Wüstite (FeO, sometimes also written as Fe0.95O) is a mineral form of mostly 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.
Wüstite 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:
FeO ⋅ Fe 2 O 3 magnetite + C graphite / diamond ⟶ 3 FeO + CO carbon monoxide {\displaystyle {\ce {{\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. Magnetite is termed a redox buffer because, until all Fe3+ present in the system is converted to Fe2+, 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 (Eh) 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 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, fulgurites and lightning-affected rock, and perhaps the mantle where reduced carbon is present, exemplified by the presence of diamond or graphite.
Effects upon silicate minerals
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. In chemically reduced rocks, magnetite may be absent due to the propensity of iron to enter olivine, and wüstite may only be present if there is an excess of iron above what can be used by silica. Thus, wüstite may only be found in silica-undersaturated compositions which are also heavily chemically reduced, satisfying both the need to remove all Fe3+ and to maintain iron outside of silicate minerals. In nature, carbonate rocks, potentially carbonatite, kimberlites, carbonate-bearing melilitic rocks, and other rare alkaline rocks may satisfy these criteria. However, wüstite is not reported in most of these rocks in nature, potentially because the redox state necessary to drive magnetite to wüstite is so rare.
Role in nitrogen fixation
Approximately 2–3% of the world's energy budget is allocated to the Haber process for ammonia (NH3) production, which relies on wüstite-derived catalysts. The industrial catalyst is derived from finely ground iron powder, which is usually obtained by reduction of high-purity magnetite (Fe3O4). The pulverized iron metal is burnt (oxidized) to give magnetite or wüstite of a defined particle size. The magnetite (or wüstite) particles are then partially reduced, removing some of the oxygen in the process. The resulting catalyst particles consist of a core of magnetite, encased in a shell of wüstite, which in turn is surrounded by an outer shell of iron metal. The catalyst maintains most of its bulk volume during the reduction, resulting in a highly porous high-surface-area material, which enhances its effectiveness as a catalyst.
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