:Iron

{{Main|Allotropes of iron}}

File:Iron-alpha-pV.svg

At least four allotropes of iron (differing atom arrangements in the solid) are known, conventionally denoted α, γ, δ, and ε.

The first three forms are observed at ordinary pressures. As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic (bcc) crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic (fcc) crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}}

The physical properties of iron at very high pressures and temperatures have also been studied extensively,{{Cite journal|vauthors=Tateno S, Hirose K |title=The Structure of Iron in Earth's Inner Core| journal=Science| volume=330| pages=359–361| publisher=American Association for the Advancement of Science| date=2010| doi=10.1126/science.1194662| issue=6002| pmid=20947762| bibcode=2010Sci...330..359T| s2cid=206528628}}{{Cite journal| first=Gaminchev| last=Chamati| title=Dynamic stability of Fe under high pressure| journal=Journal of Physics| volume=558| pages=012013| publisher=IOP Publishing| date=2014| doi=10.1088/1742-6596/558/1/012013| issue=1| bibcode=2014JPhCS.558a2013G| doi-access=free}} because of their relevance to theories about the cores of the Earth and other planets. Above approximately 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed (hcp) structure, which is also known as ε-iron. The higher-temperature γ-phase also changes into ε-iron, but does so at higher pressure.

Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K. It is supposed to have an orthorhombic or a double hcp structure.{{Cite journal| first=Reinhard| last=Boehler| title=High-pressure experiments and the phase diagram of lower mantle and core materials| journal =Reviews of Geophysics| volume=38| pages=221–45| publisher=American Geophysical Union| date=2000| doi=10.1029/1998RG000053| issue=2| bibcode=2000RvGeo..38..221B| s2cid=33458168| doi-access=free}} (Confusingly, the term "β-iron" is sometimes also used to refer to α-iron above its Curie point, when it changes from being ferromagnetic to paramagnetic, even though its crystal structure has not changed.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}})

The Earth's inner core is generally presumed to consist of an iron-nickel alloy with ε (or β) structure.{{Cite journal |last1=Stixrude |first1=Lars |last2=Wasserman |first2=Evgeny |last3=Cohen |first3=Ronald E. |date=1997-11-10 |title=Composition and temperature of Earth's inner core |journal=Journal of Geophysical Research: Solid Earth |volume=102 |issue=B11 |pages=24729–39 |doi=10.1029/97JB02125 |bibcode=1997JGR...10224729S |doi-access=free}}

File:Pure iron phase diagram (EN).svg of pure iron]]

The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus;{{sfn|Greenwood|Earnshaw|1997|p=1116}} however, they are higher than the values for the previous element manganese because that element has a half-filled 3d sub-shell and consequently its d-electrons are not easily delocalized. This same trend appears for ruthenium but not osmium.{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}}

The melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data (as of 2007) still varies by tens of gigapascals and over a thousand kelvin.{{Cite book| pages=527–41 |doi=10.1016/B978-044452748-6.00047-X|title =Mineral Physics|first1 = Reinhard|last1 = Boehler|first2= M.|last2 = Ross|chapter = Properties of Rocks and Minerals_High-Pressure Melting|publisher = Elsevier| date = 2007| series = Treatise on Geophysics| volume = 2|isbn=9780444527486}}

File:Magnetization curves.svg

Below its Curie point of {{Convert|770|C|F K|abbr=on}}, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom generally align with the spins of its neighbors, creating an overall magnetic field.{{cite book |last=Cullity |author2=C. D. Graham |title=Introduction to Magnetic Materials, 2nd|publisher=Wiley–IEEE|year=2008 |location=New York |page=116 |url=https://books.google.com/books?id=ixAe4qIGEmwC&pg=PA116 |isbn=978-0-471-47741-9}} This happens because the orbitals of those two electrons (d_z² and d_{x² − y²}) do not point toward neighboring atoms in the lattice, and therefore are not involved in metallic bonding.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}}

In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometers across,{{Cite book| chapter-url={{Google books|hoM8VJHTt24C|page=PA24|keywords=|text=|plainurl=yes}}|pages=24–28|title =Metallographer's guide: practice and procedures for irons and steels|first1 = B.L.|last1 = Bramfitt|first2= Arlan O.|last2 = Benscoter|chapter = The Iron Carbon Phase Diagram|publisher = ASM International| date = 2002| isbn = 978-0-87170-748-2}} such that the atoms in each domain have parallel spins, but some domains have other orientations. Thus a macroscopic piece of iron will have a nearly zero overall magnetic field.

Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field. This effect is exploited in devices that need to channel magnetic fields to fulfill design function, such as electrical transformers, magnetic recording heads, and electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists even after the external field is removed – thus turning the iron object into a (permanent) magnet.

Similar behavior is exhibited by some iron compounds, such as the ferrites including the mineral magnetite, a crystalline form of the mixed iron(II,III) oxide {{chem2|Fe3O4}} (although the atomic-scale mechanism, ferrimagnetism, is somewhat different). Pieces of magnetite with natural permanent magnetization (lodestones) provided the earliest compasses for navigation. Particles of magnetite were extensively used in magnetic recording media such as core memories, magnetic tapes, floppies, and disks, until they were replaced by cobalt-based materials.

{{Main|Isotopes of iron}}

Iron has four stable isotopes: ⁵⁴Fe (5.845% of natural iron), ⁵⁶Fe (91.754%), ⁵⁷Fe (2.119%) and ⁵⁸Fe (0.282%). Twenty-four artificial isotopes have also been created. Of these stable isotopes, only ⁵⁷Fe has a nuclear spin (−{{frac|1|2}}). The nuclide ⁵⁴Fe theoretically can undergo double electron capture to ⁵⁴Cr, but the process has never been observed and only a lower limit on the half-life of 4.4×10²⁰ years has been established.{{cite journal | last1=Bikit | first1=I. | last2=Krmar | first2=M. | last3=Slivka | first3=J. | last4=Vesković | first4=M. | last5=Čonkić | first5=Lj. | last6=Aničin | first6=I. | title=New results on the double β decay of iron | journal=Physical Review C | volume=58 | issue=4 | date=1998-10-01 | issn=0556-2813 | doi=10.1103/PhysRevC.58.2566 | pages=2566–2567| bibcode=1998PhRvC..58.2566B }}

⁶⁰Fe is an extinct radionuclide of long half-life (2.6 million years).{{cite journal |last1=Rugel |first1=G. |last2=Faestermann |first2=T. |last3=Knie |first3=K. |last4=Korschinek |first4=G. |last5=Poutivtsev |first5=M. |last6=Schumann |first6=D. |last7=Kivel |first7=N. |last8=Günther-Leopold |first8=I. |last9=Weinreich |first9=R.|last10=Wohlmuther |first10=M. |title=New Measurement of the ⁶⁰Fe Half-Life |journal=Physical Review Letters |volume=103 |issue=7 |page=072502 |date=2009 |doi=10.1103/PhysRevLett.103.072502 |pmid=19792637 |bibcode=2009PhRvL.103g2502R |url=https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A17743/datastream/PDF/view}} It is not found on Earth, but its ultimate decay product is its granddaughter, the stable nuclide ⁶⁰Ni.{{NUBASE2020|ref}} Much of the past work on isotopic composition of iron has focused on the nucleosynthesis of ⁶⁰Fe through studies of meteorites and ore formation. In the last decade, advances in mass spectrometry have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work is driven by the Earth and planetary science communities, although applications to biological and industrial systems are emerging.{{Cite journal|last1=Dauphas|first1=N.|last2=Rouxel|first2=O.|date=2006|title=Mass spectrometry and natural variations of iron isotopes|journal=Mass Spectrometry Reviews |volume=25 |issue=4 |pages=515–50 |doi=10.1002/mas.20078 |url=https://geosci.uchicago.edu/~dauphas/OLwebsite/PDFfiles/Dauphas_Rouxel_MSR06.pdf |pmid=16463281 |bibcode=2006MSRv...25..515D |url-status=dead|archive-url=https://web.archive.org/web/20100610095913/https://geosci.uchicago.edu/~dauphas/OLwebsite/PDFfiles/Dauphas_Rouxel_MSR06.pdf |archive-date=10 June 2010}}

In phases of the meteorites Semarkona and Chervony Kut, a correlation between the concentration of ⁶⁰Ni, the granddaughter of ⁶⁰Fe, and the abundance of the stable iron isotopes provided evidence for the existence of ⁶⁰Fe at the time of formation of the Solar System. Possibly the energy released by the decay of ⁶⁰Fe, along with that released by ²⁶Al, contributed to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of ⁶⁰Ni present in extraterrestrial material may bring further insight into the origin and early history of the Solar System.{{cite journal |title=Evidence for live 60Fe in meteorites |date=2004 |last1=Mostefaoui |first1=S. |last2=Lugmair |first2=G.W. |last3=Hoppe |first3=P. |last4=El Goresy |first4=A. |journal=New Astronomy Reviews |volume=48 |issue=1–4 |pages=155–59 |doi=10.1016/j.newar.2003.11.022 |bibcode=2004NewAR..48..155M}}

The most abundant iron isotope ⁵⁶Fe is of particular interest to nuclear scientists because it represents the most common endpoint of nucleosynthesis.{{cite journal|last1=Fewell|first1=M. P.|title=The atomic nuclide with the highest mean binding energy |journal=American Journal of Physics|volume=63|issue=7 |page=653|date=1995|doi=10.1119/1.17828 |bibcode=1995AmJPh..63..653F}} Since ⁵⁶Ni (14 alpha particles) is easily produced from lighter nuclei in the alpha process in nuclear reactions in supernovae (see silicon burning process), it is the endpoint of fusion chains inside extremely massive stars. Although adding more alpha particles is possible, but nonetheless the sequence does effectively end at ⁵⁶Ni because conditions in stellar interiors cause the competition between photodisintegration and the alpha process to favor photodisintegration around ⁵⁶Ni.{{cite journal |last=Fewell |first=M.P. |date=1995-07-01 |title=The atomic nuclide with the highest mean binding energy |journal=American Journal of Physics |volume=63 |issue=7 |pages=653–658 |doi=10.1119/1.17828 |bibcode=1995AmJPh..63..653F |issn=0002-9505}}{{cite journal |last1=Burbidge |first1=E. Margaret |author-link1=Margaret Burbidge |last2=Burbidge |first2=G.R. |author-link2=Geoffrey Burbidge |last3=Fowler |first3=William A. |author-link3=William Alfred Fowler |last4=Hoyle |first4=F. |author-link4=Fred Hoyle |date=1957-10-01 |title=Synthesis of the elements in stars |journal=Reviews of Modern Physics |volume=29 |issue=4 |pages=547–650 |bibcode=1957RvMP...29..547B |doi=10.1103/RevModPhys.29.547 |doi-access=free}} This ⁵⁶Ni, which has a half-life of about 6 days, is created in quantity in these stars, but soon decays by two successive positron emissions within supernova decay products in the supernova remnant gas cloud, first to radioactive ⁵⁶Co, and then to stable ⁵⁶Fe. As such, iron is the most abundant element in the core of red giants, and is the most abundant metal in iron meteorites and in the dense metal cores of planets such as Earth.{{sfn|Greenwood|Earnshaw|1997|p=12}} It is also very common in the universe, relative to other stable metals of approximately the same atomic weight.{{sfn|Greenwood|Earnshaw|1997|p=12}}{{cite journal |last1=Woosley |first1=S. |last2=Janka |first2=T. |title=The physics of core collapse supernovae |year=2006 |arxiv=astro-ph/0601261 |doi=10.1038/nphys172 |volume=1 |issue=3 |journal=Nature Physics |pages=147–54| bibcode=2005NatPh...1..147W |s2cid=118974639}} Iron is the sixth most abundant element in the universe, and the most common refractory element.{{cite journal

| last1=McDonald | first1=I. | last2=Sloan | first2=G. C.

| last3=Zijlstra | first3=A. A. | last4=Matsunaga | first4=N.

| last5=Matsuura | first5=M. | last6=Kraemer | first6=K. E.

| last7=Bernard-Salas | first7=J. | last8=Markwick | first8=A. J.

| title=Rusty Old Stars: A Source of the Missing Interstellar Iron?

| journal=The Astrophysical Journal Letters

| volume=717 | issue=2 | pages=L92–L97 | date=2010

| doi=10.1088/2041-8205/717/2/L92 | bibcode=2010ApJ...717L..92M |arxiv = 1005.3489 | s2cid=14437704 }}

File:Ironattenuation.PNG mass attenuation coefficient for iron]]

Although a further tiny energy gain could be extracted by synthesizing ⁶²Ni, which has a marginally higher binding energy than ⁵⁶Fe, conditions in stars are unsuitable for this process. Element production in supernovas greatly favor iron over nickel, and in any case, ⁵⁶Fe still has a lower mass per nucleon than ⁶²Ni due to its higher fraction of lighter protons.{{cite journal |title=Iron and Nickel Abundances in H~II Regions and Supernova Remnants |date=1995 |bibcode=1995AAS...186.3707B |last1=Bautista |first1= Manuel A. |last2=Pradhan |first2=Anil K. |journal=Bulletin of the American Astronomical Society |volume=27 |page=865}} Hence, elements heavier than iron require a supernova for their formation, involving rapid neutron capture by starting ⁵⁶Fe nuclei.{{sfn|Greenwood|Earnshaw|1997|p=12}}

In the far future of the universe, assuming that proton decay does not occur, cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into ⁵⁶Fe nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting all stellar-mass objects to cold spheres of pure iron.{{cite journal |title=Time without end: Physics and biology in an open universe |first=Freeman J. |last=Dyson |journal=Reviews of Modern Physics |volume=51 |issue=3 |year=1979 |pages=447–60 |doi=10.1103/RevModPhys.51.447 |bibcode = 1979RvMP...51..447D }}

Iron's abundance in rocky planets like Earth is due to its abundant production during the runaway fusion and explosion of type Ia supernovae, which scatters the iron into space.{{Cite web |last=Aron |first=Jacob |title=Supernova space bullets could have seeded Earth's iron core |url=https://www.newscientist.com/article/dn27570-supernova-space-bullets-could-have-seeded-earths-iron-core/ |access-date=2020-10-02 |website=New Scientist |language=en-US}}{{Cite web |last=Croswell |first=Ken |title=Iron in the Fire: The Little-Star Supernovae That Could |url=https://www.scientificamerican.com/article/little-star-supernovae-that-could-dwarf-stars/ |access-date=2021-01-03 |website=Scientific American |language=en}}

File:Widmanstatten hand.jpg)]]

Metallic or native iron is rarely found on the surface of the Earth because it tends to oxidize. However, both the Earth's inner and outer core, which together account for 35% of the mass of the whole Earth, are believed to consist largely of an iron alloy, possibly with nickel. Electric currents in the liquid outer core are believed to be the origin of the Earth's magnetic field. The other terrestrial planets (Mercury, Venus, and Mars) as well as the Moon are believed to have a metallic core consisting mostly of iron. The M-type asteroids are also believed to be partly or mostly made of metallic iron alloy.

