Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.[1]

Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely used by humans, e.g., as iron ores, pigments, catalysts, in thermite (see the diagram) and hemoglobin. Common rust is a form of iron(III) oxide. Iron oxides are widely used as inexpensive, durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172.

Almindeligt rust - jernoxid
Electrochemically oxidized iron (rust)


Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3.
Red and green iron oxides
Green and reddish brown stains on a limestone core sample, respectively corresponding to oxides/hydroxides of Fe2+ and Fe3+.


Thermal expansion

Iron oxide CTE (× 10-6 °C-1)
Fe2O3 14.9[7]
Fe3O4 >9.2[7]
FeO 12.1[7]


  • goethite (α-FeOOH),
  • akaganéite (β-FeOOH),
  • lepidocrocite (γ-FeOOH),
  • feroxyhyte (δ-FeOOH),
  • ferrihydrite ( approx.), or , better recast as
  • high-pressure FeOOH
  • schwertmannite (ideally or )[8]
  • green rust ( where A is Cl or 0.5SO42−)

Microbial degradation

Several species of bacteria, including Shewanella oneidensis, Geobacter sulfurreducens and Geobacter metallireducens, metabolically utilize solid iron oxides as a terminal electron acceptor, reducing Fe(III) oxides to Fe(II) containing oxides.[9]

Environmental effects

Methanogenesis replacement by iron oxide reduction

Under conditions favoring iron reduction, the process of iron oxide reduction can replace at least 80% of methane production occurring by methanogenesis.[10] This phenomenon occurs in a nitrogen-containing (N2) environment with low sulfate concentrations. Methanogenesis, an Archaean driven process, is typically the predominate form of carbon mineralization in sediments at the bottom of the ocean. Methanogenesis completes the decomposition of organic matter to methane (CH4).[10] The specific electron donor for iron oxide reduction in this situation is still under debate, but the two potential candidates include either Titanium (III) or compounds present in yeast. The predicted reactions with Titanium (III) serving as the electron donor and phenazine-1-carboxylate (PCA) serving as an electron shuttle is as follows:

Ti(III)-cit + CO2 + 8H+ → CH4 + 2H2O + Ti(IV) + cit                           ΔE = –240 + 300 mV
Ti(III)-cit + PCA (oxidized) → PCA (reduced) + Ti(IV) + cit                ΔE = –116 + 300 mV
PCA (reduced) + Fe(OH)3 → Fe2+ + PCA (oxidized)                         ΔE = –50 + 116 mV [10]

Titanium (III) is oxidized to Titanium (IV) while PCA is reduced. The reduced form of PCA can then reduce the iron hydroxide (Fe(OH)3).

Hydroxyl radical formation

On the other hand when airborne, iron oxides have been shown to harm the lung tissues of living organisms by the formation of hydroxyl radicals, leading to the creation of alkyl radicals. The following reactions occur when Fe2O3 and FeO, hereafter represented as Fe3+ and Fe2+ respectively, iron oxide particulates accumulate in the lungs.[11]

O2 + eO2• –[11]

The formation of the superoxide anion (O2• –) is catalyzed by a transmembrane enzyme called NADPH oxidase. The enzyme facilitates the transport of an electron across the plasma membrane from cytosolic NADPH to extracellular oxygen (O2) to produce O2• –. NADPH and FAD are bound to cytoplasmic binding sites on the enzyme. Two electrons from NADPH are transported to FAD which reduces it to FADH2. Then, one electron moves to one of two heme groups in the enzyme within the plane of the membrane. The second electron pushes the first electron to the second heme group so that it can associate with the first heme group. For the transfer to occur, the second heme must be bound to extracellular oxygen which is the acceptor of the electron. This enzyme can also be located within the membranes of intracellular organelles allowing the formation of O2• – to occur within organelles.[12]

2O2• – + 2H+H
+ O2 [11][13]

The formation of hydrogen peroxide (H
) can occur spontaneously when the environment has a lower pH especially at pH 7.4.[13] The enzyme superoxide dismutase can also catalyze this reaction. Once H
has been synthesized, it can diffuse through membranes to travel within and outside the cell due to its nonpolar nature.[12]

Fe2+ + H
→ Fe3+ + HO + OH
Fe3+ + H2O2 → Fe2+ + O2• – + 2H+
H2O2 + O2• – → HO + OH + O2 [11]

