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Wüstite
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.
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:
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[clarification needed]. 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.
Wüstite
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.
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:
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[clarification needed]. 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.
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