The rare iron meteorites are the main form of natural metallic iron on the Earth's surface. Items made of cold-worked meteoritic iron have been found in various archaeological sites dating from a time when iron smelting had not yet been developed; and the Inuit in Greenland have been reported to use iron from the Cape York meteorite for tools and hunting weapons.{{cite journal|last=Buchwald |first= V F| title = On the Use of Iron by the Eskimos in Greenland| journal = Materials Characterization| volume = 29| issue = 2| year = 1992 | pages = 139–176 | doi = 10.1016/1044-5803(92)90112-U }} About 1 in 20 meteorites consist of the unique iron-nickel minerals taenite (35–80% iron) and kamacite (90–95% iron).{{Cite book |url={{Google books|QDU7AAAAIAAJ|page=PA152|keywords=|text=|plainurl=yes}} |page=152 |title=Planet earth: cosmology, geology, and the evolution of life and environment |publisher=Cambridge University Press|first=Cesare|last=Emiliani |date=1992 |isbn=978-0-521-40949-0 |bibcode=1992pecg.book.....E}} Native iron is also rarely found in basalts that have formed from magmas that have come into contact with carbon-rich sedimentary rocks, which have reduced the oxygen fugacity sufficiently for iron to crystallize. This is known as telluric iron and is described from a few localities, such as Disko Island in West Greenland, Yakutia in Russia and Bühl in Germany.{{Cite journal |last1=Pernet-Fisher |first1=J. |last2=Day |first2=J.M.D. |last3=Howarth |first3=G.H. |last4=Ryabov |first4=V.V. |last5=Taylor |first5=L.A. |date=2017 |title=Atmospheric outgassing and native-iron formation during carbonaceous sediment–basalt melt interactions |url=https://www.researchgate.net/publication/312455203 |journal=Earth and Planetary Science Letters |volume=460 |pages=201–212 |doi=10.1016/j.epsl.2016.12.022|bibcode=2017E&PSL.460..201P |doi-access=free }}

Ferropericlase {{chem2|(Mg,Fe)O}}, a solid solution of periclase (MgO) and wüstite (FeO), makes up about 20% of the volume of the lower mantle of the Earth, which makes it the second most abundant mineral phase in that region after silicate perovskite {{chem2|(Mg,Fe)SiO3}}; it also is the major host for iron in the lower mantle.Stark, Anne M. (20 September 2007) [https://web.archive.org/web/20100527235247/https://publicaffairs.llnl.gov/news/news_releases/2007/NR-07-09-03.html Researchers locate mantle's spin transition zone, leading to clues about earth's structure]. Lawrence Livermore National Laboratory At the bottom of the transition zone of the mantle, the reaction γ-{{chem2|(Mg,Fe)2[SiO4] ↔ (Mg,Fe)[SiO3] + (Mg,Fe)O}} transforms γ-olivine into a mixture of silicate perovskite and ferropericlase and vice versa. In the literature, this mineral phase of the lower mantle is also often called magnesiowüstite.[https://www.mindat.org/min-35903.html Ferropericlase]. Mindat.org Silicate perovskite may form up to 93% of the lower mantle,{{cite journal |last=Murakami |first=M. |author2=Ohishi Y. |author3=Hirao N. |author4=Hirose K. |year=2012 |title=A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data |journal=Nature |volume=485 |issue=7396 |pages=90–94|bibcode=2012Natur.485...90M |doi=10.1038/nature11004 |pmid=22552097 |s2cid=4387193}} and the magnesium iron form, {{chem2|(Mg,Fe)SiO3}}, is considered to be the most abundant mineral in the Earth, making up 38% of its volume.{{cite journal|last1=Sharp|first1=T.|title=Bridgmanite – named at last |journal=Science |date=27 November 2014 |volume=346 |issue=6213 |pages=1057–58 |doi=10.1126/science.1261887 |pmid=25430755 |bibcode=2014Sci...346.1057S |s2cid=206563252}}

File:Roussillon_sentier_des_ocres2.JPG]]

While iron is the most abundant element on Earth, most of this iron is concentrated in the inner and outer cores.{{Cite journal|last1=Kong|first1=L. T.|last2=Li|first2=J. F.|last3=Shi|first3=Q. W.|last4=Huang|first4=H. J.|last5=Zhao|first5=K.|date=2012-03-06|title=Dynamical stability of iron under high-temperature and high-pressure conditions|journal=EPL|volume=97|issue=5|pages=56004p1–56004p5|doi=10.1209/0295-5075/97/56004|bibcode=2012EL.....9756004K|s2cid=121861429 }}{{Cite journal|last1=Gaminchev|first1=K. G.|last2=Chamati|first2=H.|date=2014-12-03|title=Dynamic stability of Fe under high pressure|journal=J. Phys.|volume=558|issue=1|pages=012013(1–7)|doi=10.1088/1742-6596/558/1/012013|bibcode=2014JPhCS.558a2013G|doi-access=free}} The fraction of iron that is in Earth's crust only amounts to about 5% of the overall mass of the crust and is thus only the fourth most abundant element in that layer (after oxygen, silicon, and aluminium).{{Cite journal|name-list-style=amp|date=1980|title=Chemical composition of Earth, Venus, and Mercury|journal=Proc. Natl. Acad. Sci.|volume=77|issue=12|pages=6973–77|bibcode=1980PNAS...77.6973M|doi=10.1073/pnas.77.12.6973|pmc=350422|pmid=16592930|last1=Morgan |first1= John W. |last2=Anders |first2= Edward |doi-access=free}}

Most of the iron in the crust is combined with various other elements to form many iron minerals. An important class is the iron oxide minerals such as hematite (Fe₂O₃), magnetite (Fe₃O₄), and siderite (FeCO₃), which are the major ores of iron. Many igneous rocks also contain the sulfide minerals pyrrhotite and pentlandite.{{cite web|url=https://www.mindat.org/min-3328.html |publisher=Mindat.org| title=Pyrrhotite|access-date=2009-07-07}}Klein, Cornelis and Cornelius S. Hurlbut, Jr. (1985) Manual of Mineralogy, Wiley, 20th ed, pp. 278–79 {{ISBN|0-471-80580-7}} During weathering, iron tends to leach from sulfide deposits as the sulfate and from silicate deposits as the bicarbonate. Both of these are oxidized in aqueous solution and precipitate in even mildly elevated pH as iron(III) oxide.{{sfn|Greenwood|Earnshaw|1997|p=1071}}

File:Outcropping banded iron formation - panoramio.jpg

Large deposits of iron are banded iron formations, a type of rock consisting of repeated thin layers of iron oxides alternating with bands of iron-poor shale and chert. The banded iron formations were laid down in the time between {{Ma|3700}} and {{Ma|1800}}.{{Cite journal| first1 = T. W.|last2 = Reinhard|title = Early Earth: Oxygen for heavy-metal fans|journal = Nature|volume = 461|issue = 7261|pages = 179–181|date = 2009|last1 = Lyons|doi = 10.1038/461179a|pmid = 19741692|first2 = C. T.|bibcode=2009Natur.461..179L|s2cid = 205049360|doi-access = free}}{{Cite journal| first1 = P.|title = Paleoecological Significance of the Banded Iron-Formation|journal = Economic Geology|volume = 68|last1 = Cloud|pages = 1135–43|date = 1973|doi = 10.2113/gsecongeo.68.7.1135| issue = 7| bibcode=1973EcGeo..68.1135C }}

Materials containing finely ground iron(III) oxides or oxide-hydroxides, such as ochre, have been used as yellow, red, and brown pigments since pre-historical times. They contribute as well to the color of various rocks and clays, including entire geological formations like the Painted Hills in Oregon and the Buntsandstein ("colored sandstone", British Bunter).Dickinson, Robert E. (1964). Germany: A regional and economic geography (2nd ed.). London: Methuen. Through Eisensandstein (a jurassic 'iron sandstone', e.g. from Donzdorf in Germany)[https://web.archive.org/web/20040913020231/https://www.lgrb.uni-freiburg.de/lgrb/Fachbereiche/rohstoffgeologie/grundlagen/lagerstaetten/naturwerksteine Naturwerksteine in Baden-Württemberg.] Landesamt für Geologie, Rohstoffe und Bergbau, Baden-Württemberg and Bath stone in the UK, iron compounds are responsible for the yellowish color of many historical buildings and sculptures.{{cite web|url=https://minervaconservation.com/articles/talesfromtheriverbank.html|title=Tales From The Riverbank|publisher=Minerva Stone Conservation|access-date=22 September 2015|archive-date=28 September 2015|archive-url=https://web.archive.org/web/20150928031602/http://www.minervaconservation.com/articles/talesfromtheriverbank.html|url-status=dead}} The proverbial red color of the surface of Mars is derived from an iron oxide-rich regolith.{{Cite journal|last2=Morris|first2=R. V.|last3=Souza|first3=P. A.|last4=Rodionov|first4=D.|last5=Schröder|first5=C.|date=2007|title=Two earth years of Mössbauer studies of the surface of Mars with MIMOS II|journal=Hyperfine Interactions|volume=170|issue=1–3|pages=169–77|bibcode=2006HyInt.170..169K|doi=10.1007/s10751-007-9508-5|last1=Klingelhöfer|first1=G.|s2cid=98227499}}

Significant amounts of iron occur in the iron sulfide mineral pyrite (FeS₂), but it is difficult to extract iron from it and it is therefore not exploited.{{cite book | last1=Winderlich | first1=R. | last2=Peter | first2=W. | title=Lehrbuch der Chemie für Höhere Lehranstalten : Einheitsausgabe für Unter- und Oberstufe | publication-place=Wiesbaden | date=1954 | isbn=978-3-663-04370-6 | oclc=913701506 | language=de|page=75 |publisher=Vieweg+Teubner Verlag }} In fact, iron is so common that production generally focuses only on ores with very high quantities of it.{{cite book | last=Bertau | first=Martin | title=Industrielle Anorganische Chemie | publisher=Wiley-VCH | publication-place=Weinheim | date=2013 | isbn=978-3-527-64956-3 | oclc=855858511 | language=de|page=696}}

According to the International Resource Panel's Metal Stocks in Society report, the global stock of iron in use in society is 2,200 kg per capita. More-developed countries differ in this respect from less-developed countries (7,000–14,000 vs 2,000 kg per capita).[https://archive.today/20120914112201/https://www.unep.org/resourcepanel/Publications/MetalStocks/tabid/56054/Default.aspx Metal Stocks in Society: Scientific synthesis], 2010, International Resource Panel, UNEP

Ocean science demonstrated the role of the iron in the ancient seas in both marine biota and climate.{{cite journal | last=Stoll | first=Heather | title=30 years of the iron hypothesis of ice ages | journal=Nature | publisher=Springer Science and Business Media LLC | volume=578 | issue=7795 | date=2020-02-17 | issn=0028-0836 | doi=10.1038/d41586-020-00393-x | pages=370–371| pmid=32066927 | bibcode=2020Natur.578..370S | s2cid=211139074 }}

{{Main|Iron compounds}}

Iron shows the characteristic chemical properties of the transition metals, namely the ability to form variable oxidation states differing by steps of one and a very large coordination and organometallic chemistry: indeed, it was the discovery of an iron compound, ferrocene, that revolutionalized the latter field in the 1950s.{{sfn|Greenwood|Earnshaw|1997|p=905}} Iron is sometimes considered as a prototype for the entire block of transition metals, due to its abundance and the immense role it has played in the technological progress of humanity.{{sfn|Greenwood|Earnshaw|1997|p=1070}} Its 26 electrons are arranged in the configuration [Ar]3d⁶4s², of which the 3d and 4s electrons are relatively close in energy, and thus a number of electrons can be ionized.{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}}

Iron forms compounds mainly in the oxidation states +2 (iron(II), "ferrous") and +3 (iron(III), "ferric"). Iron also occurs in higher oxidation states, e.g., the purple potassium ferrate (K₂FeO₄), which contains iron in its +6 oxidation state. The anion [FeO₄]^– with iron in its +7 oxidation state, along with an iron(V)-peroxo isomer, has been detected by infrared spectroscopy at 4 K after cocondensation of laser-ablated Fe atoms with a mixture of O₂/Ar.{{Cite journal|last1=Lu|first1=Jun-Bo|last2=Jian|first2=Jiwen|last3=Huang|first3=Wei|last4=Lin|first4=Hailu|last5=Li|first5=Jun|last6=Zhou|first6=Mingfei|date=2016-11-16|title=Experimental and theoretical identification of the Fe(VII) oxidation state in FeO₄⁻|journal=Phys. Chem. Chem. Phys.|volume=18|issue=45|pages=31125–31131|doi=10.1039/c6cp06753k|pmid=27812577|bibcode=2016PCCP...1831125L}} Iron(IV) is a common intermediate in many biochemical oxidation reactions.{{Cite journal|doi = 10.1021/ar700027f|title = High-Valent Iron(IV)–Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions|date = 2007|last1 = Nam|first1 = Wonwoo|journal = Accounts of Chemical Research|volume = 40|pages = 522–531|pmid = 17469792|issue = 7|url = https://cbs.ewha.ac.kr/pub/data/2007_07.pdf|access-date = 22 February 2022|archive-date = 15 June 2021|archive-url = https://web.archive.org/web/20210615123946/http://cbs.ewha.ac.kr/pub/data/2007_07.pdf|url-status = dead}}{{Cite book|publisher = Walter de Gruyter|date = 1985|edition = 91–100|pages = 1125–46|isbn = 3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first1 = Arnold F.|last1 = Holleman|last2 = Wiberg|first2 = Egon|last3 = Wiberg|first3 = Nils|chapter = Iron| language = de}} Numerous organoiron compounds contain formal oxidation states of +1, 0, −1, or even −2. The oxidation states and other bonding properties are often assessed using the technique of Mössbauer spectroscopy.{{Cite book| chapter = Mössbauer Spectroscopy and the Coordination Chemistry of Iron|first1 = William Michael|last1 = Reiff|first2 = Gary J.|last2 = Long |title = Mössbauer spectroscopy applied to inorganic chemistry|publisher = Springer|date = 1984|isbn = 978-0-306-41647-7|pages = 245–83}} Many mixed valence compounds contain both iron(II) and iron(III) centers, such as magnetite and Prussian blue ({{chem2|Fe4(Fe[CN]6)3}}). The latter is used as the traditional "blue" in blueprints.{{Cite book| chapter = An introduction in monochrome|pages = 11–19|first = Mike|last = Ware|publisher = NMSI Trading Ltd|title = Cyanotype: the history, science and art of photographic printing in Prussian blue|isbn = 978-1-900747-07-3|date = 1999| chapter-url={{Google books|C-7I69gFIbMC|page=PA11|keywords=|text=|plainurl=yes}}}}

Iron is the first of the transition metals that cannot reach its group oxidation state of +8, although its heavier congeners ruthenium and osmium can, with ruthenium having more difficulty than osmium.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}} Ruthenium exhibits an aqueous cationic chemistry in its low oxidation states similar to that of iron, but osmium does not, favoring high oxidation states in which it forms anionic complexes.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}} In the second half of the 3d transition series, vertical similarities down the groups compete with the horizontal similarities of iron with its neighbors cobalt and nickel in the periodic table, which are also ferromagnetic at room temperature and share similar chemistry. As such, iron, cobalt, and nickel are sometimes grouped together as the iron triad.{{sfn|Greenwood|Earnshaw|1997|p=1070}}

Unlike many other metals, iron does not form amalgams with mercury. As a result, mercury is traded in standardized 76 pound flasks (34 kg) made of iron.{{Cite book|title = Hand-book of chemistry|volume = 6| first1 = Leopold|last1 = Gmelin|author-link = Leopold Gmelin|pages = 128–29| chapter = Mercury and Iron|chapter-url={{Google books|nosMAAAAYAAJ|page=PA128|keywords=|text=|plainurl=yes}}|publisher = Cavendish Society|date = 1852}}

Iron is by far the most reactive element in its group; it is pyrophoric when finely divided and dissolves easily in dilute acids, giving Fe²⁺. However, it does not react with concentrated nitric acid and other oxidizing acids due to the formation of an impervious oxide layer, which can nevertheless react with hydrochloric acid.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}} High-purity iron, called electrolytic iron, is considered to be resistant to rust, due to its oxide layer.

