 MH magnetite-hematite

4 Fe3O4 + O2 = 6 Fe2O3

NiNiO nickel-nickel oxide

2 Ni + O2 = 2 NiO

FMQ fayalite-magnetite-quartz

3 Fe2SiO4 + O2 = 2 Fe3O4 + 3 SiO2

WM wustite-magnetite

3 Fe1−xO + O2 ~ Fe3O4

IW iron-wustite

2(1-x) Fe + O2 = 2 Fe1−xO

QIF quartz-iron-fayalite

2 Fe + SiO2 + O2 = Fe2SiO4

The ratio of Fe2+ to Fe3+ 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 Fe3+ and hematite is a likely mineral in iron-bearing rocks.