text: The ratio of Fe to Fe within a rock determines, in part, the silicate mineral and oxide mineral assemblage of the rock. Within a rock of a given chemical composition, iron enters minerals based on the bulk chemical composition and the mineral phases which are stable at that temperature and pressure. For instance, at redox conditions more oxidizing than the MH (magnetite-hematite) buffer, at least much of the iron is likely to be present as Fe and hematite is a likely mineral in iron-bearing rocks. Iron may only enter minerals such as olivine if it is present as Fe; Fe cannot enter the lattice of fayalite olivine. Elements in olivine such as magnesium, however, stabilize olivine containing Fe to conditions more oxidizing than those required for fayalite stability. Solid solution between magnetite and the titanium-bearing endmember, ulvospinel, enlarges the stability field of magnetite. Likewise, at conditions more reducing than the IW (iron-wustite) buffer, minerals such as pyroxene can still contain Fe. The redox buffers therefore are only approximate guides to the proportions of Fe and Fe in minerals and rocks.