Fayalite (Fe₂SiO₄) is the iron end-member of the olivine group, completing the forsterite-fayalite series. Its dark green to brown color comes from divalent iron substituting for magnesium. Fayalite-rich olivines occur in Fe-rich igneous rocks (gabbros, anorthosites) and fayalitic slags from historical iron smelting. The Faial Island (Azores) type-locality and lunar mare basalts are classic occurrences.
Key Facts
- Mohs hardness 6.5–7.
- Orthorhombic; forms continuous series with forsterite.
- Color darkens with increasing iron content (forsterite is light olive; pure fayalite is dark green-brown).
- Stable in Fe-rich, Mg-poor magmas (gabbros, ferro-syenites).
- Lunar mare basalts contain abundant fayalite olivine.
Notable Localities
Faial Island (Azores, Portugal) is the type-locality. Mourne Mountains (Ireland) and Rockport (Massachusetts) yield collector specimens.
