Wüstite

Wüstite (FeO) is a mineral form of iron(II) oxide found with meteorites and native iron. It has a grey colour with a greenish tint in reflected light. Wüstite crystallizes in the isometric-hexoctahedral crystal system in opaque to translucent metallic grains. It has a Mohs hardness of 5 to 5.5 and a specific gravity of 5.88. Wüstite is a typical example of a non-stoichiometric compound. Wüstite was named after Fritz Wüst (1860–1938), a German metallurgist and founding director of the Kaiser-Wilhelm-Institut für Eisenforschung (presently Max Planck Institute for Iron Research GmbH). In addition to its type locality in Germany, it has been reported from Disko Island, Greenland; the Jharia coalfield, Jharkhand, India; and as inclusions in diamonds in a number of kimberlite pipes. It also is reported from deep sea manganese nodules. Its presence indicates a highly reducing environment. Mineral redox buffer Iron minerals on the earth's surface are typically richly oxidized, forming hematite, with Fe3+ state, or in somewhat less oxidizing environments, magnetite, with a mixture of Fe3+ and Fe2+. Wüstite, in geochemistry, defines a redox buffer of oxidation within rocks at which point the rock is so reduced that Fe3+, 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 Fe3+ ions in magnetite to Fe2+ ions. An example reaction is presented below: \underset{magnetite}{FeO.Fe2O3} + \underset{graphite/diamond}{C} -> {3FeO} + \underset{carbon\ monoxide}{CO} 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.