The ratio of Fe2+ to Fe3+ within a rock determines, in part, the silicate 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. Iron may only enter minerals such as pyroxene and olivine if it is present as Fe2+; Fe3+ cannot enter the lattice of fayalite olivine and thus for every two Fe3+ ions, one Fe2+ is used and one molecule of magnetite is created.