{{multiple image

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| caption1 = Ferrous or iron(II) oxide, {{chem2|FeO}}

| image2 = Iron(III)-oxide-sample.jpg

| caption2 = Ferric or iron(III) oxide {{chem2|Fe2O3}}

| image3 = Fe3O4.JPG

| caption3 = Ferrosoferric or iron(II,III) oxide {{chem2|Fe3O4}}

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Iron forms various oxide and hydroxide compounds; the most common are iron(II,III) oxide (Fe₃O₄), and iron(III) oxide (Fe₂O₃). Iron(II) oxide also exists, though it is unstable at room temperature. Despite their names, they are actually all non-stoichiometric compounds whose compositions may vary.{{sfn|Greenwood|Earnshaw|1997|p=1079}} These oxides are the principal ores for the production of iron (see bloomery and blast furnace). They are also used in the production of ferrites, useful magnetic storage media in computers, and pigments. The best known sulfide is iron pyrite (FeS₂), also known as fool's gold owing to its golden luster. It is not an iron(IV) compound, but is actually an iron(II) polysulfide containing Fe²⁺ and {{chem|S|2|2-}} ions in a distorted sodium chloride structure.{{sfn|Greenwood|Earnshaw|1997|p=1079}}

File:Pourbaix Diagram of Iron.svg of iron]]

File:Iron(III) chloride hexahydrate.jpg

The binary ferrous and ferric halides are well-known. The ferrous halides typically arise from treating iron metal with the corresponding hydrohalic acid to give the corresponding hydrated salts.

:Fe + 2 HX → FeX₂ + H₂ (X = F, Cl, Br, I)

Iron reacts with fluorine, chlorine, and bromine to give the corresponding ferric halides, ferric chloride being the most common.{{sfn|Greenwood|Earnshaw|1997|pp=1082–84}}

:2 Fe + 3 X₂ → 2 FeX₃ (X = F, Cl, Br)

Ferric iodide is an exception, being thermodynamically unstable due to the oxidizing power of Fe³⁺ and the high reducing power of I⁻:{{sfn|Greenwood|Earnshaw|1997|pp=1082–84}}

:2 I⁻ + 2 Fe³⁺ → I₂ + 2 Fe²⁺ (E⁰ = +0.23 V)

Ferric iodide, a black solid, is not stable in ordinary conditions, but can be prepared through the reaction of iron pentacarbonyl with iodine and carbon monoxide in the presence of hexane and light at the temperature of −20 °C, with oxygen and water excluded.{{sfn|Greenwood|Earnshaw|1997|pp=1082–84}} Complexes of ferric iodide with some soft bases are known to be stable compounds.Siegfried Pohl, Ulrich Bierbach, Wolfgang Saak; "FeI3SC(NMe2)2, a Neutral Thiourea Complex of Iron(III) Iodide", Angewandte Chemie International Edition in English (1989) 28 (6), 776–777. https://doi.org/10.1002/anie.198907761Nicholas A. Barnes, Stephen M.Godfrey, Nicholas Ho, Charles A.McAuliffe, Robin G.Pritchard; "Facile synthesis of a rare example of an iron(III) iodide complex, [FeI3(AsMe3)2], from the reaction of Me3AsI2 with unactivated iron powder", Polyhedron (2013) 55, 67–72. https://doi.org/10.1016/j.poly.2013.02.066

File:Ferrate_and_permanganate_solution.jpg (right)]]

The standard reduction potentials in acidic aqueous solution for some common iron ions are given below:{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}}

:

The red-purple tetrahedral ferrate(VI) anion is such a strong oxidizing agent that it oxidizes ammonia to nitrogen (N₂) and water to oxygen:{{sfn|Greenwood|Earnshaw|1997|pp=1082–84}}

:4 {{chem|FeO|4|2-}} + 34 {{chem|H|2|O}} → 4 {{chem2|[Fe(H2O)6](3+)}} + 20 {{chem|OH|-}} + 3 O₂

The pale-violet hexaquo complex {{chem2|[Fe(H2O)6](3+)}} is an acid such that above pH 0 it is fully hydrolyzed:{{sfn|Greenwood|Earnshaw|1997|pp=1088–91}}

:

File:Iron(II)-sulfate-heptahydrate-sample.jpg heptahydrate]]

As pH rises above 0 the above yellow hydrolyzed species form and as it rises above 2–3, reddish-brown hydrous iron(III) oxide precipitates out of solution. Although Fe³⁺ has a d⁵ configuration, its absorption spectrum is not like that of Mn²⁺ with its weak, spin-forbidden d–d bands, because Fe³⁺ has higher positive charge and is more polarizing, lowering the energy of its ligand-to-metal charge transfer absorptions. Thus, all the above complexes are rather strongly colored, with the single exception of the hexaquo ion – and even that has a spectrum dominated by charge transfer in the near ultraviolet region.{{sfn|Greenwood|Earnshaw|1997|pp=1088–91}} On the other hand, the pale green iron(II) hexaquo ion {{chem2|[Fe(H2O)6](2+)}} does not undergo appreciable hydrolysis. Carbon dioxide is not evolved when carbonate anions are added, which instead results in white iron(II) carbonate being precipitated out. In excess carbon dioxide this forms the slightly soluble bicarbonate, which occurs commonly in groundwater, but it oxidises quickly in air to form iron(III) oxide that accounts for the brown deposits present in a sizeable number of streams.{{sfn|Greenwood|Earnshaw|1997|pp=1091–97}}

Due to its electronic structure, iron has a very large coordination and organometallic chemistry.

File:2-isomers-of-ferrioxalate.svg of the ferrioxalate ion]]

Many coordination compounds of iron are known. A typical six-coordinate anion is hexachloroferrate(III), [FeCl₆]³⁻, found in the mixed salt tetrakis(methylammonium) hexachloroferrate(III) chloride.{{cite journal|last1 = Clausen|first1 = C.A.|last2 = Good|first2 = M.L.|year = 1968|title = Stabilization of the hexachloroferrate(III) anion by the methylammonium cation|journal = Inorganic Chemistry |volume = 7|issue = 12|pages = 2662–63|doi = 10.1021/ic50070a047}}{{cite journal|last1 = James|first1 = B.D.|first2 = M.|last2 = Bakalova|first3 = J.|last3 = Lieseganga|first4 = W.M.|last4 = Reiff|first5 = D.C.R.|last5 = Hockless|first6 = B.W.|last6 = Skelton|first7 = A.H.|last7 = White|year = 1996|title = The hexachloroferrate(III) anion stabilized in hydrogen bonded packing arrangements. A comparison of the X-ray crystal structures and low temperature magnetism of tetrakis(methylammonium) hexachloroferrate(III) chloride (I) and tetrakis(hexamethylenediammonium) hexachloroferrate(III) tetrachloroferrate(III) tetrachloride (II)|journal = Inorganica Chimica Acta|volume = 247|issue = 2|pages = 169–74|doi = 10.1016/0020-1693(95)04955-X}} Complexes with multiple bidentate ligands have geometric isomers. For example, the trans-chlorohydridobis(bis-1,2-(diphenylphosphino)ethane)iron(II) complex is used as a starting material for compounds with the {{chem2|Fe(dppe)2}} moiety.{{cite book|last1 = Giannoccaro|first1 = P.|last2 = Sacco|first2 = A.| title=Inorganic Syntheses | chapter=Bis[Ethylenebis(Diphenylphosphine)]-Hydridoiron Complexes|year = 1977|volume = 17|pages = 69–72|doi = 10.1002/9780470132487.ch19|isbn = 978-0-470-13248-7}}{{cite journal|last1 = Lee|first1 = J.|last2 = Jung|first2 = G.|last3 = Lee|first3 = S.W.|title = Structure of trans-chlorohydridobis(diphenylphosphinoethane)iron(II)|journal = Bull. Korean Chem. Soc.|year = 1998|volume = 19|issue = 2|pages = 267–69|url = https://www.koreascience.or.kr/article/ArticleFullRecord.jsp?cn=JCGMCS_1998_v19n2_267|doi = 10.1007/BF02698412|s2cid = 35665289}} The ferrioxalate ion with three oxalate ligands displays helical chirality with its two non-superposable geometries labelled Λ (lambda) for the left-handed screw axis and Δ (delta) for the right-handed screw axis, in line with IUPAC conventions.{{sfn|Greenwood|Earnshaw|1997|pp=1088–91}} Potassium ferrioxalate is used in chemical actinometry and along with its sodium salt undergoes photoreduction applied in old-style photographic processes. The dihydrate of iron(II) oxalate has a polymeric structure with co-planar oxalate ions bridging between iron centres with the water of crystallisation located forming the caps of each octahedron, as illustrated below.{{cite journal|first1 = Takuya|last1 = Echigo|first2 = Mitsuyoshi|last2 = Kimata|title = Single-crystal X-ray diffraction and spectroscopic studies on humboldtine and lindbergite: weak Jahn–Teller effect of Fe²⁺ ion|journal = Phys. Chem. Miner.|year = 2008|volume = 35|issue = 8|pages = 467–75|doi= 10.1007/s00269-008-0241-7|bibcode = 2008PCM....35..467E|s2cid = 98739882}}

File:Fe(C2O4)(H2O)2-chain-from-xtal-2008-CM-3D-balls.png

File:Pentaaqua(thiocyanato)iron(III) chloride.jpg

Iron(III) complexes are quite similar to those of chromium(III) with the exception of iron(III)'s preference for O-donor instead of N-donor ligands. The latter tend to be rather more unstable than iron(II) complexes and often dissociate in water. Many Fe–O complexes show intense colors and are used as tests for phenols or enols. For example, in the ferric chloride test, used to determine the presence of phenols, iron(III) chloride reacts with a phenol to form a deep violet complex:{{sfn|Greenwood|Earnshaw|1997|pp=1088–91}}

:3 ArOH + FeCl₃ → Fe(OAr)₃ + 3 HCl (Ar = aryl)

Among the halide and pseudohalide complexes, fluoro complexes of iron(III) are the most stable, with the colorless [FeF₅(H₂O)]²⁻ being the most stable in aqueous solution. Chloro complexes are less stable and favor tetrahedral coordination as in [FeCl₄]⁻; [FeBr₄]⁻ and [FeI₄]⁻ are reduced easily to iron(II). Thiocyanate is a common test for the presence of iron(III) as it forms the blood-red [Fe(SCN)(H₂O)₅]²⁺. Like manganese(II), most iron(III) complexes are high-spin, the exceptions being those with ligands that are high in the spectrochemical series such as cyanide. An example of a low-spin iron(III) complex is [Fe(CN)₆]³⁻. Iron shows a great variety of electronic spin states, including every possible spin quantum number value for a d-block element from 0 (diamagnetic) to {{frac|5|2}} (5 unpaired electrons). This value is always half the number of unpaired electrons. Complexes with zero to two unpaired electrons are considered low-spin and those with four or five are considered high-spin.{{sfn|Greenwood|Earnshaw|1997|p=1079}}

Iron(II) complexes are less stable than iron(III) complexes but the preference for O-donor ligands is less marked, so that for example {{chem2|[Fe(NH3)6](2+)}} is known while {{chem2|[Fe(NH3)6](3+)}} is not. They have a tendency to be oxidized to iron(III) but this can be moderated by low pH and the specific ligands used.{{sfn|Greenwood|Earnshaw|1997|pp=1091–97}}

File:Sample of iron pentacarbonyl.jpg

Organoiron chemistry is the study of organometallic compounds of iron, where carbon atoms are covalently bound to the metal atom. They are many and varied, including cyanide complexes, carbonyl complexes, sandwich and half-sandwich compounds.

File:Prussian blue.jpg

Prussian blue or "ferric ferrocyanide", Fe₄[Fe(CN)₆]₃, is an old and well-known iron-cyanide complex, extensively used as pigment and in several other applications. Its formation can be used as a simple wet chemistry test to distinguish between aqueous solutions of Fe²⁺ and Fe³⁺ as they react (respectively) with potassium ferricyanide and potassium ferrocyanide to form Prussian blue.

Another old example of an organoiron compound is iron pentacarbonyl, Fe(CO)₅, in which a neutral iron atom is bound to the carbon atoms of five carbon monoxide molecules. The compound can be used to make carbonyl iron powder, a highly reactive form of metallic iron. Thermolysis of iron pentacarbonyl gives triiron dodecacarbonyl, {{chem2|Fe3(CO)12}}, a complex with a cluster of three iron atoms at its core. Collman's reagent, disodium tetracarbonylferrate, is a useful reagent for organic chemistry; it contains iron in the −2 oxidation state. Cyclopentadienyliron dicarbonyl dimer contains iron in the rare +1 oxidation state.{{Greenwood&Earnshaw1st|pages=1282–86}}.

{{multiple image

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| image1 = Ferrocene.svg

| image2 = Photo of Ferrocene (powdered).JPG

| footer = Structural formula of ferrocene and a powdered sample

}}

A landmark in this field was the discovery in 1951 of the remarkably stable sandwich compound ferrocene {{chem2|Fe(C5H5)2}}, by Pauson and Kealy{{Cite journal |last1= Kealy |first1=T.J. |last2= Pauson |first2=P.L. |year= 1951 |title= A New Type of Organo-Iron Compound |journal= Nature |volume= 168 |issue= 4285 |pages= 1039–40 |doi= 10.1038/1681039b0|bibcode = 1951Natur.168.1039K |s2cid=4181383 }} and independently by Miller and colleagues,{{cite journal| last1=Miller|first1= S. A.|last2=Tebboth|first2= J. A.|last3= Tremaine|first3= J. F.|journal= J. Chem. Soc.|year=1952| pages= 632–635| title=114. Dicyclopentadienyliron |doi=10.1039/JR9520000632}} whose surprising molecular structure was determined only a year later by Woodward and Wilkinson{{cite journal |first1= G. |last1=Wilkinson|author1-link=Geoffrey Wilkinson |first2=M.|last2= Rosenblum |first3=M. C.|last3= Whiting|first4= R. B. |last4=Woodward|author4-link=Robert Burns Woodward |title = The Structure of Iron Bis-Cyclopentadienyl |journal = J. Am. Chem. Soc. |year = 1952|volume = 74 |pages = 2125–2126 |doi = 10.1021/ja01128a527 |issue = 8|bibcode=1952JAChS..74.2125W }} and Fischer.{{Cite journal|last=Okuda|first=Jun|date=2016-12-28|title=Ferrocene – 65 Years After|journal=European Journal of Inorganic Chemistry|volume=2017|issue=2|pages=217–219|doi=10.1002/ejic.201601323|issn=1434-1948|doi-access=free}}

Ferrocene is still one of the most important tools and models in this class.{{sfn|Greenwood|Earnshaw|1997|p=1104}}

Iron-centered organometallic species are used as catalysts. The Knölker complex, for example, is a transfer hydrogenation catalyst for ketones.{{cite journal|last=Bullock|first=R.M.|date=11 September 2007|title=An Iron Catalyst for Ketone Hydrogenations under Mild Conditions|journal=Angew. Chem. Int. Ed.|volume=46|issue=39|pages=7360–63|doi=10.1002/anie.200703053|pmid=17847139|doi-access=free}}

The iron compounds produced on the largest scale in industry are iron(II) sulfate (FeSO₄·7H₂O) and iron(III) chloride (FeCl₃). The former is one of the most readily available sources of iron(II), but is less stable to aerial oxidation than Mohr's salt ({{chem2|(NH4)2Fe(SO4)2*6H2O}}). Iron(II) compounds tend to be oxidized to iron(III) compounds in the air.

{{Main|History of ferrous metallurgy}}

Iron is one of the elements undoubtedly known to the ancient world.{{sfn|Weeks|1968|p=4}} It has been worked, or wrought, for millennia. However, iron artefacts of great age are much rarer than objects made of gold or silver due to the ease with which iron corrodes.{{sfn|Weeks|1968|p=29}} The technology developed slowly, and even after the discovery of smelting it took many centuries for iron to replace bronze as the metal of choice for tools and weapons.

