Wüstite
(FeO) is a mineral form of iron(II) oxide found with meteorites and native iron. It has a gray color 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 for 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). [1]
In addition to the 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.
Wüstite Redox Buffer
Main article: Mineral redox buffer
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:
FeO.Fe2O3 + C --> 3FeO + CO
magnetite + graphite/diamond --> wüstite + carbon monoxide
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.
Once the Fe3+ 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 and perhaps the mantle where reduced carbon is present, exemplified by the presence of diamond and/or graphite.