The formula for magnetite is more accurately written as FeO·Fe2O3 than as Fe3O4. Magnetite is one part FeO and one part Fe2O3, rather than a solid solution of wüstite and hematite. The magnetite is termed a "redox buffer" because until all Fe3+ magnetite is converted to Fe2+ the oxide mineral assemblage of iron remains wüstite-magnetite, and furthermore the redox state of the rock remains at the same level of oxygen fugacity. This is similar to buffering in the H+/OH− acid–base system of water.