Fe2+ is oxidized to Fe3+ when it donates an electron to H2O2, thus, reducing H2O2 and forming a hydroxyl radical (HO) in the process. H2O2 can then reduce Fe3+ to Fe2+ by donating an electron to it to create O2• –. O2• – can then be used to make more H2O2 by the process previously shown perpetuating the cycle, or it can react with H2O2 to form more hydroxyl radicals. Hydroxyl radicals have been shown to increase cellular oxidative stress and attack cell membranes as well as the cell genomes.[11]

HO + RH → R + H2O [11]

The HO radical produced from the above reactions with iron can abstract a hydrogen atom (H) from molecules containing an R-H bond where the R is a group attached to the rest of the molecule, in this case H, at a carbon (C).[11]

See also


  1. ^ Cornell, RM; Schwertmann, U (2003). The iron oxides: structure, properties, reactions, occurrences and uses. Wiley VCH. ISBN 3-527-30274-3.
  2. ^ Hu, Qingyang; Kim, Duck Young; Yang, Wenge; Yang, Liuxiang; Meng, Yue; Zhang, Li; Mao, Ho-Kwang (June 2016). "FeO2 and (FeO)OH under deep lower-mantle conditions and Earth's oxygen–hydrogen cycles". Nature. 534 (7606): 241–244. Bibcode:2016Natur.534..241H. doi:10.1038/nature18018. ISSN 1476-4687.
  3. ^ "Discovery of the recoverable high-pressure iron oxide Fe4O5". Proceedings of the National Academy of Sciences. 108 (42): 17281–17285. Oct 2011. Bibcode:2011PNAS..10817281L. doi:10.1073/pnas.1107573108. PMC 3198347.
  4. ^ "Synthesis of Fe5O6".
  5. ^ a b "Structural complexity of simple Fe2O3 at high pressures and temperatures".
  6. ^ "The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions".
  7. ^ a b c Fakouri Hasanabadi, M.; Kokabi, A.H.; Nemati, A.; Zinatlou Ajabshir, S. (February 2017). "Interactions near the triple-phase boundaries metal/glass/air in planar solid oxide fuel cells". International Journal of Hydrogen Energy. 42 (8): 5306–5314. doi:10.1016/j.ijhydene.2017.01.065. ISSN 0360-3199.
  8. ^ Mindat
  9. ^ Bretschger, O.; Obraztsova, A.; Sturm, C. A.; Chang, I. S.; Gorby, Y. A.; Reed, S. B.; Culley, D. E.; Reardon, C. L.; Barua, S.; Romine, M. F.; Zhou, J.; Beliaev, A. S.; Bouhenni, R.; Saffarini, D.; Mansfeld, F.; Kim, B.-H.; Fredrickson, J. K.; Nealson, K. H. (20 July 2007). "Current Production and Metal Oxide Reduction by Shewanella oneidensis MR-1 Wild Type and Mutants". Applied and Environmental Microbiology. 73 (21): 7003–7012. doi:10.1128/AEM.01087-07. PMC 2223255.
  10. ^ a b c Sivan, O.; Shusta, S. S.; Valentine, D. L. (2016-03-01). "Methanogens rapidly transition from methane production to iron reduction". Geobiology. 14 (2): 190–203. doi:10.1111/gbi.12172. ISSN 1472-4669.
  11. ^ a b c d e f g Hartwig, A.; MAK Commission 2016 (July 25, 2016). "Iron oxides (inhalable fraction) [MAK Value Documentation, 2011]". The MAK Collection for Occupational Health and Safety. Wiley-VCH Verlag GmbH & Co. KGaA. 1: 1804–1869. doi:10.1002/3527600418.mb0209fste5116.
  12. ^ a b Bedard, Karen; Krause, Karl-Heinz (2007-01-01). "The NOX Family of ROS-Generating NADPH Oxidases: Physiology and Pathophysiology". Physiological Reviews. 87 (1): 245–313. doi:10.1152/physrev.00044.2005. ISSN 0031-9333. PMID 17237347.
  13. ^ a b Chapple, Iain L. C.; Matthews, John B. (2007-02-01). "The role of reactive oxygen and antioxidant species in periodontal tissue destruction". Periodontology 2000. 43 (1): 160–232. doi:10.1111/j.1600-0757.2006.00178.x. ISSN 1600-0757.