File:Meteorite iron harpoon.jpg. The iron edge covers a narwhal tusk harpoon using meteorite iron from the Cape York meteorite, one of the largest iron meteorites known.]]

Beads made from meteoric iron in 3500 BC or earlier were found in Gerzeh, Egypt by G. A. Wainwright.{{sfn|Weeks|1968|p=31}} The beads contain 7.5% nickel, which is a signature of meteoric origin since iron found in the Earth's crust generally has only minuscule nickel impurities.

Meteoric iron was highly regarded due to its origin in the heavens and was often used to forge weapons and tools.{{sfn|Weeks|1968|p=31}} For example, a dagger made of meteoric iron was found in the tomb of Tutankhamun, containing similar proportions of iron, cobalt, and nickel to a meteorite discovered in the area, deposited by an ancient meteor shower.{{cite journal|last=Bjorkman|first=Judith Kingston|title=Meteors and Meteorites in the ancient Near East |journal=Meteoritics |date=1973 |pages=91–132 |doi=10.1111/j.1945-5100.1973.tb00146.x |volume=8|issue=2|bibcode=1973Metic...8...91B}}{{cite journal|doi=10.1111/maps.12664 |bibcode=2016M&PS...51.1301C|title=The meteoritic origin of Tutankhamun's iron dagger blade|journal=Meteoritics & Planetary Science|volume=51|issue=7|pages=1301–09|year=2016|last1=Comelli|first1=Daniela|last2=d'Orazio|first2=Massimo|last3=Folco|first3=Luigi|last4=El-Halwagy|first4=Mahmud|last5=Frizzi|first5=Tommaso|last6=Alberti|first6=Roberto|last7=Capogrosso|first7=Valentina|last8=Elnaggar|first8=Abdelrazek|last9=Hassan|first9=Hala|last10=Nevin|first10=Austin|last11=Porcelli|first11=Franco|last12=Rashed|first12=Mohamed G|last13=Valentini|first13=Gianluca|doi-access=free}}{{cite news | last=Walsh | first=Declan | title=King Tut's Dagger Made of 'Iron From the Sky,' Researchers Say | newspaper=The New York Times | quote=the blade's composition of iron, nickel and cobalt was an approximate match for a meteorite that landed in northern Egypt. The result "strongly suggests an extraterrestrial origin" | date=2 June 2016 | url=https://www.nytimes.com/2016/06/03/world/middleeast/king-tuts-dagger-made-of-iron-from-the-sky-researchers-say.html |archive-url=https://ghostarchive.org/archive/20220103/https://www.nytimes.com/2016/06/03/world/middleeast/king-tuts-dagger-made-of-iron-from-the-sky-researchers-say.html |archive-date=2022-01-03 |url-access=subscription |url-status=live | access-date=4 June 2016}}{{cbignore}} Items that were likely made of iron by Egyptians date from 3000 to 2500 BC.{{sfn|Weeks|1968|p=29}}

Meteoritic iron is comparably soft and ductile and easily cold forged but may get brittle when heated because of the nickel content.{{Cite book|url={{Google books|-CQ4AQAAIAAJ|page=PA492|keywords=|text=|plainurl=yes}}|title=Technisches wörterbuch oder Handbuch der Gewerbskunde ... : Bearb. nach Dr. Andrew Ure's Dictionary of arts, manufactures and mines|last=Ure|first=Andrew|date=1843|publisher=G. Haase|page=492|language=de}}

{{Main|Wrought iron}}

{{Further|Ancient iron production}}

File:Mars symbol (fixed width).svg has been used since antiquity to represent iron.]] File:QtubIronPillar.JPG is an example of the iron extraction and processing methodologies of early India.]]

The first iron production started in the Middle Bronze Age, but it took several centuries before iron displaced bronze. Samples of smelted iron from Asmar, Mesopotamia and Tall Chagar Bazaar in northern Syria were made sometime between 3000 and 2700 BC.{{sfn|Weeks|1968|p=32}} The Hittites established an empire in north-central Anatolia around 1600 BC. They appear to be the first to understand the production of iron from its ores and regard it highly in their society.McNutt, Paula (1990 1). The Forging of Israel: Iron Technology, Symbolism and Tradition in Ancient Society. A&C Black. The Hittites began to smelt iron between 1500 and 1200 BC and the practice spread to the rest of the Near East after their empire fell in 1180 BC.{{sfn|Weeks|1968|p=32}} The subsequent period is called the Iron Age.

Artifacts of smelted iron are found in India dating from 1800 to 1200 BC,{{cite web| url = https://antiquity.ac.uk/projgall/tewari/tewari.pdf|first = Rakesh|last = Tewari|title = The origins of Iron Working in India: New evidence from the Central Ganga plain and the Eastern Vindhyas|publisher = State Archaeological Department|access-date = 23 May 2010}} and in the Levant from about 1500 BC (suggesting smelting in Anatolia or the Caucasus).{{Cite journal|doi=10.1080/00438243.1989.9980081|last=Photos|first = E.|title=The Question of Meteoritic versus Smelted Nickel-Rich Iron: Archaeological Evidence and Experimental Results|journal=World Archaeology |volume=20 |issue=3 |date=1989 |pages=403–21|publisher=Taylor & Francis, Ltd.|jstor = 124562}}{{Cite book| last = Muhly|first = James D.|chapter = Metalworking/Mining in the Levant|pages = 174–83|title =Near Eastern Archaeology IN: Eisenbrauns |editor = Lake, Richard Winona |date = 2003|volume = 180}} Alleged references (compare history of metallurgy in South Asia) to iron in the Indian Vedas have been used for claims of a very early usage of iron in India respectively to date the texts as such. The rigveda term ayas (metal) refers to copper, while iron which is called as śyāma ayas, literally "black copper", first is mentioned in the post-rigvedic Atharvaveda.Witzel, Michael (2001), [https://www.people.fas.harvard.edu/~witzel/EJVS-7-3.pdf "Autochthonous Aryans? The Evidence from Old Indian and Iranian Texts"], in Electronic Journal of Vedic Studies (EJVS) 7-3, pp. 1–93

Some archaeological evidence suggests iron was smelted in Zimbabwe and southeast Africa as early as the eighth century BC.Weeks, p. 33, quoting Cline, Walter (1937) "Mining and Metallurgy in Negro Africa", George Banta Publishing Co., Menasha, Wis., pp. 17–23. Iron working was introduced to Greece in the late 11th century BC, from which it spread quickly throughout Europe.Riederer, Josef; Wartke, Ralf-B. (2009) "Iron", Cancik, Hubert; Schneider, Helmuth (eds.): Brill's New Pauly, Brill.

File:1255 - Keramikos Museum, Athens - Iron tool - Photo by Giovanni Dall'Orto, Nov 12 2009.jpg

The spread of ironworking in Central and Western Europe is associated with Celtic expansion. According to Pliny the Elder, iron use was common in the Roman era.{{sfn|Weeks|1968|p=31}} In the lands of what is now considered China, iron appears approximately 700–500 BC.Sawyer, Ralph D. and Sawyer, Mei-chün (1993). The Seven Military Classics of Ancient China. Boulder: Westview. {{ISBN|0-465-00304-4}}. p. 10. Iron smelting may have been introduced into China through Central Asia.Pigott, Vincent C. (1999). The Archaeometallurgy of the Asian Old World. Philadelphia: University of Pennsylvania Museum of Archaeology and Anthropology. {{ISBN|0-924171-34-0}}, p. 8. The earliest evidence of the use of a blast furnace in China dates to the 1st century AD,{{cite book|last=Golas |first= Peter J. |title=Science and Civilisation in China: Volume 5, Chemistry and Chemical Technology, Part 13, Mining|url={{Google books|TSiII7s2wLkC|page=PA152|keywords=|text=|plainurl=yes}}|date= 1999|publisher=Cambridge University Press|isbn=978-0-521-58000-7|page=152|quote=earliest blast furnace discovered in China from about the first century AD}} and cupola furnaces were used as early as the Warring States period (403–221 BC).Pigott, Vincent C. (1999). The Archaeometallurgy of the Asian Old World. Philadelphia: University of Pennsylvania Museum of Archaeology and Anthropology. {{ISBN|0-924171-34-0}}, p. 191. Usage of the blast and cupola furnace remained widespread during the Tang and Song dynasties.{{cite book|title=The Coming of the Ages of Steel|url={{Google books|uMwUAAAAIAAJ|page=PA54|keywords=|text=|plainurl=yes}}|publisher=Brill Archive|page=54|date=1961}}

During the Industrial Revolution in Britain, Henry Cort began refining iron from pig iron to wrought iron (or bar iron) using innovative production systems. In 1783 he patented the puddling process for refining iron ore. It was later improved by others, including Joseph Hall.{{cite journal|doi=10.1179/tns.1977.011|title=Dry and Wet Puddling|journal=Transactions of the Newcomen Society|volume=49|pages=156–57|year=2014|last1=Mott|first1=R.A}}

{{Main|Cast iron}}

Cast iron was first produced in China during 5th century BC,{{Cite journal|last=Wagner |first= Donald B.|url=https://hist-met.org/images/Journal_PDFs/37_1_p_25_Wagner.pdf|title=Chinese blast furnaces from the 10th to the 14th century|journal=Historical Metallurgy|volume=37|issue=1|date=2003|pages=25–37|access-date=7 January 2018|archive-url=https://web.archive.org/web/20180107175015/https://hist-met.org/images/Journal_PDFs/37_1_p_25_Wagner.pdf|archive-date=7 January 2018|url-status=dead}} originally published in {{Cite journal|first =Donald B.|last =Wagner|title=Chinese blast furnaces from the 10th to the 14th century|journal=West Asian Science, Technology, and Medicine|volume=18 |date=2001|pages=41–74|doi =10.1163/26669323-01801008}} but was hardly in Europe until the medieval period.Giannichedda, Enrico (2007): [https://books.google.com/books?id=LAgxAJNXhFwC&pg=PA200 "Metal production in Late Antiquity"], in Technology in Transition AD 300–650 Lavan, L.; Zanini, E. and Sarantis, A.(eds.), Brill, Leiden; {{ISBN|90-04-16549-5}}, p. 200.{{Cite book| title = Chemistry, Precision and Design|publisher = A Beka Book, Inc.|first1 = Verne|last1 =Biddle|first2= Gregory|last2 =Parker}} The earliest cast iron artifacts were discovered by archaeologists in what is now modern Luhe County, Jiangsu in China. Cast iron was used in ancient China for warfare, agriculture, and architecture.{{cite book|last=Wagner |first= Donald B. |title=Iron and Steel in Ancient China|date=1993|publisher=Brill|isbn=978-90-04-09632-5|pages=335–340}} During the medieval period, means were found in Europe of producing wrought iron from cast iron (in this context known as pig iron) using finery forges. For all these processes, charcoal was required as fuel.{{sfn|Greenwood|Earnshaw|1997|p=1072}}

File:Philipp Jakob Loutherbourg d. J. 002.jpg, 1801. Blast furnaces light the iron making town of Coalbrookdale.]]

Medieval blast furnaces were about {{convert|10|ft|m}} tall and made of fireproof brick; forced air was usually provided by hand-operated bellows. Modern blast furnaces have grown much bigger, with hearths fourteen meters in diameter that allow them to produce thousands of tons of iron each day, but essentially operate in much the same way as they did during medieval times.{{sfn|Greenwood|Earnshaw|1997|p=1072}}

In 1709, Abraham Darby I established a coke-fired blast furnace to produce cast iron, replacing charcoal, although continuing to use blast furnaces. The ensuing availability of inexpensive iron was one of the factors leading to the Industrial Revolution. Toward the end of the 18th century, cast iron began to replace wrought iron for certain purposes, because it was cheaper. Carbon content in iron was not implicated as the reason for the differences in properties of wrought iron, cast iron, and steel until the 18th century.{{sfn|Weeks|1968|p=32}}

Since iron was becoming cheaper and more plentiful, it also became a major structural material following the building of the innovative first iron bridge in 1778. This bridge still stands today as a monument to the role iron played in the Industrial Revolution. Following this, iron was used in rails, boats, ships, aqueducts, and buildings, as well as in iron cylinders in steam engines.{{sfn|Greenwood|Earnshaw|1997|p=1072}} Railways have been central to the formation of modernity and ideas of progressSchivelbusch, G. (1986) The Railway Journey: Industrialization and Perception of Time and Space in the 19th Century. Oxford: Berg. and various languages refer to railways as iron road (e.g. French {{Lang|fr|chemin de fer}}, German {{Lang|de|Eisenbahn}}, Turkish {{Lang|tr|demiryolu}}, Russian {{Lang|ru|железная дорога}}, Chinese, Japanese, and Korean 鐵道, Vietnamese {{Lang|vi|đường sắt}}).

{{Main|Steel}}

{{See also|Steelmaking}}

Steel (with smaller carbon content than pig iron but more than wrought iron) was first produced in antiquity by using a bloomery. Blacksmiths in Luristan in western Persia were making good steel by 1000 BC.{{sfn|Weeks|1968|p=32}} Then improved versions, Wootz steel by India and Damascus steel were developed around 300 BC and AD 500 respectively. These methods were specialized, and so steel did not become a major commodity until the 1850s.Spoerl, Joseph S. [https://www.anselm.edu/homepage/dbanach/h-carnegie-steel.htm A Brief History of Iron and Steel Production] {{webarchive|url=https://web.archive.org/web/20100602031459/https://www.anselm.edu/homepage/dbanach/h-carnegie-steel.htm |date=2 June 2010 }}. Saint Anselm College

New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to produce mild steel. This made steel much more economical, thereby leading to wrought iron no longer being produced in large quantities.{{cite book | url={{Google books|fUmTX8yKU4gC|page=PA190|keywords=|text=|plainurl=yes}} | pages = 190–91 | title = Encyclopedia of the Elements: Technical Data – History – Processing – Applications | isbn = 978-3-527-61234-5 | last1 = Enghag | first1 = Per | date = 8 January 2008| publisher = John Wiley & Sons }}

In 1774, Antoine Lavoisier used the reaction of water steam with metallic iron inside an incandescent iron tube to produce hydrogen in his experiments leading to the demonstration of the conservation of mass, which was instrumental in changing chemistry from a qualitative science to a quantitative one.{{cite journal|doi=10.1021/ed052p658|title=An historical note on the conservation of mass|journal=Journal of Chemical Education|volume=52|issue=10|page=658|year=1975|last1=Whitaker|first1=Robert D|bibcode=1975JChEd..52..658W}}

File:Eisernes Kreuz (1914).jpg brooch from WWI. ]]

Iron plays a certain role in mythology and has found various usage as a metaphor and in folklore. The Greek poet Hesiod's Works and Days (lines 109–201) lists different ages of man named after metals like gold, silver, bronze and iron to account for successive ages of humanity.{{cite journal |doi=10.1086/366027 |jstor=268960 |title=Work, Justice, and Hesiod's Five Ages |journal=Classical Philology |volume=69 |issue=1 |pages=1–16 |year=1974 |last1=Fontenrose |first1=Joseph |s2cid=161808359}} The Iron Age was closely related with Rome, and in Ovid's Metamorphoses

{{Blockquote|text=The Virtues, in despair, quit the earth; and the depravity of man becomes universal and complete. Hard steel succeeded then.|sign=Ovid|source=Metamorphoses, Book I, Iron age, line 160 ff}}

An example of the importance of iron's symbolic role may be found in the German Campaign of 1813. Frederick William III commissioned then the first Iron Cross as military decoration. Berlin iron jewellery reached its peak production between 1813 and 1815, when the Prussian royal family urged citizens to donate gold and silver jewellery for military funding. The inscription Ich gab Gold für Eisen (I gave gold for iron) was used as well in later war efforts.Schmidt, Eva (1981) Der preußische Eisenkunstguss. (Art of Prussian cast iron) Technik, Geschichte, Werke, Künstler. Verlag Mann, Berlin, {{ISBN|3-7861-1130-8}}

File:Iron powder.JPG]]

For a few limited purposes when it is needed, pure iron is produced in the laboratory in small quantities by reducing the pure oxide or hydroxide with hydrogen, or forming iron pentacarbonyl and heating it to 250 °C so that it decomposes to form pure iron powder.{{sfn|Greenwood|Earnshaw|1997|p=1071}} Another method is electrolysis of ferrous chloride onto an iron cathode.Lux, H. (1963) "Metallic Iron" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. G. Brauer (ed.), Academic Press, NY. Vol. 2. pp. 1490–91.

