Iron minerals on the Earths surface are typically richly oxidized, forming hematite, with Fe state, or in somewhat less oxidizing environments, magnetite, with a mixture of Fe and Fe. Wüstite, in geochemistry, defines a redox buffer' of oxidation within rocks at which point the rock is so reduced that Fe, and thus hematite, is absent.
As the redox state of a rock is further reduced, magnetite is converted to wüstite. This occurs by conversion of the Fe ions in magnetite to Fe ions. An example reaction is presented below:
The formula for magnetite is more accurately written as FeO·FeO than as FeO. Magnetite is one part FeO and one part FeO, rather than a solid solution of wüstite and hematite. Magnetite is termed a redox buffer because, until all Fe present in the system is converted to Fe, the oxide mineral assemblage of iron remains wüstite-magnetite. Furthermore, the redox state of the rock remains at the same level of oxygen fugacity. Considering buffering the redox potential (E) in the Fe–O redox system, this can be compared to buffering the pH in the H/OH acid–base system of water.
Once the Fe is consumed, then oxygen must be stripped from the system to further reduce it and wüstite is converted to native iron. The oxide mineral equilibrium assemblage of the rock becomes wüstite–magnetite–iron.
In nature, the only natural systems which are chemically reduced enough to even attain a wüstite–magnetite composition are rare, including carbonate-rich skarns, meteorites, fulgurites and lightning-affected rock, and perhaps the mantle where reduced carbon is present, exemplified by the presence of diamond or graphite.