It is isomorphous with the iron-rich end-member, fayalite. Iron-rich olivine (fayalite) favors production of magnetite-magnesite-silica compositions. Fayalite transforms to spinel at pressures below . Mafic minerals may include aegirine, fayalite, aenigmatite, ilmenite, and sodic amphibole (often arfvedsonite or ferrorichterite). Fayalite (FeSiO) is the most common mineral found in ancient slag. The most common minerals in the ignimbrites are feldspar and clinopyroxene, while fayalite is rare. When fayalite is replete with CaO, monticellite and pyroxene form. Olivine is a solid solution of forsterite, the magnesium-endmember, and fayalite, the iron-endmember.