
Wüstite (
Fe O, sometimes also written as Fe
0.95O) is a mineral form of mostly
iron(II) oxide found with
meteorite
A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...
s and
native iron. It has a grey colour with a greenish tint in reflected
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
. Wüstite crystallizes in the
isometric-hexoctahedral crystal system in opaque to translucent metallic grains. It has a
Mohs hardness
The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fair ...
of 5 to 5.5 and a
specific gravity
Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...
of 5.88. Wüstite is a typical example of a
non-stoichiometric compound
Non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); most often, in s ...
.
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
Jharkhand (; ) is a States and union territories of India, state in East India, eastern India. The state shares its border with the states of West Bengal to the east, Chhattisgarh to the west, Uttar Pradesh to the northwest, Bihar to the north ...
, India; and as inclusions in
diamond
Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...
s in a number of
kimberlite
Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known as the main host matrix for diamonds. It is named after the town of Kimberley, Northern Cape, Kimberley in South Africa, where the discovery of an 83.5-Car ...
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
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
, with Fe
3+ state, or in somewhat less oxidizing environments,
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
, with a mixture of Fe
3+ and Fe
2+. Wüstite, in geochemistry, defines a ''
redox buffer'' of oxidation within rocks at which point the rock is so reduced that Fe
3+, 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 Fe
3+ ions in magnetite to Fe
2+ ions. An example reaction is presented below:
:
\underset + \underset -> + \underset
The formula for magnetite is more accurately written as FeO·Fe
2O
3 than as Fe
3O
4. Magnetite is one part FeO and one part Fe
2O
3, rather than a
solid solution
A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...
of wüstite and
hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
. Magnetite is termed a ''redox buffer'' because, until all Fe
3+ present in the system is converted to Fe
2+, the oxide mineral assemblage of
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
remains wüstite-magnetite. Furthermore, the redox state of the rock remains at the same level of
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
fugacity. Considering buffering the
redox potential (E
h) 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 Fe
3+ 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
Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...
or
graphite
Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
.
Effects upon silicate minerals
The ratio of Fe
2+ to Fe
3+ 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
The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
if it is present as Fe
2+; Fe
3+ cannot enter the lattice of
fayalite
Fayalite (, commonly abbreviated to Fa) is the iron-rich endmember, end-member of the olivine solid solution, solid-solution series. In common with all minerals in the olivine, olivine group, fayalite crystallizes in the orthorhombic system (spac ...
olivine and thus for every two Fe
3+ ions, one Fe
2+ 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 Fe
3+ and to maintain iron outside of silicate minerals.
In nature, carbonate rocks, potentially
carbonatite
Carbonatite () is a type of intrusive rock, intrusive or extrusive rock, extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geoche ...
,
kimberlite
Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known as the main host matrix for diamonds. It is named after the town of Kimberley, Northern Cape, Kimberley in South Africa, where the discovery of an 83.5-Car ...
s, 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
The Haber process, also called the Haber–Bosch process, is the main industrial procedure for the ammonia production, production of ammonia. It converts atmospheric nitrogen (N2) to ammonia (NH3) by a reaction with hydrogen (H2) using finely di ...
for
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
() 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
Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
(Fe
3O
4). 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
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
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.
Historical use
According to Vagn Fabritius Buchwald, wüstite was an important component during the
Iron Age
The Iron Age () is the final epoch of the three historical Metal Ages, after the Chalcolithic and Bronze Age. It has also been considered as the final age of the three-age division starting with prehistory (before recorded history) and progre ...
to facilitate the process of
forge welding
Forge welding (FOW), also called fire welding, is a solid-state welding process that joins two pieces of metal by heating them to a high temperature and then hammering them together. It may also consist of heating and forcing the metals together ...
. 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 wüstite 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 silica in the sand reacts with the wüstite to form
fayalite
Fayalite (, commonly abbreviated to Fa) is the iron-rich endmember, end-member of the olivine solid solution, solid-solution series. In common with all minerals in the olivine, olivine group, fayalite crystallizes in the orthorhombic system (spac ...
, 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
The Bronze Age () was a historical period characterised principally by the use of bronze tools and the development of complex urban societies, as well as the adoption of writing in some areas. The Bronze Age is the middle principal period of ...
and into the modern.
Related minerals
Wüstite forms a solid solution with
periclase (
MgO), and iron substitutes for magnesium. Periclase, when hydrated, forms
brucite
Brucite is the mineral form of magnesium hydroxide, with the chemical formula Magnesium, Mg(hydroxyl, OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal Vein (geology), vein mineral in metamorphosed li ...
(Mg(O
H)
2), a common product of
serpentinite metamorphic reactions.
Oxidation and hydration of wüstite forms
goethite
Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α- polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...
and
limonite
Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary qu ...
.
Zinc, aluminium, and other metals may substitute for iron in wüstite.
Wüstite in
dolomite skarns may be related to
siderite (iron(II) carbonate),
wollastonite
Wollastonite is a calcium Silicate minerals, inosilicate mineral (calcium, Casilicon, Sioxygen, O3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or D ...
,
enstatite,
diopside, and
magnesite.
See also
*
Normative mineralogy
References
Mineral Data Pub. PDF fileAccessed 3/5/2006
Accessed 3/5/2006
Accessed 3/5/2006
Webmineral dataAccessed 3/5/2006
{{DEFAULTSORT:Wustite
Cubic minerals
Iron(II) minerals
Minerals in space group 225
Non-stoichiometric compounds
Oxide minerals
Minerals described in 1927