External links


Hematite, also spelled as haematite, is the mineral form of iron(III) oxide (Fe2O3), one of several iron oxides. It is the oldest known iron oxide mineral that has ever formed on earth, and is widespread in rocks and soils. Hematite crystallizes in the rhombohedral lattice system, and it has the same crystal structure as ilmenite and corundum. Hematite and ilmenite form a complete solid solution at temperatures above 950 °C (1,740 °F).

Hematite is colored black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. Varieties include kidney ore, martite (pseudomorphs after magnetite), iron rose and specularite (specular hematite). While the forms of hematite vary, they all have a rust-red streak. Hematite is harder than pure iron, but much more brittle. Maghemite is a hematite- and magnetite-related oxide mineral.

Huge deposits of hematite are found in banded iron formations. Gray hematite is typically found in places that can have still standing water or mineral hot springs, such as those in Yellowstone National Park in North America. The mineral can precipitate out of water and collect in layers at the bottom of a lake, spring, or other standing water. Hematite can also occur without water, however, usually as the result of volcanic activity.

Clay-sized hematite crystals can also occur as a secondary mineral formed by weathering processes in soil, and along with other iron oxides or oxyhydroxides such as goethite, is responsible for the red color of many tropical, ancient, or otherwise highly weathered soils.


Inceptisols are a soil order in USDA soil taxonomy. They form quickly through alteration of parent material. They are more developed than Entisols. They have no accumulation of clays, iron oxide, aluminium oxide or organic matter. They have an ochric or umbric horizon and a cambic subsurface horizon.

In the World Reference Base for Soil Resources (WRB), most Inceptisols are Cambisols or Umbrisols. Some may be Nitisols. Many Aquepts belong to Gleysols and Stagnosols.

Indian red (color)

Indian red is a pigment composed of naturally occurring iron oxides that is widely used in India. Other shades of iron oxides include Venetian Red, English Red, and Kobe, all shown below.

Chestnut is a color similar to but separate and distinct from Indian red.

Iron(II) oxide

Iron(II) oxide or ferrous oxide is the inorganic compound with the formula FeO. Its mineral form is known as wüstite. One of several iron oxides, it is a black-colored powder that is sometimes confused with rust, the latter of which consists of hydrated iron(III) oxide (ferric oxide). Iron(II) oxide also refers to a family of related non-stoichiometric compounds, which are typically iron deficient with compositions ranging from Fe0.84O to Fe0.95O.

Iron(II,III) oxide

Iron(II,III) oxide is the chemical compound with formula Fe3O4. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe2O3) also known as hematite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙ Fe2O3. This iron oxide is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is ferrimagnetic, but is sometimes incorrectly described as ferromagnetic. Its most extensive use is as a black pigment. For this purpose, it is synthesised rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production.

Iron(III) oxide

Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main source of iron for the steel industry. Fe2O3 is readily attacked by acids. Iron(III) oxide is often called rust, and to some extent this label is useful, because rust shares several properties and has a similar composition. To a chemist, rust is considered an ill-defined material, described as hydrated ferric oxide.

Iron(III) oxide-hydroxide

A number of chemicals are dubbed iron(III) oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous (FeO(OH)) or hydrated (FeO(OH)·nH2O) forms. The monohydrate (FeO(OH)·H2O) might otherwise be described as iron(III) hydroxide (Fe(OH)3), and is also known as hydrated iron oxide or yellow iron oxide.

Iron oxide copper gold ore deposits

Iron oxide copper gold ore deposits (IOCG) are important and highly valuable concentrations of copper, gold and uranium ores hosted within iron oxide dominant gangue assemblages which share a common genetic origin.

These ore bodies range from around 10 million tonnes of contained ore, to 4,000 million tonnes or more, and have a grade of between 0.2% and 5% copper, with gold contents ranging from 0.1 to >3 grams per tonne (parts per million). These ore bodies tend to express as cone-like, blanket-like breccia sheets within granitic margins, or as long ribbon-like breccia or massive iron oxide deposits within faults or shears.

The tremendous size, relatively simple metallurgy and relatively high grade of IOCG deposits can produce extremely profitable mines.

Iron oxide copper-gold deposits are also often associated with other valuable trace elements such as uranium, bismuth and rare-earth metals, although these accessories are typically subordinate to copper and gold in economic terms.

