Wüstite
| Wüstite | |
|---|---|
| General | |
| Category | Oxide minerals |
|
Formula (repeating unit) |
FeO |
| Strunz classification | 4.AB.25 |
| Crystal system | Cubic |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m) |
| Identification | |
| Color | Greyish white to yellow or brown; colorless in thin section |
| Crystal habit | Pyramidic, prismatic |
| Cleavage | {001} perfect |
| Fracture | Subconchoidal to rough |
| Mohs scale hardness | 5 - 5.5 |
| Specific gravity | 5.88 |
| Density | 5.7 g/cm³ |
| Refractive index | 1.735 to 2.32 in synthetic crystals |
| Pleochroism | None |
| Solubility | Soluble in dilute HCl |
| Other characteristics | Forms solid solution with periclase |
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).
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, 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 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.
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