{{See also|Iron ore}}

Nowadays, the industrial production of iron or steel consists of two main stages. In the first stage, iron ore is reduced with coke in a blast furnace, and the molten metal is separated from gross impurities such as silicate minerals. This stage yields an alloy – pig iron – that contains relatively large amounts of carbon. In the second stage, the amount of carbon in the pig iron is lowered by oxidation to yield wrought iron, steel, or cast iron.{{sfn|Greenwood|Earnshaw|1997|p=1073}} Other metals can be added at this stage to form alloy steels.

{{Main|Blast furnace}}

The blast furnace is loaded with iron ores, usually hematite {{chem2|Fe2O3}} or magnetite {{chem2|Fe3O4}}, along with coke (coal that has been separately baked to remove volatile components) and flux (limestone or dolomite). "Blasts" of air pre-heated to 900 °C (sometimes with oxygen enrichment) is blown through the mixture, in sufficient amount to turn the carbon into carbon monoxide:{{sfn|Greenwood|Earnshaw|1997|p=1073}}

:{{chem2 | 2 C + O2 -> 2 CO }}

This reaction raises the temperature to about 2000 °C. The carbon monoxide reduces the iron ore to metallic iron:{{sfn|Greenwood|Earnshaw|1997|p=1073}}

:{{chem2 | Fe2O3 + 3 CO -> 2 Fe + 3 CO2 }}

Some iron in the high-temperature lower region of the furnace reacts directly with the coke:{{sfn|Greenwood|Earnshaw|1997|p=1073}}

:{{chem2 | 2 Fe2O3 + 3 C -> 4 Fe + 3 CO2 }}

The flux removes silicaceous minerals in the ore, which would otherwise clog the furnace: The heat of the furnace decomposes the carbonates to calcium oxide, which reacts with any excess silica to form a slag composed of calcium silicate {{chem2|CaSiO3}} or other products. At the furnace's temperature, the metal and the slag are both molten. They collect at the bottom as two immiscible liquid layers (with the slag on top), that are then easily separated.{{sfn|Greenwood|Earnshaw|1997|p=1073}} The slag can be used as a material in road construction or to improve mineral-poor soils for agriculture.

Steelmaking thus remains one of the largest industrial contributors of CO₂ emissions in the world.{{Cite journal |last1=Wang |first1=Peng |last2=Ryberg |first2=Morten |last3=Yang |first3=Yi |last4=Feng |first4=Kuishuang |last5=Kara |first5=Sami |last6=Hauschild |first6=Michael |last7=Chen |first7=Wei-Qiang |date=2021-04-06 |title=Efficiency stagnation in global steel production urges joint supply- and demand-side mitigation efforts |journal=Nature Communications |volume=12 |issue=1 |pages=2066 |doi=10.1038/s41467-021-22245-6 |issn=2041-1723 |pmc=8024266 |pmid=33824307|bibcode=2021NatCo..12.2066W }}

File:Chinese Fining and Blast Furnace.jpg|17th century Chinese illustration of workers at a blast furnace, making wrought iron from pig ironSong Yingxing (1637): The Tiangong Kaiwu encyclopedia.

File:Iron-Making.jpg|How iron was extracted in the 19th century

File:Geography of Ohio - DPLA - aaba7b3295ff6973b6fd1e23e33cde14 (page 111) (cropped).jpg|Iron furnace in Columbus, Ohio, 1922

{{Main|Steelmaking|Ironworks}}

The pig iron produced by the blast furnace process contains up to 4–5% carbon (by mass), with small amounts of other impurities like sulfur, magnesium, phosphorus, and manganese. This high level of carbon makes it relatively weak and brittle. Reducing the amount of carbon to 0.002–2.1% produces steel, which may be up to 1000 times harder than pure iron. A great variety of steel articles can then be made by cold working, hot rolling, forging, machining, etc. Removing the impurities from pig iron, but leaving 2–4% carbon, results in cast iron, which is cast by foundries into articles such as stoves, pipes, radiators, lamp-posts, and rails.{{sfn|Greenwood|Earnshaw|1997|p=1073}}

Steel products often undergo various heat treatments after they are forged to shape. Annealing consists of heating them to 700–800 °C for several hours and then gradual cooling. It makes the steel softer and more workable.Verhoeven, J.D. (1975) Fundamentals of Physical Metallurgy, Wiley, New York, p. 326

File:LightningVolt Iron Ore Pellets.jpg|This heap of iron ore pellets will be used in steel production.

File:Melted raw-iron.jpg|A pot of molten iron being used to make steel

Owing to environmental concerns, alternative methods of processing iron have been developed. "Direct iron reduction" reduces iron ore to a ferrous lump called "sponge" iron or "direct" iron that is suitable for steelmaking. Two main reactions comprise the direct reduction process:

Natural gas is partially oxidized (with heat and a catalyst):

:{{chem2 | 2 CH4 + O2 -> 2 CO + 4 H2 }}

Iron ore is then treated with these gases in a furnace, producing solid sponge iron:

:{{chem2 | Fe2O3 + CO + 2 H2 -> 2 Fe + CO2 + 2 H2O }}

Silica is removed by adding a limestone flux as described above.

{{Main|Thermite}}

Ignition of a mixture of aluminium powder and iron oxide yields metallic iron via the thermite reaction:

:{{chem2 | Fe2O3 + 2 Al -> 2 Fe + Al2O3 }}

Alternatively pig iron may be made into steel (with up to about 2% carbon) or wrought iron (commercially pure iron). Various processes have been used for this, including finery forges, puddling furnaces, Bessemer converters, open hearth furnaces, basic oxygen furnaces, and electric arc furnaces. In all cases, the objective is to oxidize some or all of the carbon, together with other impurities. On the other hand, other metals may be added to make alloy steels.{{sfn|Greenwood|Earnshaw|1997|p=1072}}

Molten oxide electrolysis (MOE) uses electrolysis of molten iron oxide to yield metallic iron. It is studied in laboratory-scale experiments and is proposed as a method for industrial iron production that has no direct emissions of carbon dioxide. It uses a liquid iron cathode, an anode formed from an alloy of chromium, aluminium and iron,{{cite journal |last1=Allanore |first1=Antoine |last2=Yin |first2=Lan |last3=Sadoway |first3=Donald R. |title=A new anode material for oxygen evolution in molten oxide electrolysis |journal=Nature |volume=497 |date=2013 |issue=7449 |issn=0028-0836 |doi=10.1038/nature12134 |pages=353–356|pmid=23657254 |bibcode=2013Natur.497..353A |hdl=1721.1/82073 |hdl-access=free }} and the electrolyte is a mixture of molten metal oxides into which iron ore is dissolved. The current keeps the electrolyte molten and reduces the iron oxide. Oxygen gas is produced in addition to liquid iron. The only carbon dioxide emissions come from any fossil fuel-generated electricity used to heat and reduce the metal.{{cite journal |last1=Wiencke |first1=Jan |last2=Lavelaine |first2=Hervé |last3=Panteix |first3=Pierre-Jean |last4=Petitjean |first4=Carine |last5=Rapin |first5=Christophe |title=Electrolysis of iron in a molten oxide electrolyte |journal=Journal of Applied Electrochemistry |volume=48 |issue=1 |date=2018 |issn=0021-891X |doi=10.1007/s10800-017-1143-5 |pages=115–126|doi-access=free }}{{cite journal |last1=Fan |first1=Zhiyuan |last2=Friedmann |first2=S. Julio |title=Low-carbon production of iron and steel: Technology options, economic assessment, and policy |journal=Joule |volume=5 |date=2021 |issue=4 |doi=10.1016/j.joule.2021.02.018 |pages=829–862|doi-access=free |bibcode=2021Joule...5..829F }}{{cite news |last1=Gallucci |first1=Maria |title=Boston Metal gets big funding boost to make green steel |url=https://www.canarymedia.com/articles/clean-industry/boston-metal-gets-big-funding-boost-to-make-green-steel |access-date=11 March 2024 |work=Canary Media |publisher=Rocky Mountain Institute |date=September 7, 2023}}

Iron is the most widely used of all the metals, accounting for over 90% of worldwide metal production. Its low cost and high strength often make it the material of choice to withstand stress or transmit forces, such as the construction of machinery and machine tools, rails, automobiles, ship hulls, concrete reinforcing bars, and the load-carrying framework of buildings. Since pure iron is quite soft, it is most commonly combined with alloying elements to make steel.{{sfn|Greenwood|Earnshaw|1997|pp=1070–71}}

The mechanical properties of iron and its alloys are extremely relevant to their structural applications. Those properties can be evaluated in various ways, including the Brinell test, the Rockwell test and the Vickers hardness test.

The properties of pure iron are often used to calibrate measurements or to compare tests.{{cite web| url=https://mdmetric.com/tech/hardnessconversion.html| title=Hardness Conversion Chart| access-date=23 May 2010| publisher=Maryland Metrics| url-status=dead| archive-url=https://web.archive.org/web/20150618071701/https://mdmetric.com/tech/hardnessconversion.html| archive-date=18 June 2015}} However, the mechanical properties of iron are significantly affected by the sample's purity: pure, single crystals of iron are actually softer than aluminium, and the purest industrially produced iron (99.99%) has a hardness of 20–30 Brinell.{{Cite journal| title=Properties of Various Pure Irons: Study on pure iron I| url=https://ci.nii.ac.jp/naid/110001459778/en| volume=50| issue=1| pages=42–47| journal=Tetsu-to-Hagane| first1 = Kusakawa|last1 = Takaji|first2 = Otani|last2 =Toshikatsu| date=1964| doi=10.2355/tetsutohagane1955.50.1_42| doi-access=free}} The pure iron (99.9%～99.999%), especially called electrolytic iron, is industrially produced by electrolytic refining.

An increase in the carbon content will cause a significant increase in the hardness and tensile strength of iron. Maximum hardness of 65 R_c is achieved with a 0.6% carbon content, although the alloy has low tensile strength.{{Cite book| url={{Google books|LgB5dkmPML0C|page=PA218|keywords=|text=|plainurl=yes}}| page=218| title=Materials Science and Engineering|first=V.| last= Raghavan| publisher =PHI Learning Pvt. Ltd.|isbn=81-203-2455-2 |date=2004}} Because of the softness of iron, it is much easier to work with than its heavier congeners ruthenium and osmium.{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}}

{{See also|Steel}}

File:Iron carbon phase diagram.svg

α-Iron is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C).{{Cite book|url={{Google books|xv420pEC2qMC|page=PA183|keywords=|text=|plainurl=yes}}| page=183| title=Concise encyclopedia of the structure of materials| first=John Wilson|last = Martin| publisher=Elsevier| date= 2007|isbn=978-0-08-045127-5}} Austenite (γ-iron) is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146 °C). This form of iron is used in the type of stainless steel used for making cutlery, and hospital and food-service equipment.

Commercially available iron is classified based on purity and the abundance of additives. Pig iron has 3.5–4.5% carbon{{Cite book|last1 = Camp|first1 = James McIntyre|last2 = Francis |first2 = Charles Blaine|title = The Making, Shaping and Treating of Steel|publisher = Carnegie Steel Company |date=1920|location = Pittsburgh|pages = 173–74|url={{Google books|P9MxAAAAMAAJ|keywords=|text=|plain-url=yes}}|isbn = 1-147-64423-3}} and contains varying amounts of contaminants such as sulfur, silicon and phosphorus. Pig iron is not a saleable product, but rather an intermediate step in the production of cast iron and steel. The reduction of contaminants in pig iron that negatively affect material properties, such as sulfur and phosphorus, yields cast iron containing 2–4% carbon, 1–6% silicon, and small amounts of manganese.{{sfn|Greenwood|Earnshaw|1997|p=1073}} Pig iron has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together.{{sfn|Greenwood|Earnshaw|1997|pp=1075–79}} Its mechanical properties vary greatly and depend on the form the carbon takes in the alloy.{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}}

"White" cast irons contain their carbon in the form of cementite, or iron carbide (Fe₃C).{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}} This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken iron carbide, a very pale, silvery, shiny material, hence the appellation. Cooling a mixture of iron with 0.8% carbon slowly below 723 °C to room temperature results in separate, alternating layers of cementite and α-iron, which is soft and malleable and is called pearlite for its appearance. Rapid cooling, on the other hand, does not allow time for this separation and creates hard and brittle martensite. The steel can then be tempered by reheating to a temperature in between, changing the proportions of pearlite and martensite. The end product below 0.8% carbon content is a pearlite-αFe mixture, and that above 0.8% carbon content is a pearlite-cementite mixture.{{sfn|Greenwood|Earnshaw|1997|pp=1074–75}}

In gray iron the carbon exists as separate, fine flakes of graphite, and also renders the material brittle due to the sharp edged flakes of graphite that produce stress concentration sites within the material.{{Citation | last1 = Smith | first1 = William F. | last2 = Hashemi | first2 = Javad | title = Foundations of Materials Science and Engineering | edition = 4th | year = 2006 | publisher = McGraw-Hill | isbn = 0-07-295358-6 | postscript =. |page=431}} A newer variant of gray iron, referred to as ductile iron, is specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, reducing the stress concentrations and vastly increasing the toughness and strength of the material.

Wrought iron contains less than 0.25% carbon but large amounts of slag that give it a fibrous characteristic. Wrought iron is more corrosion resistant than steel. It has been almost completely replaced by mild steel, which corrodes more readily than wrought iron, but is cheaper and more widely available. Carbon steel contains 2.0% carbon or less,{{cite web|title=Classification of Carbon and Low-Alloy Steels |url=https://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=62 |access-date=5 January 2008 |url-status=dead |archive-date=2 January 2011 |archive-url=https://web.archive.org/web/20110102110320/http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=62}} with small amounts of manganese, sulfur, phosphorus, and silicon. Alloy steels contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc. Their alloy content raises their cost, and so they are usually only employed for specialist uses. One common alloy steel, though, is stainless steel. Recent developments in ferrous metallurgy have produced a growing range of microalloyed steels, also termed 'HSLA' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.{{cite web |title=HSLA Steel |date=2002-11-15 |url=https://machinedesign.com/BasicsOfDesignEngineeringItem/717/65970/HSLASteel.aspx |access-date=2008-10-11 |archive-url=https://web.archive.org/web/20091230082918/https://machinedesign.com/article/hsla-steel-1115 |archive-date=30 December 2009 |url-status=dead}}{{cite book |last=Oberg |first=E. |title=Machinery's Handbook |place=New York |publisher=Industrial Press |edition=25th |year=1996 |display-authors=etal |pages=440–42 |bibcode=1984msh..book.....R}}

Alloys with high purity elemental makeups (such as alloys of electrolytic iron) have specifically enhanced properties such as ductility, tensile strength, toughness, fatigue strength, heat resistance, and corrosion resistance.