Some examples include the Olympic Dam, South Australia, and Candelaria, Chile, deposits.

Iron oxide nanoparticle

Iron oxide nanoparticles are iron oxide particles with diameters between about 1 and 100 nanometers. The two main forms are magnetite (Fe3O4) and its oxidized form maghemite (γ-Fe2O3). They have attracted extensive interest due to their superparamagnetic properties and their potential applications in many fields (although Co and Ni are also highly magnetic materials, they are toxic and easily oxidized).

Applications of iron oxide nanoparticles include terabit magnetic storage devices, catalysis, sensors, superparamagnetic relaxometry (SPMR), and high-sensitivity biomolecular magnetic resonance imaging (MRI) for medical diagnosis and therapeutics. These applications require coating of the nanoparticles by agents such as long-chain fatty acids, alkyl-substituted amines and diols.

Manganese oxide

Manganese oxide is any of a variety of manganese oxides and hydroxides. These include

Manganese(II) oxide, MnO (aka Ferrite Grade)

Manganese(II,III) oxide, Mn3O4

Manganese(III) oxide, Mn2O3

Manganese dioxide, (manganese(IV) oxide), MnO2

Manganese(VI) oxide, MnO3

Manganese(VII) oxide, Mn2O7It may refer more specifically to the following manganese minerals:






PyrolusiteManganese may also form mixed oxides with other metals such as Fe, Nb, Ta, ... :

Bixbyite, a manganese iron oxide mineral

Jacobsite, a manganese iron oxide mineral

Columbite, also called niobite, niobite-tantalite and columbate

Tantalite, a mineral group close to columbite

Coltan, a mixture of columbite and tantalite

Galaxite, a spinel mineral

Todorokite, a rare complex hydrous manganese oxide mineral


Ochre (British English) ( OH-kər; from Ancient Greek: ὤχρα, from ὠχρός, ōkhrós, pale) or ocher (American English) is a natural clay earth pigment which is a mixture of ferric oxide and varying amounts of clay and sand. It ranges in colour from yellow to deep orange or brown. It is also the name of the colours produced by this pigment, especially a light brownish-yellow. A variant of ochre containing a large amount of hematite, or dehydrated iron oxide, has a reddish tint known as "red ochre" (or, in some dialects, ruddle).

The word ochre also describes clays colored with iron oxide, derived during the extraction of tin and copper.

Ordinary chondrite

The ordinary chondrites (sometimes called the O chondrites) are a class of stony chondritic meteorites. They are by far the most numerous group and comprise about 87% of all finds. Hence, they have been dubbed "ordinary". The ordinary chondrites are thought to have originated from three parent asteroids, with the fragments making up the H chondrite, L chondrite and LL chondrite groups respectively.


Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of iron oxide (Fe2O3). Other colors, such as yellow, green, blue and orange, are due to other minerals.

When sandstone is cemented to quartzite, the individual quartz grains recrystallize along with the former cementing material to form an interlocking mosaic of quartz crystals. Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism. The grainy, sandpaper-like surface becomes glassy in appearance. Minor amounts of former cementing materials, iron oxide, silica, carbonate and clay, often migrate during recrystallization and metamorphosis. This causes streaks and lenses to form within the quartzite.

Orthoquartzite is a very pure quartz sandstone composed of usually well-rounded quartz grains cemented by silica. Orthoquartzite is often 99% SiO2 with only very minor amounts of iron oxide and trace resistant minerals such as zircon, rutile and magnetite. Although few fossils are normally present, the original texture and sedimentary structures are preserved.

The term is also traditionally used for quartz-cemented quartz arenites, and both usages are found in the literature. The typical distinction between the two (since each is a gradation into the other) is a metamorphic quartzite is so highly cemented, diagenetically altered, and metamorphosized so that it will fracture and break across grain boundaries, not around them.

Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. The nearly pure silica content of the rock provides little for soil; therefore, the quartzite ridges are often bare or covered only with a very thin layer of soil and little (if any) vegetation.


Rust is an iron oxide, a usually red oxide formed by the redox reaction of iron and oxygen in the presence of water or air moisture. Several forms of rust are distinguishable both visually and by spectroscopy, and form under different circumstances. Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3).

