Orthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur in pigeonite). Ferrosillite is rarely found in nature because under most geological conditions fayalite + quartz is more stable than ferrosillite: Fe2Si2O3 → Fe2SiO4 + SiO2. See Pyroxene entry for more information.

Some or all content above used with permission from J. H. Wittke.