Apart from traditional applications, iron is also used for protection from ionizing radiation. Although it is lighter than another traditional protection material, lead, it is much stronger mechanically.{{cite web |url=https://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-13033.pdf |title=Radiation Shielding at High-Energy Electron and Proton Accelerators |last1=Rokni |first1=Sayed H. |last2=Cossairt |first2=J. Donald |last3=Liu |first3=James C. |date=January 2008 |access-date=6 August 2016}}

The main disadvantage of iron and steel is that pure iron, and most of its alloys, suffer badly from rust if not protected in some way, a cost amounting to over 1% of the world's economy.{{sfn|Greenwood|Earnshaw|1997|p=1076}} Painting, galvanization, passivation, plastic coating and bluing are all used to protect iron from rust by excluding water and oxygen or by cathodic protection. The mechanism of the rusting of iron is as follows:{{sfn|Greenwood|Earnshaw|1997|p=1076}}

:Cathode: 3 O₂ + 6 H₂O + 12 e⁻ → 12 OH⁻

:Anode: 4 Fe → 4 Fe²⁺ + 8 e⁻; 4 Fe²⁺ → 4 Fe³⁺ + 4 e⁻

:Overall: 4 Fe + 3 O₂ + 6 H₂O → 4 Fe³⁺ + 12 OH⁻ → 4 Fe(OH)₃ or 4 FeO(OH) + 4 H₂O

The electrolyte is usually iron(II) sulfate in urban areas (formed when atmospheric sulfur dioxide attacks iron), and salt particles in the atmosphere in seaside areas.{{sfn|Greenwood|Earnshaw|1997|p=1076}}

Because Fe is inexpensive and nontoxic, much effort has been devoted to the development of Fe-based catalysts and reagents. Iron is however less common as a catalyst in commercial processes than more expensive metals.{{cite journal |doi=10.1021/acscentsci.6b00272|title=Iron Catalysis in Organic Synthesis: A Critical Assessment of What It Takes to Make This Base Metal a Multitasking Champion |year=2016 |last1=Fürstner |first1=Alois |journal=ACS Central Science |volume=2 |issue=11 |pages=778–789 |pmid=27981231 |pmc=5140022 }} In biology, Fe-containing enzymes are pervasive.{{cite journal |doi=10.1126/science.abc3183|title=Using nature's blueprint to expand catalysis with Earth-abundant metals |year=2020 |last1=Bullock |first1=R. Morris |display-authors=etal |journal=Science |volume=369 |issue=6505 |pages=eabc3183 |pmid=32792370 |pmc=7875315 }}

Iron catalysts are traditionally used in the Haber–Bosch process for the production of ammonia and the Fischer–Tropsch process for conversion of carbon monoxide to hydrocarbons for fuels and lubricants.{{Cite book| title = Surface science: foundations of catalysis and nanoscience|first = Kurt W.|last = Kolasinski|isbn = 978-0-471-49244-3| publisher =John Wiley and Sons|date = 2002|chapter-url={{Google books|OA7L1l6oHAYC|page=PR15|keywords=|text=|plainurl=yes}}|chapter = Where are Heterogenous Reactions Important|pages = 15–16}} Powdered iron in an acidic medium is used in the Bechamp reduction, the conversion of nitrobenzene to aniline.{{Cite book| chapter-url={{Google books|BiywGdlot9kC|page=PA167|keywords=|text=|plainurl=yes}}|chapter = Nitrobenzene and Nitrotoluene |isbn = 978-0-8247-2481-8|publisher = CRC Press|date = 1989|first = John J.|last = McKetta|title = Encyclopedia of Chemical Processing and Design: Volume 31 – Natural Gas Liquids and Natural Gasoline to Offshore Process Piping: High Performance Alloys|pages = 166–67}}

Iron(III) oxide mixed with aluminium powder can be ignited to create a thermite reaction, used in welding large iron parts (like rails) and purifying ores. Iron(III) oxide and oxyhydroxide are used as reddish and ocher pigments.

Iron(III) chloride finds use in water purification and sewage treatment, in the dyeing of cloth, as a coloring agent in paints, as an additive in animal feed, and as an etchant for copper in the manufacture of printed circuit boards.{{Cite encyclopedia| doi = 10.1002/14356007.a14_591| encyclopedia = Ullmann's Encyclopedia of Industrial Chemistry| date = 2000| last1 = Wildermuth| first1 = Egon| last2 = Stark| first2 = Hans| last3 = Friedrich| first3 = Gabriele| last4 = Ebenhöch| first4 = Franz Ludwig| last5 = Kühborth| first5 = Brigitte| last6 = Silver| first6 = Jack| last7 = Rituper| first7 = Rafael| isbn = 3-527-30673-0| chapter = Iron Compounds}} It can also be dissolved in alcohol to form tincture of iron, which is used as a medicine to stop bleeding in canaries.{{cite book |last=Stroud |first=Robert |date=1933 |title=Diseases of Canaries |publisher=Canary Publishers Company |page=203 |isbn=978-1-4465-4656-7|title-link=Diseases of Canaries }}

Iron(II) sulfate is used as a precursor to other iron compounds. It is also used to reduce chromate in cement. It is used to fortify foods and treat iron deficiency anemia. Iron(III) sulfate is used in settling minute sewage particles in tank water. Iron(II) chloride is used as a reducing flocculating agent, in the formation of iron complexes and magnetic iron oxides, and as a reducing agent in organic synthesis.

Sodium nitroprusside is a drug used as a vasodilator. It is on the World Health Organization's List of Essential Medicines.{{cite book | vauthors = ((World Health Organization)) | title = World Health Organization model list of essential medicines: 22nd list (2021) | year = 2021 | hdl = 10665/345533 | author-link = World Health Organization | publisher = World Health Organization | location = Geneva | id = WHO/MHP/HPS/EML/2021.02 | hdl-access=free}}

{{Main|Iron in biology}}

Iron is required for life.{{cite book |last1=Dlouhy |first1=Adrienne C. |last2=Outten |first2=Caryn E. |chapter=The Iron Metallome in Eukaryotic Organisms |editor1-first=Lucia |editor1-last=Banci |series=Metal Ions in Life Sciences |volume=12 |pages=241–78 |title=Metallomics and the Cell |date=2013 |publisher=Springer |isbn=978-94-007-5560-4|doi=10.1007/978-94-007-5561-1_8|pmid=23595675 |pmc=3924584}} electronic-book {{ISBN|978-94-007-5561-1}}

{{cite book |first1=Gereon M. |last1=Yee |first2=William B. |last2=Tolman |editor=Peter M.H. Kroneck |editor2=Martha E. Sosa Torres |title=Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases |chapter=Transition Metal Complexes and the Activation of Dioxygen |series=Metal Ions in Life Sciences |volume=15 |year=2015 |publisher=Springer |pages=131–204 |doi=10.1007/978-3-319-12415-5_5 |pmid=25707468|isbn=978-3-319-12414-8 }}

The iron–sulfur clusters are pervasive and include nitrogenase, the enzymes responsible for biological nitrogen fixation. Iron-containing proteins participate in transport, storage and use of oxygen. Iron proteins are involved in electron transfer.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}}

File:Heme B.svg; in the protein additional ligand(s) are attached to Fe.]]

Examples of iron-containing proteins in higher organisms include hemoglobin, cytochrome (see high-valent iron), and catalase.{{Cite book| first1 = S.J.|last1 = Lippard|first2 = J.M.|last2 = Berg|title = Principles of Bioinorganic Chemistry|publisher = University Science Books|place = Mill Valley|date = 1994|isbn = 0-935702-73-3}} The average adult human contains about 0.005% body weight of iron, or about four grams, of which three quarters is in hemoglobin—a level that remains constant despite only about one milligram of iron being absorbed each day,{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} because the human body recycles its hemoglobin for the iron content.{{Cite journal

| last1 = Kikuchi | first1 = G.

| last2 = Yoshida | first2 = T.

| last3 = Noguchi | first3 = M.

| doi = 10.1016/j.bbrc.2005.08.020

| title = Heme oxygenase and heme degradation

| journal = Biochemical and Biophysical Research Communications

| volume = 338

| issue = 1

| pages = 558–67

| year = 2005

| pmid = 16115609

}}

Microbial growth may be assisted by oxidation of iron(II) or by reduction of iron(III).{{cite book |doi=10.1515/9783110589771-006 |chapter=Contents of Volumes in the Metal Ions in Life Sciences Series |title=Metals, Microbes, and Minerals - the Biogeochemical Side of Life |year=2021 |pages=xxv-xlvi |publisher=De Gruyter |isbn=9783110589771|s2cid=196704759 }}

Iron acquisition poses a problem for aerobic organisms because ferric iron is poorly soluble near neutral pH. Thus, these organisms have developed means to absorb iron as complexes, sometimes taking up ferrous iron before oxidising it back to ferric iron. In particular, bacteria have evolved very high-affinity sequestering agents called siderophores.{{Cite journal |pmid=7592901 |doi=10.1074/jbc.270.45.26723 |date=1995 |last1=Neilands |first1=J.B. |title=Siderophores: structure and function of microbial iron transport compounds |volume=270 |issue=45 |pages=26723–26 |journal=The Journal of Biological Chemistry |doi-access=free}}{{Cite journal |doi=10.1146/annurev.bi.50.070181.003435 |title=Microbial Iron Compounds |date=1981 |last1=Neilands |first1=J.B. |journal=Annual Review of Biochemistry |volume=50 |pages=715–31 |pmid=6455965|issue=1}}{{Cite journal| doi=10.1023/A:1020218608266 |date=2002 |last1=Boukhalfa |first1=Hakim |last2=Crumbliss |first2=Alvin L. |journal=BioMetals |volume=15 |issue=4 |pages=325–39 |pmid=12405526 |title=Chemical aspects of siderophore mediated iron transport |s2cid=19697776}}

After uptake in human cells, iron storage is precisely regulated.{{Cite journal |title=Tumor necrosis factor-α-induced iron sequestration and oxidative stress in human endothelial cells |last11=Nakanishi |first11=T. |last10=Suzuki |first10=K. |first9=H. |last9=Eguchi |first8=M. |last8=Izumi |first7=Y. |last7=Hasuike |first6=K. |last6=Miyagawa |first5=R. |last5=Moriguchi |first4=K. |last4=Ito |first3=Y. |last3=Otaki |first2=T. |last2=Ookawara |first1=M. |last1=Nanami |pmid=16224057 |journal=Arteriosclerosis, Thrombosis, and Vascular Biology |date=2005 |volume=25 |issue=12 |pages=2495–501 |doi=10.1161/01.ATV.0000190610.63878.20 |doi-access=free}} A major component of this regulation is the protein transferrin, which binds iron ions absorbed from the duodenum and carries it in the blood to cells.{{Cite journal|doi=10.1371/journal.pbio.0000079|title=How Mammals Acquire and Distribute Iron Needed for Oxygen-Based Metabolism|date=2003|last=Rouault|first = Tracey A.|author-link=Tracey Rouault|journal=PLOS Biology |volume=1 |issue=3 |pages=e9 |pmid=14691550|pmc=300689 |doi-access=free }} Transferrin contains Fe³⁺ in the middle of a distorted octahedron, bonded to one nitrogen, three oxygens and a chelating carbonate anion that traps the Fe³⁺ ion: it has such a high stability constant that it is very effective at taking up Fe³⁺ ions even from the most stable complexes. At the bone marrow, transferrin is reduced from Fe³⁺ to Fe²⁺ and stored as ferritin to be incorporated into hemoglobin.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}}

The most commonly known and studied bioinorganic iron compounds (biological iron molecules) are the heme proteins: examples are hemoglobin, myoglobin, and cytochrome P450. These compounds participate in transporting gases, building enzymes, and transferring electrons.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} Metalloproteins are a group of proteins with metal ion cofactors. Some examples of iron metalloproteins are ferritin and rubredoxin.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} Many enzymes vital to life contain iron, such as catalase,{{cite web |vauthors=Boon EM, Downs A, Marcey D |title=Proposed Mechanism of Catalase |work=Catalase: H₂O₂: H₂O₂ Oxidoreductase: Catalase Structural Tutorial |url=https://biology.kenyon.edu/BMB/Chime/catalase/frames/cattx.htm#Proposed%20Mechanism%20of%20Catalase |access-date=2007-02-11}} lipoxygenases,{{cite journal |vauthors=Boyington JC, Gaffney BJ, Amzel LM |title=The three-dimensional structure of an arachidonic acid 15-lipoxygenase |journal=Science |volume=260 |issue=5113 |pages=1482–86 |year=1993 |pmid=8502991 |doi=10.1126/science.8502991 |bibcode=1993Sci...260.1482B}} and IRE-BP.{{cite journal |last1=Gray |first1=N.K. |last2=Hentze |first2=M.W. |title=Iron regulatory protein prevents binding of the 43S translation pre-initiation complex to ferritin and eALAS mRNAs |journal=EMBO J. |volume=13 |number=16 |pages=3882–91 |date=August 1994 |pmc=395301 |pmid=8070415 |doi=10.1002/j.1460-2075.1994.tb06699.x}}

Hemoglobin is an oxygen carrier that occurs in red blood cells and contributes their color, transporting oxygen in the arteries from the lungs to the muscles where it is transferred to myoglobin, which stores it until it is needed for the metabolic oxidation of glucose, generating energy. Here the hemoglobin binds to carbon dioxide, produced when glucose is oxidized, which is transported through the veins by hemoglobin (predominantly as bicarbonate anions) back to the lungs where it is exhaled.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} In hemoglobin, the iron is in one of four heme groups and has six possible coordination sites; four are occupied by nitrogen atoms in a porphyrin ring, the fifth by an imidazole nitrogen in a histidine residue of one of the protein chains attached to the heme group, and the sixth is reserved for the oxygen molecule it can reversibly bind to.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} When hemoglobin is not attached to oxygen (and is then called deoxyhemoglobin), the Fe²⁺ ion at the center of the heme group (in the hydrophobic protein interior) is in a high-spin configuration. It is thus too large to fit inside the porphyrin ring, which bends instead into a dome with the Fe²⁺ ion about 55 picometers above it. In this configuration, the sixth coordination site reserved for the oxygen is blocked by another histidine residue.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}}

When deoxyhemoglobin picks up an oxygen molecule, this histidine residue moves away and returns once the oxygen is securely attached to form a hydrogen bond with it. This results in the Fe²⁺ ion switching to a low-spin configuration, resulting in a 20% decrease in ionic radius so that now it can fit into the porphyrin ring, which becomes planar.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} Additionally, this hydrogen bonding results in the tilting of the oxygen molecule, resulting in a Fe–O–O bond angle of around 120° that avoids the formation of Fe–O–Fe or Fe–O₂–Fe bridges that would lead to electron transfer, the oxidation of Fe²⁺ to Fe³⁺, and the destruction of hemoglobin. This results in a movement of all the protein chains that leads to the other subunits of hemoglobin changing shape to a form with larger oxygen affinity. Thus, when deoxyhemoglobin takes up oxygen, its affinity for more oxygen increases, and vice versa.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} Myoglobin, on the other hand, contains only one heme group and hence this cooperative effect cannot occur. Thus, while hemoglobin is almost saturated with oxygen in the high partial pressures of oxygen found in the lungs, its affinity for oxygen is much lower than that of myoglobin, which oxygenates even at low partial pressures of oxygen found in muscle tissue.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} As described by the Bohr effect (named after Christian Bohr, the father of Niels Bohr), the oxygen affinity of hemoglobin diminishes in the presence of carbon dioxide.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}}

File:Carboxyhemoglobin from 1AJ9.png, showing the carbonyl ligand at the apical position, trans to the histidine residue{{cite journal | journal = Acta Crystallogr. D | title = Human Carboxyhemoglobin at 2.2 Å Resolution: Structure and Solvent Comparisons of R-State, R2-State and T-State Hemoglobins |author1=Gregory B. Vásquez |author2=Xinhua Ji |author3=Clara Fronticelli |author4=Gary L. Gilliland | doi = 10.1107/S0907444997012250 | pmid = 9761903 | volume = 54 | issue = 3 | pages = 355–66 | year = 1998| doi-access = free | bibcode = 1998AcCrD..54..355V }}]]

