than potassium. The forsterite-fayalite minerals also occur in dolomitic limestones, marbles, and metamorphosed iron-rich sediments. These minerals are relatively infusible, not melting below 1,500 °C (2,700 °F), and are sometimes used in the manufacture of refractory brick. For detailed physical properties, see olivine (table).