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
Crystal structure of Wüstite

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

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:

${\displaystyle {\ce {{\underset {magnetite}{FeO.Fe2O3}}+{\underset {graphite/diamond}{C}}->{3FeO}+{\underset {carbon\ monoxide}{CO}}}}}$

The formula for magnetite is more accurately written as ${\displaystyle {{\ce {FeO.Fe2O3}}}}$ 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, fulgurites and lightning-affected rock, and perhaps the mantle where reduced carbon is present, exemplified by the presence of diamond and/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.

Historical use

According to Vagn Fabritius Buchwald, wustite was an important component during the Iron age to facilitate the process of forge welding. In ancient times, when blacksmithing was performed using a charcoal forge, the deep charcoal pit in which the steel or iron was placed provided a highly-reducing, virtually oxygen-free environment, producing a thin wustite layer on the metal. At the welding temperature, the iron becomes highly reactive with oxygen, and will spark and form thick layers of slag when exposed to the air, which makes welding the iron or steel nearly impossible. To solve this problem, ancient blacksmiths would toss small amounts of sand onto the white-hot metal. The silicon in the sand then reacts with the wustite to form fayalite, which melts just below the welding temperature. This produced an effective flux that shielded the metal from oxygen and helped extract oxides and impurities, leaving a pure surface that can weld readily. Although the ancients had no knowledge of how this worked, the ability to weld iron contributed to the movement out of the Bronze age and into the modern.[2]

Related minerals

Wüstite forms a solid solution with periclase (MgO), and Fe substitutes for Mg. Periclase, when hydrated, forms brucite (Mg(OH)2), a common product of serpentinite metamorphic reactions.

Oxidation of wüstite forms goethite-limonite.

Zinc, aluminium and other transition metals may substitute for Fe in wüstite.

Wüstite in dolomite skarns may be related to siderite (Fe-carbonate), wollastonite, enstatite, diopside and magnesite.