Carbon monoxide and phosphorus trifluoride are poisonous to humans because they bind to hemoglobin similarly to oxygen, but with much more strength, so that oxygen can no longer be transported throughout the body. Hemoglobin bound to carbon monoxide is known as carboxyhemoglobin. This effect also plays a minor role in the toxicity of cyanide, but there the major effect is by far its interference with the proper functioning of the electron transport protein cytochrome a.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} The cytochrome proteins also involve heme groups and are involved in the metabolic oxidation of glucose by oxygen. The sixth coordination site is then occupied by either another imidazole nitrogen or a methionine sulfur, so that these proteins are largely inert to oxygen—with the exception of cytochrome a, which bonds directly to oxygen and thus is very easily poisoned by cyanide.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}} Here, the electron transfer takes place as the iron remains in low spin but changes between the +2 and +3 oxidation states. Since the reduction potential of each step is slightly greater than the previous one, the energy is released step-by-step and can thus be stored in adenosine triphosphate. Cytochrome a is slightly distinct, as it occurs at the mitochondrial membrane, binds directly to oxygen, and transports protons as well as electrons, as follows:{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}}

:4 Cytc²⁺ + O₂ + 8H{{su|p=+|b=inside}} → 4 Cytc³⁺ + 2 H₂O + 4H{{su|p=+|b=outside}}

Although the heme proteins are the most important class of iron-containing proteins, the iron–sulfur proteins are also very important, being involved in electron transfer, which is possible since iron can exist stably in either the +2 or +3 oxidation states. These have one, two, four, or eight iron atoms that are each approximately tetrahedrally coordinated to four sulfur atoms; because of this tetrahedral coordination, they always have high-spin iron. The simplest of such compounds is rubredoxin, which has only one iron atom coordinated to four sulfur atoms from cysteine residues in the surrounding peptide chains. Another important class of iron–sulfur proteins is the ferredoxins, which have multiple iron atoms. Transferrin does not belong to either of these classes.{{sfn|Greenwood|Earnshaw|1997|pp=1098–104}}

The ability of sea mussels to maintain their grip on rocks in the ocean is facilitated by their use of organometallic iron-based bonds in their protein-rich cuticles. Based on synthetic replicas, the presence of iron in these structures increased elastic modulus 770 times, tensile strength 58 times, and toughness 92 times. The amount of stress required to permanently damage them increased 76 times.{{Cite journal| first = K| last = Sanderson|title = Mussels' iron grip inspires strong and stretchy polymer| journal = Chemical & Engineering News|page=8|volume = 95| issue = 44| publisher = American Chemical Society| date = 2017| url=https://cen.acs.org/articles/95/i44/Mussels-iron-grip-inspires-strong-stretchy-polymer.html|access-date=2 November 2017| doi=10.1021/cen-09544-notw3}}

Iron is pervasive, but particularly rich sources of dietary iron include red meat, oysters, beans, poultry, fish, leaf vegetables, watercress, tofu, and blackstrap molasses. Bread and breakfast cereals are sometimes specifically fortified with iron.[https://www.eatwell.gov.uk/healthissues/irondeficiency/ Food Standards Agency – Eat well, be well – Iron deficiency] {{webarchive|url=https://web.archive.org/web/20060808184739/https://www.eatwell.gov.uk/healthissues/irondeficiency/ |date=8 August 2006 }}. Eatwell.gov.uk (5 March 2012). Retrieved on 27 June 2012.

Iron provided by dietary supplements is often found as iron(II) fumarate, although iron(II) sulfate is cheaper and is absorbed equally well. Elemental iron, or reduced iron, despite being absorbed at only one-third to two-thirds the efficiency (relative to iron sulfate),{{cite journal|last1=Hoppe|first1=M.|last2=Hulthén|first2=L.|last3=Hallberg|first3=L.|title=The relative bioavailability in humans of elemental iron powders for use in food fortification|journal=European Journal of Nutrition|volume=45|issue=1|pages=37–44|date=2005|pmid=15864409|doi=10.1007/s00394-005-0560-0|s2cid=42983904}} is often added to foods such as breakfast cereals or enriched wheat flour. Iron is most available to the body when chelated to amino acids{{Cite journal|title=Effectiveness of treatment of iron-deficiency anemia in infants and young children with ferrous bis-glycinate chelate |journal=Nutrition |volume=17 |issue=5 |pages=381–4 |date=2001 |pmid=11377130| doi = 10.1016/S0899-9007(01)00519-6 |last1=Pineda |first1=O. |last2=Ashmead |first2=H. D.}} and is also available for use as a common iron supplement. Glycine, the least expensive amino acid, is most often used to produce iron glycinate supplements.{{Cite book|last = Ashmead |first = H. DeWayne |date = 1989 |title = Conversations on Chelation and Mineral Nutrition |publisher = Keats Publishing |isbn = 0-87983-501-X}}

The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for iron in 2001. The current EAR for iron for women ages 14{{nbnd}}18 is 7.9 mg/day, 8.1 mg/day for ages 19{{nbnd}}50 and 5.0 mg/day thereafter (postmenopause). For men, the EAR is 6.0 mg/day for ages 19 and up. The RDA is 15.0 mg/day for women ages 15{{nbnd}}18, 18.0 mg/day for ages 19{{nbnd}}50 and 8.0 mg/day thereafter. For men, 8.0 mg/day for ages 19 and up. RDAs are higher than EARs so as to identify amounts that will cover people with higher-than-average requirements. RDA for pregnancy is 27 mg/day and, for lactation, 9 mg/day. For children ages 1{{nbnd}}3 years 7 mg/day, 10 mg/day for ages 4–8 and 8 mg/day for ages 9{{nbnd}}13. As for safety, the IOM also sets Tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of iron, the UL is set at 45 mg/day. Collectively the EARs, RDAs and ULs are referred to as Dietary Reference Intakes.{{cite book|chapter= Iron|chapter-url= https://www.nal.usda.gov/sites/default/files/fnic_uploads//290-393_150.pdf|title= Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Iron|publisher= National Academy Press|year= 2001|pages= 290–393|pmid= 25057538|isbn= 0-309-07279-4|author1= Institute of Medicine (US) Panel on Micronutrients|access-date= 9 March 2017|archive-date= 9 September 2017|archive-url= https://web.archive.org/web/20170909191057/https://www.nal.usda.gov/sites/default/files/fnic_uploads//290-393_150.pdf|url-status= dead}}

The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL are defined the same as in the United States. For women the PRI is 13 mg/day ages 15{{nbnd}}17 years, 16 mg/day for women ages 18 and up who are premenopausal and 11 mg/day postmenopausal. For pregnancy and lactation, 16 mg/day. For men the PRI is 11 mg/day ages 15 and older. For children ages 1 to 14, the PRI increases from 7 to 11 mg/day. The PRIs are higher than the U.S. RDAs, with the exception of pregnancy.{{cite web | title = Overview on Dietary Reference Values for the EU population as derived by the EFSA Panel on Dietetic Products, Nutrition and Allergies| year = 2017| url = https://www.efsa.europa.eu/sites/default/files/assets/DRV_Summary_tables_jan_17.pdf|work=European Food Safety Authority}} The EFSA reviewed the same safety question did not establish a UL.{{cite web| title = Tolerable Upper Intake Levels For Vitamins And Minerals| publisher = European Food Safety Authority| year = 2006| url = https://www.efsa.europa.eu/sites/default/files/efsa_rep/blobserver_assets/ndatolerableuil.pdf}}

Infants may require iron supplements if they are bottle-fed cow's milk.{{cite web |url=https://bodyandhealth.canada.com/condition_info_details.asp?disease_id=274 |title=Iron Deficiency Anemia |publisher=MediResource |access-date=17 December 2008 |archive-date=16 December 2008 |archive-url=https://web.archive.org/web/20081216132821/http://bodyandhealth.canada.com/condition_info_details.asp?disease_id=274 |url-status=dead }} Frequent blood donors are at risk of low iron levels and are often advised to supplement their iron intake.{{Cite journal| doi= 10.1016/0925-5710(95)00426-2|pmid= 8867722|date= 1996|last1= Milman|first1=N.|title= Serum ferritin in Danes: studies of iron status from infancy to old age, during blood donation and pregnancy|volume= 63|issue= 2|pages= 103–35|journal= International Journal of Hematology|doi-access= free}}

For U.S. food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV). For iron labeling purposes, 100% of the Daily Value was 18 mg, and {{as of|2016|May|27|lc=y|df=US}} remained unchanged at 18 mg.{{Cite web|url=https://www.gpo.gov/fdsys/pkg/FR-2016-05-27/pdf/2016-11867.pdf|title=Federal Register May 27, 2016 Food Labeling: Revision of the Nutrition and Supplement Facts Labels. FR page 33982.}}{{cite web | title=Daily Value Reference of the Dietary Supplement Label Database (DSLD) | website=Dietary Supplement Label Database (DSLD) | url=https://www.dsld.nlm.nih.gov/dsld/dailyvalue.jsp | access-date=16 May 2020 | archive-date=7 April 2020 | archive-url=https://web.archive.org/web/20200407073956/https://dsld.nlm.nih.gov/dsld/dailyvalue.jsp | url-status=dead }} A table of the old and new adult daily values is provided at Reference Daily Intake.

{{Main|Iron deficiency}}

Iron deficiency is the most common nutritional deficiency in the world.{{cite journal |author=Centers for Disease Control and Prevention |title=Iron deficiency – United States, 1999–2000 |journal=MMWR |date=2002 |volume=51 |issue=40 |pages=897–99 |url=https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5140a1.htm|pmid=12418542}}{{cite book|first1=Robert C. |last1=Hider |first2=Xiaole|last2=Kong |editor=Astrid Sigel, Helmut Sigel and Roland K.O. Sigel |title=Interrelations between Essential Metal Ions and Human Diseases|series=Metal Ions in Life Sciences|volume=13 |year=2013|publisher=Springer|pages=229–94 |chapter=Chapter 8. Iron: Effect of Overload and Deficiency|doi=10.1007/978-94-007-7500-8_8|pmid=24470094|isbn=978-94-007-7499-5 }}{{cite book |last1=Dlouhy |first1=Adrienne C. |last2=Outten |first2=Caryn E. |chapter=The Iron Metallome in Eukaryotic Organisms |editor1-first=Lucia|editor1-last=Banci |series=Metal Ions in Life Sciences |volume=12 |title=Metallomics and the Cell |year=2013 |pages=241–78 |publisher=Springer |isbn=978-94-007-5560-4 |doi=10.1007/978-94-007-5561-1_8|pmid=23595675 |pmc=3924584 }} electronic-book {{ISBN|978-94-007-5561-1}} When loss of iron is not adequately compensated by adequate dietary iron intake, a state of latent iron deficiency occurs, which over time leads to iron-deficiency anemia if left untreated, which is characterised by an insufficient number of red blood cells and an insufficient amount of hemoglobin.{{cite journal |author=CDC Centers for Disease Control and Prevention |title=Recommendations to Prevent and Control Iron Deficiency in the United States |journal=Morbidity and Mortality Weekly Report |date=3 April 1998 |volume=47 |issue=RR3 |page=1 |url=https://www.cdc.gov/mmwr/preview/mmwrhtml/00051880.htm |access-date=12 August 2014}} Children, pre-menopausal women (women of child-bearing age), and people with poor diet are most susceptible to the disease. Most cases of iron-deficiency anemia are mild, but if not treated can cause problems like fast or irregular heartbeat, complications during pregnancy, and delayed growth in infants and children.{{cite web|author=Centers for Disease Control and Prevention|title=Iron and Iron Deficiency |url=https://www.cdc.gov/nutrition/everyone/basics/vitamins/iron.html|access-date=12 August 2014}}

The brain is resistant to acute iron deficiency due to the slow transport of iron through the blood brain barrier.{{Cite journal |last1=Youdim |first1=M. B. |last2=Ben-Shachar |first2=D. |last3=Yehuda |first3=S. |date=September 1989 |title=Putative biological mechanisms of the effect of iron deficiency on brain biochemistry and behavior |journal=The American Journal of Clinical Nutrition |volume=50 |issue=3 Suppl |pages=607–615; discussion 615–617 |doi=10.1093/ajcn/50.3.607 |issn=0002-9165 |pmid=2773840|doi-access=free }} Acute fluctuations in iron status (marked by serum ferritin levels) do not reflect brain iron status, but prolonged nutritional iron deficiency is suspected to reduce brain iron concentrations over time.{{Cite journal |last1=Erikson |first1=K. M. |last2=Pinero |first2=D. J. |last3=Connor |first3=J. R. |last4=Beard |first4=J. L. |date=October 1997 |title=Regional brain iron, ferritin and transferrin concentrations during iron deficiency and iron repletion in developing rats |journal=The Journal of Nutrition |volume=127 |issue=10 |pages=2030–2038 |doi=10.1093/jn/127.10.2030 |issn=0022-3166 |pmid=9311961|doi-access=free }}{{Cite journal |last1=Unger |first1=Erica L. |last2=Bianco |first2=Laura E. |last3=Jones |first3=Byron C. |last4=Allen |first4=Richard P. |last5=Earley |first5=Christopher J. |date=November 2014 |title=Low brain iron effects and reversibility on striatal dopamine dynamics |journal=Experimental Neurology |language=en |volume=261 |pages=462–468 |doi=10.1016/j.expneurol.2014.06.023 |pmc=4318655 |pmid=24999026}} In the brain, iron plays a role in oxygen transport, myelin synthesis, mitochondrial respiration, and as a cofactor for neurotransmitter synthesis and metabolism.{{Cite journal |last1=Ward |first1=Roberta J. |last2=Zucca |first2=Fabio A. |last3=Duyn |first3=Jeff H. |last4=Crichton |first4=Robert R. |last5=Zecca |first5=Luigi |date=October 2014 |title=The role of iron in brain ageing and neurodegenerative disorders |journal=The Lancet. Neurology |volume=13 |issue=10 |pages=1045–1060 |doi=10.1016/S1474-4422(14)70117-6 |issn=1474-4465 |pmc=5672917 |pmid=25231526}} Animal models of nutritional iron deficiency report biomolecular changes resembling those seen in Parkinson's and Huntington's disease.{{Cite journal |last1=Pino |first1=Jessica M. V. |last2=da Luz |first2=Marcio H. M. |last3=Antunes |first3=Hanna K. M. |last4=Giampá |first4=Sara Q. de Campos |last5=Martins |first5=Vilma R. |last6=Lee |first6=Kil S. |date=2017-05-17 |title=Iron-Restricted Diet Affects Brain Ferritin Levels, Dopamine Metabolism and Cellular Prion Protein in a Region-Specific Manner |journal=Frontiers in Molecular Neuroscience |volume=10 |pages=145 |doi=10.3389/fnmol.2017.00145 |issn=1662-5099 |pmc=5434142 |pmid=28567002 |doi-access=free }}{{Cite journal |last1=Beard |first1=John |last2=Erikson |first2=Keith M. |last3=Jones |first3=Byron C. |date=2003-04-01 |title=Neonatal Iron Deficiency Results in Irreversible Changes in Dopamine Function in Rats |journal=The Journal of Nutrition |language=en |volume=133 |issue=4 |pages=1174–1179 |doi=10.1093/jn/133.4.1174 |pmid=12672939 |issn=0022-3166|doi-access=free }} However, age-related accumulation of iron in the brain has also been linked to the development of Parkinson's.{{cite journal |author1=Dominic J. Hare |author2=Kay L. Double |title=Iron and dopamine: a toxic couple |journal=Brain |volume=139 |issue=4 |date=April 2016 |pages=1026–1035 |doi=10.1093/brain/aww022|pmid=26962053 |doi-access=free }}