Given sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust and disintegrate. Surface rust is flaky and

friable, and it provides no protection to the underlying iron, unlike the formation of patina on copper surfaces. Rusting is the common term for corrosion of iron and its alloys, such as steel. Many other metals undergo similar corrosion, but the resulting oxides are not commonly called rust.Other forms of rust exist, like the result of reactions between iron and chloride in an environment deprived of oxygen. Rebar used in underwater concrete pillars, which generates green rust, is an example. Although rusting is generally a negative aspect of iron, a particular form of rusting, known as "stable rust," causes the object to have a thin coating of rust over the top, and if kept in low relative humidity, makes the "stable" layer protective to the iron below, but not to the extent of other oxides, such as aluminum.

Rust (color)

Rust is an orange-red-brown color resembling iron oxide. It is a commonly used color in stage lighting, and appears roughly the same color as photographic safelights when used over a standard tungsten light source. The color is number 777 in the Lee Filters swatch book.

The first recorded use of rust as a color name in English was in 1692.


Sienna (from Italian: terra di Siena, "Siena earth") is an earth pigment containing iron oxide and manganese oxide. In its natural state, it is yellow-brown and is called raw sienna. When heated, it becomes a reddish brown and is called burnt sienna. It takes its name from the city-state of Siena, where it was produced during the Renaissance. Along with ochre and umber, it was one of the first pigments to be used by humans, and is found in many cave paintings. Since the Renaissance, it has been one of the brown pigments most widely used by artists.

The first recorded use of sienna as a colour name in English was in 1760.


Sinopia (also known as sinoper, named after the now Turkish city Sinop) is a dark reddish-brown natural earth pigment, whose reddish colour comes from hematite, a dehydrated form of iron oxide. It was widely used in Classical Antiquity and the Middle Ages for painting, and during the Renaissance it was often used on the rough initial layer of plaster for the underdrawing for a fresco. The word came to be used both for the pigment and for the preparatory drawing itself, which may be revealed when a fresco is stripped from its wall for transfer.

During the Middle Ages synopia in Latin and Italian came to mean simply a red ochre. It entered the English language as the word sinoper, meaning a red earth colour.Sinopia is a colour in various modern colour systems.


Umber is a natural brown or reddish-brown earth pigment that contains iron oxide and manganese oxide. Umber is darker than the other similar earth pigments, ochre and sienna.In its natural form, it is called raw umber. When heated (calcinated), the color becomes more intense, and then becomes known as burnt umber.

The name comes from terra d'ombra, or earth of Umbria, the Italian name of the pigment. Umbria is a mountainous region in central Italy where the pigment was originally extracted. The word also may be related to the Latin word Umbra.Umber is not one precise color, but a range of different colors, from medium to dark, from yellowish to reddish to grayish. The color of the natural earth depends upon the amount of iron oxide and manganese in the clay. Umber earth pigments contain between five and twenty percent manganese oxide, which accounts for their being a darker color than yellow ochre or sienna.

Commercial colors vary depending upon the manufacturer or the color list. Not all umber pigments contain natural earths; some contain synthetic iron and manganese oxide, indicated on the label. Pigments containing the natural umber earths indicate them on the label as PBr7 (Pigment brown 7), following the Colour Index International system.

The color shown in the box at right is one of the many commercial varieties of umber, from the ISCC-NBS color list: ISCC-NBS Dictionary of Color Names (1955)—Color Sample of Umber (color sample #61).

Venetian red

Venetian red is a light and warm (somewhat unsaturated) pigment that is a darker shade of scarlet, derived from nearly pure ferric oxide (Fe2O3) of the hematite type. Modern versions are frequently made with synthetic red iron oxide.

Historically, Venetian red was a red earth color often used in Italian Renaissance paintings. It was also called sinopia, because the best-quality pigment came from the port of Sinop in northern Turkey. It was the major ingredient in the pigment called cinabrese, described by the 15th-century Italian painter and writer Cennino Cennini in his handbook on painting, Il Libro dell'Arte. Cennini recommended mixing Venetian red with lime white, in proportions of two to one, to paint the skin tones of faces, hands and nudes.The first recorded use of Venetian red as a color name in English was in 1753. Venetian red was the defining colour used by the British Army since the end of the English Civil War until its replacement with khaki, in the 1890s, mainly noted as being the primary colour of a Redcoat's dress, during the 18th and 19th centuries.

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