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Electrochemically oxidized iron (rust)

Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. [1]

Iron oxides and oxyhydroxides are widespread in nature and play an important role in many geological and biological processes. They are used as iron ores, pigments, catalysts, and in thermite, and occur in hemoglobin. Iron oxides are inexpensive and 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.


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

Iron oxides feature as ferrous ( Fe(II)) or ferric ( Fe(III)) or both. They adopt octahedral or tetrahedral coordination geometry. Only a few oxides are significant at the earth's surface, particularly wüstite, magnetite, and hematite.

Thermal expansion

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


  • goethite (α-FeOOH),
  • akaganéite (β-FeOOH),
  • lepidocrocite (γ-FeOOH),
  • feroxyhyte (δ-FeOOH),
  • ferrihydrite (Fe5HO8 · 4 H2O approx., or 5 Fe2O3 · 9 H2O, better recast as FeOOH · 0.4 H2O)
  • high-pressure pyrite-structured FeOOH. [7] Once dehydration is triggered, this phase may form FeO2Hx (0 < x < 1). [8]
  • green rust (FeIII
    OH3x + yz (A)z where A is Cl or 0.5 SO2−4)


In blast furnaces and related factories, iron oxides are converted to the metal. Typical reducing agents are various forms of carbon. A representative reaction starts with ferric oxide: [9]

2 Fe2O3 + 3 C → 4 Fe + 3 CO2

In nature

Iron is stored in many organisms in the form of ferritin, which is a ferrous oxide encased in a solubilizing protein sheath. [10]

Species of bacteria, including Shewanella oneidensis, Geobacter sulfurreducens and Geobacter metallireducens, use iron oxides as terminal electron acceptors. [11]


Almost all iron ores are oxides, so in that sense these materials are important precursors to iron metal and its many alloys.

Iron oxides are important pigments, coming in a variety of colors (black, red, yellow). Among their many advantages, they are inexpensive, strongly colored, and nontoxic. [12]

Magnetite is a component of magnetic recording tapes.

See also


  1. ^ Cornell., RM.; Schwertmann, U (2003). The iron oxides: structure, properties, reactions, occurrences and. Wiley VCH. ISBN  978-3-527-30274-1.
  2. ^ Lavina, B.; Dera, P.; Kim, E.; Meng, Y.; Downs, R. T.; Weck, P. F.; Sutton, S. R.; Zhao, Y. (Oct 2011). "Discovery of the recoverable high-pressure iron oxide Fe4O5". Proceedings of the National Academy of Sciences. 108 (42): 17281–17285. Bibcode: 2011PNAS..10817281L. doi: 10.1073/pnas.1107573108. PMC  3198347. PMID  21969537.
  3. ^ Lavina, Barbara; Meng, Yue (2015). "Synthesis of Fe5O6". Science Advances. 1 (5): e1400260. doi: 10.1126/sciadv.1400260. PMC  4640612. PMID  26601196.
  4. ^ a b Bykova, E.; Dubrovinsky, L.; Dubrovinskaia, N.; Bykov, M.; McCammon, C.; Ovsyannikov, S. V.; Liermann, H. -P.; Kupenko, I.; Chumakov, A. I.; Rüffer, R.; Hanfland, M.; Prakapenka, V. (2016). "Structural complexity of simple Fe2O3 at high pressures and temperatures". Nature Communications. 7: 10661. Bibcode: 2016NatCo...710661B. doi: 10.1038/ncomms10661. PMC  4753252. PMID  26864300.
  5. ^ Merlini, Marco; Hanfland, Michael; Salamat, Ashkan; Petitgirard, Sylvain; Müller, Harald (2015). "The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions". American Mineralogist. 100 (8–9): 2001–2004. Bibcode: 2015AmMin.100.2001M. doi: 10.2138/am-2015-5369. S2CID  54496448.
  6. ^ 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.
  7. ^ Nishi, Masayuki; Kuwayama, Yasuhiro; Tsuchiya, Jun; Tsuchiya, Taku (2017). "The pyrite-type high-pressure form of FeOOH". Nature. 547 (7662): 205–208. Bibcode: 2017Natur.547..205N. doi: 10.1038/nature22823. ISSN  1476-4687. PMID  28678774. S2CID  205257075.
  8. ^ Hu, Qingyang; Kim, Duckyoung; Liu, Jin; Meng, Yue; Liuxiang, Yang; Zhang, Dongzhou; Mao, Wendy L.; Mao, Ho-kwang (2017). "Dehydrogenation of goethite in Earth's deep lower mantle". Proceedings of the National Academy of Sciences. 114 (7): 1498–1501. Bibcode: 2017PNAS..114.1498H. doi: 10.1073/pnas.1620644114. PMC  5320987. PMID  28143928.
  9. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1072. ISBN  978-0-08-037941-8.
  10. ^ Honarmand Ebrahimi, Kourosh; Hagedoorn, Peter-Leon; Hagen, Wilfred R. (2015). "Unity in the Biochemistry of the Iron-Storage Proteins Ferritin and Bacterioferritin". Chemical Reviews. 115 (1): 295–326. doi: 10.1021/cr5004908. PMID  25418839.
  11. ^ 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. Bibcode: 2007ApEnM..73.7003B. doi: 10.1128/AEM.01087-07. PMC  2223255. PMID  17644630.
  12. ^ Buxbaum, Gunter; Printzen, Helmut; Mansmann, Manfred; Räde, Dieter; Trenczek, Gerhard; Wilhelm, Volker; Schwarz, Stefanie; Wienand, Henning; Adel (2009). "Pigments, Inorganic, 3. Colored Pigments". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi: 10.1002/14356007.n20_n02. ISBN  978-3527306732.

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