{{Main|Iron overload}}

Iron uptake is tightly regulated by the human body, which has no regulated physiological means of excreting iron. Only small amounts of iron are lost daily due to mucosal and skin epithelial cell sloughing, so control of iron levels is primarily accomplished by regulating uptake.{{cite book|author1=Ramzi S. Cotran|author2=Vinay Kumar|author3=Tucker Collins|author4=Stanley Leonard Robbins|title=Robbins pathologic basis of disease|url={{Google books|kdhrAAAAMAAJ|keywords=|text=|plainurl=yes}}|access-date= 27 June 2012|date=1999|publisher=Saunders|isbn=978-0-7216-7335-6}} Regulation of iron uptake is impaired in some people as a result of a genetic defect that maps to the HLA-H gene region on chromosome 6 and leads to abnormally low levels of hepcidin, a key regulator of the entry of iron into the circulatory system in mammals.{{cite journal|author=Ganz T|title=Hepcidin, a key regulator of iron metabolism and mediator of anemia of inflammation|journal=Blood|volume=102|issue=3|pages=783–8|date=August 2003|pmid=12663437|doi=10.1182/blood-2003-03-0672|s2cid=28909635|doi-access=free}} In these people, excessive iron intake can result in iron overload disorders, known medically as hemochromatosis. Many people have an undiagnosed genetic susceptibility to iron overload, and are not aware of a family history of the problem. For this reason, people should not take iron supplements unless they suffer from iron deficiency and have consulted a doctor. Hemochromatosis is estimated to be the cause of 0.3–0.8% of all metabolic diseases of Caucasians.{{Cite journal|title=Hereditary hemochromatosis|journal=Rev Méd Interne|date=2000 |volume=21 |issue=11 |pages=961–71 |doi=10.1016/S0248-8663(00)00252-6 |pmid=11109593|last1=Durupt|first1=S.|last2=Durieu|first2=I.|last3=Nové-Josserand|first3=R.|last4=Bencharif|first4=L.|last5=Rousset|first5=H.|last6=Vital Durand|first6=D.}}

Overdoses of ingested iron can cause excessive levels of free iron in the blood. High blood levels of free ferrous iron react with peroxides to produce highly reactive free radicals that can damage DNA, proteins, lipids, and other cellular components. Iron toxicity occurs when the cell contains free iron, which generally occurs when iron levels exceed the availability of transferrin to bind the iron. Damage to the cells of the gastrointestinal tract can also prevent them from regulating iron absorption, leading to further increases in blood levels. Iron typically damages cells in the heart, liver and elsewhere, causing adverse effects that include coma, metabolic acidosis, shock, liver failure, coagulopathy, long-term organ damage, and even death. Humans experience iron toxicity when the iron exceeds 20 milligrams for every kilogram of body mass; 60 milligrams per kilogram is considered a lethal dose.{{cite web|url=https://www.emedicine.com/emerg/topic285.htm|title=Toxicity, Iron

| publisher = Medscape|access-date=23 May 2010}} Overconsumption of iron, often the result of children eating large quantities of ferrous sulfate tablets intended for adult consumption, is one of the most common toxicological causes of death in children under six. The Dietary Reference Intake (DRI) sets the Tolerable Upper Intake Level (UL) for adults at 45 mg/day. For children under fourteen years old the UL is 40 mg/day.{{citation|title=Dietary Reference Intakes (DRIs): Recommended Intakes for Individuals |publisher=Food and Nutrition Board, Institute of Medicine, National Academies |year=2004 |url=https://www.iom.edu/Global/News%20Announcements/~/media/Files/Activity%20Files/Nutrition/DRIs/DRI_Summary_Listing.pdf |access-date=2009-06-09 |url-status=dead |archive-url=https://web.archive.org/web/20130314000722/https://www.iom.edu/Global/News%20Announcements/~/media/Files/Activity%20Files/Nutrition/DRIs/DRI_Summary_Listing.pdf |archive-date=14 March 2013 }}

The medical management of iron toxicity is complicated, and can include use of a specific chelating agent called deferoxamine to bind and expel excess iron from the body.{{Cite journal| last1 =Cheney|first1 =K.| last2 =Gumbiner|first2 =C.| last3 = Benson|first3 =B.| last4 = Tenenbein|first4 =M.|title=Survival after a severe iron poisoning treated with intermittent infusions of deferoxamine |journal=J Toxicol Clin Toxicol |volume=33 |issue=1 |pages=61–66 |date=1995 |pmid=7837315 |doi=10.3109/15563659509020217}}{{Cite journal| last = Tenenbein|first = M.|title=Benefits of parenteral deferoxamine for acute iron poisoning |journal=J Toxicol Clin Toxicol |volume=34 |issue=5 |pages=485–89 |date=1996 |pmid=8800185 |doi=10.3109/15563659609028005}}{{cite journal |vauthors=Wu H, Wu T, Xu X, Wang J, Wang J | title = Iron toxicity in mice with collagenase-induced intracerebral hemorrhage | journal = J Cereb Blood Flow Metab | volume = 31 | issue = 5 | pages = 1243–50 |date=May 2011 | pmid = 21102602 | doi =10.1038/jcbfm.2010.209 | pmc=3099628}}

Some research has suggested that low thalamic iron levels may play a role in the pathophysiology of ADHD.{{cite journal |last1=Robberecht |first1=Harry |display-authors=etal |title=Magnesium, Iron, Zinc, Copper and Selenium Status in Attention-Deficit/Hyperactivity Disorder (ADHD) |journal=Molecules |year=2020 |volume=25 |issue=19 |page=4440 |doi=10.3390/molecules25194440 |pmid=32992575 |pmc=7583976|doi-access=free }} Some researchers have found that iron supplementation can be effective especially in the inattentive subtype of the disorder.{{cite journal |last1=Soto-Insuga |first1=V |display-authors=etal |title=[Role of iron in the treatment of attention deficit-hyperactivity disorder] |journal=An Pediatr (Barc) |date=2013 |volume=79 |issue=4 |pages=230–235 |doi=10.1016/j.anpedi.2013.02.008 |pmid=23582950}}

Some researchers in the 2000s suggested a link between low levels of iron in the blood and ADHD. A 2012 study found no such correlation.{{Cite journal |last1=Donfrancesco |first1=Renato |last2=Parisi |first2=Pasquale |last3=Vanacore |first3=Nicola |last4=Martines |first4=Francesca |last5=Sargentini |first5=Vittorio |last6=Cortese |first6=Samuele |date=May 2013 |title=Iron and ADHD: Time to Move Beyond Serum Ferritin Levels |journal=Journal of Attention Disorders |language=en |volume=17 |issue=4 |pages=347–357 |doi=10.1177/1087054711430712 |pmid=22290693 |s2cid=22445593 |issn=1087-0547}}

The role of iron in cancer defense can be described as a "double-edged sword" because of its pervasive presence in non-pathological processes.{{cite book|last1=Thévenod|first1=Frank

|editor1-last=Sigel|editor1-first=Astrid|editor2-last=Sigel|editor2-first=Helmut|editor3-last=Freisinger|editor3-first=Eva|editor4-last=Sigel|editor4-first=Roland K. O.

|title=Metallo-Drugs: Development and Action of Anticancer Agents

|date=2018

|doi= 10.1515/9783110470734-021

|pmid=29394034

|publisher=de Gruyter GmbH

|location=Berlin

|chapter= Chapter 15. Iron and Its Role in Cancer Defense: A Double-Edged Sword

|series=Metal Ions in Life Sciences 18

|volume=18

|pages= 437–67}}

People having chemotherapy may develop iron deficiency and anemia, for which intravenous iron therapy is used to restore iron levels.{{cite journal|pmid=24275533|year=2014|last1=Beguin|first1=Y|title=Epidemiological and nonclinical studies investigating effects of iron in carcinogenesis--a critical review|journal=Critical Reviews in Oncology/Hematology|volume=89|issue=1|pages=1–15|last2=Aapro|first2=M|last3=Ludwig|first3=H|last4=Mizzen|first4=L|last5=Osterborg|first5=A|doi=10.1016/j.critrevonc.2013.10.008|doi-access=free}} Iron overload, which may occur from high consumption of red meat, may initiate tumor growth and increase susceptibility to cancer onset, particularly for colorectal cancer.

Iron plays an essential role in marine systems and can act as a limiting nutrient for planktonic activity.{{cite journal | last1 = Morel | first1 = F.M.M. | last2 = Hudson | first2 = R.J.M. | last3 = Price | first3 = N.M. | year = 1991 | title = Limitation of productivity by trace metals in the sea | url = | journal = Limnology and Oceanography | volume = 36 | issue = 8| pages = 1742–1755 | doi = 10.4319/lo.1991.36.8.1742 | bibcode = 1991LimOc..36.1742M }} Because of this, too much of a decrease in iron may lead to a decrease in growth rates in phytoplanktonic organisms such as diatoms.{{cite journal | last1 = Brezezinski | first1 = M.A. | last2 = Baines | first2 = S.B. | last3 = Balch | first3 = W.M. | last4 = Beucher | first4 = C.P. | last5 = Chai | first5 = F. | last6 = Dugdale | first6 = R.C. | last7 = Krause | first7 = J.W. | last8 = Landry | first8 = M.R. | last9 = Marchi | first9 = A. | last10 = Measures | first10 = C.I. | last11 = Nelson | first11 = D.M. | last12 = Parker | first12 = A.E. | last13 = Poulton | first13 = A.J. | last14 = Selph | first14 = K.E. | last15 = Strutton | first15 = P.G. | last16 = Taylor | first16 = A.G. | last17 = Twining | first17 = B.S. | year = 2011 | title = Co-limitation of diatoms by iron and silicic acid in the equatorial Pacific | url = | journal = Deep-Sea Research Part II: Topical Studies in Oceanography | volume = 58 | issue = 3–4| pages = 493–511 | doi = 10.1016/j.dsr2.2010.08.005 | bibcode = 2011DSRII..58..493B }} Iron can also be oxidized by marine microbes under conditions that are high in iron and low in oxygen.{{cite journal | last1 = Field | first1 = E. K. | last2 = Kato | first2 = S. | last3 = Findlay | first3 = A. J. | last4 = MacDonald | first4 = D. J. | last5 = Chiu | first5 = B. K. | last6 = Luther | first6 = G. W. | last7 = Chan | first7 = C. S. | year = 2016 | title = Planktonic marine iron oxidizers drive iron mineralization under low-oxygen conditions | url = | journal = Geobiology | volume = 14 | issue = 5| pages = 499–508 | doi = 10.1111/gbi.12189 | pmid = 27384464 | bibcode = 2016Gbio...14..499F }}

Iron can enter marine systems through adjoining rivers and directly from the atmosphere. Once iron enters the ocean, it can be distributed throughout the water column through ocean mixing and through recycling on the cellular level.{{cite journal | last1 = Wells | first1 = M.L. | last2 = Price | first2 = N.M. | last3 = Bruland | first3 = K.W. | year = 1995 | title = Iron chemistry in seawater and its relationship to phytoplankton: a workshop report | url = | journal = Marine Chemistry | volume = 48 | issue = 2| pages = 157–182 | doi = 10.1016/0304-4203(94)00055-I | bibcode = 1995MarCh..48..157W }} In the arctic, sea ice plays a major role in the store and distribution of iron in the ocean, depleting oceanic iron as it freezes in the winter and releasing it back into the water when thawing occurs in the summer.{{cite journal | last1 = Lannuzel | first1 = D. | last2 = Vancoppenolle | first2 = M. | last3 = van der Merwe | first3 = P. | last4 = de Jong | first4 = J. | last5 = Meiners | first5 = K.M. | last6 = Grotti | first6 = M. | last7 = Nishioska | first7 = J. | last8 = Schoemann | year = 2016 | title = Iron in sea ice: Review and new insights | url = | journal = Elementa: Science of the Anthropocene | volume = 4 | issue = | page = 000130 | doi = 10.12952/journal.elementa.000130 | bibcode = 2016EleSA...4.0130L }} The iron cycle can fluctuate the forms of iron from aqueous to particle forms altering the availability of iron to primary producers.{{cite journal | last1 = Raiswell | first1 = R | year = 2011 | title = Iron Transport from the Continents to the Open Ocean: The Aging–Rejuvenation Cycle | url = | journal = Elements | volume = 7 | issue = 2| pages = 101–106 | doi = 10.2113/gselements.7.2.101 | bibcode = 2011Eleme...7..101R }} Increased light and warmth increases the amount of iron that is in forms that are usable by primary producers.{{cite journal | last1 = Tagliabue | first1 = A. | last2 = Bopp | first2 = L. | last3 = Aumont | first3 = O. | last4 = Arrigo | first4 = K.R. | year = 2009 | title = Influence of light and temperature on the marine iron cycle: From theoretical to global modeling | url = https://hal.science/hal-00413634/file/2008GB003214.pdf| journal = Global Biogeochemical Cycles | volume = 23| issue = 2| page = | doi = 10.1029/2008GB003214 | bibcode = 2009GBioC..23.2017T }}

{{Portal|Chemistry}}

• Economically important iron deposits include:
• Carajás Mine in the state of Pará, Brazil, is thought to be the largest iron deposit in the world.
• El Mutún in Bolivia, where 10% of the world's accessible iron ore is located.
• Hamersley Basin is the largest iron ore deposit in Australia.
• Kiirunavaara in Sweden, where one of the world's largest deposits of iron ore is located
• The Mesabi Iron Range is the chief iron ore mining district in the United States.
• Iron and steel industry
• Iron cycle
• Iron nanoparticle
• Iron–platinum nanoparticle
• Iron fertilization – proposed fertilization of oceans to stimulate phytoplankton growth
• Iron-oxidizing bacteria
• List of countries by iron production
• Pelletising – process of creation of iron ore pellets
• Rustproof iron
• Steel

{{reflist}}

{{div col | colwidth = 30em | small = yes}}

• {{Greenwood&Earnshaw2nd}}
• {{Cite book

| author-last = Weeks

| author-first = Mary Elvira

| author-link = Mary Elvira Weeks

| author-last2 = Leichester

| author-first2 = Henry M.

| date = 1968

| title = Discovery of the elements

| url = https://archive.org/details/discoveryofeleme07edunse

| url-access = registration

| publisher = Journal of Chemical Education

| location = Easton, PA

| chapter = Elements known to the ancients

| pages = [https://archive.org/details/discoveryofeleme07edunse/page/29 29]–40

| lccn = 68-15217

| ref = CITEREFWeeks1968

| isbn = 0-7661-3872-0

}}

{{div col end}}

{{div col | colwidth = 30em | small = yes}}

• H.R. Schubert, History of the British Iron and Steel Industry ... to 1775 AD (Routledge, London, 1957)
• R.F. Tylecote, History of Metallurgy (Institute of Materials, London 1992).
• R.F. Tylecote, "Iron in the Industrial Revolution" in J. Day and R.F. Tylecote, The Industrial Revolution in Metals (Institute of Materials 1991), 200–60.

{{div col end}}

{{Wikiquote}}

{{Wiktionary|iron}}

{{Commons|Iron}}

{{NIE Poster|Iron}}

• [https://education.jlab.org/itselemental/ele026.html It's Elemental – Iron]
• [https://www.periodicvideos.com/videos/026.htm Iron] at The Periodic Table of Videos (University of Nottingham)
• [https://books.google.com/books?id=brpx-LtdCLYC&pg=frontcover Metallurgy for the non-Metallurgist]
• [https://mysite.du.edu/~jcalvert/phys/iron.htm Iron] by J. B. Calvert

{{Periodic table (navbox)}}

{{Iron compounds}}

{{Authority control}}

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