View source for Iron

{{CMG}}
==Overview==
'''Iron''' is a [[chemical element]] with symbol '''Fe''' (from {{lang-la|ferrum}}) and [[atomic number]] 26. It is a [[metal]] in the [[first transition series]].<ref>http://www.iupac.org/fileadmin/user_upload/news/IUPAC_Periodic_Table-1May13.pdf</ref> It is by mass the most common element on [[Earth]], forming much of Earth's [[outer core|outer]] and [[inner core]]. It is the fourth most common [[Abundance of elements in Earth's crust|element in the Earth's crust]]. Its abundance in [[rocky planets]] like Earth is due to its abundant production by [[Nuclear fusion|fusion]] in high-mass [[stars]], where the production of [[nickel-56]] (which decays to the most common isotope of iron) is the last [[nuclear fusion reaction]] that is [[exothermic]]. Consequently, [[Radioactivity|radioactive]] [[nickel]] is the last element to be produced before the violent collapse of a [[type II supernova|supernova]] scatters [[Precursor (chemistry)|precursor]] [[radionuclide]] of iron into space.

Like other [[group 8 element]]s, iron exists in a wide range of [[oxidation state]]s, −2 to +6, although +2 and +3 are the most common. Elemental iron occurs in [[meteoroid]]s and other low [[oxygen]] environments, but is reactive to oxygen and [[water]]. Fresh iron surfaces appear lustrous silvery-gray, but [[oxidize]] in normal air to give [[hydrate]]d [[iron oxide]]s, commonly known as [[rust]]. Unlike many other metals which form [[Passivation (chemistry)|passivating]] oxide layers, iron oxides occupy more volume than the metal and thus flake off, exposing fresh surfaces for corrosion.

Iron metal has been used since [[iron age|ancient times]], although [[copper]] [[alloys]], which have lower melting temperatures, were used even earlier in human history. Pure iron is relatively soft, but is unobtainable by [[smelting]]. The material is significantly hardened and strengthened by impurities, in particular [[carbon]], from the smelting process. A certain proportion of carbon (between 0.002% and 2.1%) produces [[steel]], which may be up to 1000&nbsp;times harder than pure iron. Crude iron metal is produced in [[blast furnace]]s, where ore is reduced by [[coke (fuel)|coke]] to [[pig iron]], which has a high carbon content. Further refinement with oxygen reduces the carbon content to the correct proportion to make steel. Steels and low carbon iron [[alloy]]s along with other metals ([[alloy steel]]s) are by far the most common metals in industrial use, due to their great range of desirable properties and the widespread abundance of iron-bearing rock.

Iron chemical compounds have many uses. Iron oxide mixed with aluminium powder can be ignited to create a [[Thermite|thermite reaction]], used in welding and purifying ores. Iron forms binary compounds with the [[halogens]] and the [[chalcogens]]. Among its organometallic compounds is [[ferrocene]], the first [[sandwich compound]] discovered.

[[Iron#Biological role|Iron plays an important role in biology]], forming complexes with molecular oxygen in [[hemoglobin]] and [[myoglobin]]; these two compounds are common [[oxygen transport]] proteins in vertebrates. Iron is also the metal at the active site of many important [[redox]] [[enzymes]] dealing with [[cellular respiration]] and [[oxidation and reduction]] in plants and animals.

==Characteristics==

===Mechanical properties===
{|class="wikitable" style="float:left; clear:left; margin-right:1em; margin-top:0;"
|+ Characteristic values of [[tensile strength]] (TS) and [[Brinell hardness]] (BH) of different forms of iron.<ref name=pure>{{Cite book| url=http://books.google.com/?id=-Ll6qjWB-RUC&pg=PA164| pages=164–167| title=Handbook of materials and techniques for vacuum devices|last=Kohl|first= Walter H.| publisher=Springer| date=1995| isbn=1-56396-387-6}}</ref><ref name=corr>{{Cite book| url=http://www.gorni.eng.br/e/Gorni_SFHTHandbook.pdf| title=ASM Handbook – Mechanical Testing and Evaluation|publisher= ASM International| volume= 8| date= 2000| page= 275| isbn=0-87170-389-0|editor=Kuhn, Howard and Medlin, Dana (prepared under the direction of the ASM International Handbook Committee)}}</ref>
!Material
!TS <br />(MPa)
!BH <br />([[Brinell scale|Brinell]])
|-
|[[Monocrystalline whisker|Iron whiskers]]
|11000
|
|-
|Ausformed (hardened) <br>steel
|2930
|850–1200
|-
|[[Martensite|Martensitic steel]]
|2070
|600
|-
|[[Bainite|Bainitic steel]]
|1380
|400
|-
|[[Pearlite|Pearlitic steel]]
|1200
|350
|-
|Cold-worked iron
|690
|200
|-
|Small-grain iron
|340
|100
|-
|Carbon-containing iron
|140
|40
|-
|Pure, single-crystal iron
|10
|3
|}

The mechanical properties of iron and its alloys can be evaluated using a variety of tests, including the [[Brinell scale|Brinell test]], [[Rockwell scale|Rockwell test]] and the [[Vickers hardness test]]. The data on iron is so consistent that it is often used to calibrate measurements or to compare tests.<ref name=corr/><ref>{{cite web| url=http://mdmetric.com/tech/hardnessconversion.html| title=Hardness Conversion Chart|accessdate=23 May 2010|publisher=Maryland Metrics}}</ref> However, the mechanical properties of iron are significantly affected by the sample's purity: pure research-purpose single crystals of iron are actually softer than aluminium,<ref name=pure/> and the purest industrially produced iron (99.99%) has a hardness of 20–30&nbsp;Brinell.<ref>{{Cite journal| title=Properties of Various Pure Irons: Study on pure iron I| url=http://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}}</ref> An increase in the carbon content of the iron will initially cause a significant corresponding increase in the iron's hardness and tensile strength. Maximum hardness of [[Rockwell scale|65 R<sub>c</sub>]] is achieved with a 0.6% carbon content, although this produces a metal with a low tensile strength.<ref>{{Cite book| url=http://books.google.com/?id=LgB5dkmPML0C&pg=PA218| page=218| title=Materials Science and Engineering|first=V.| last= Raghavan| publisher =PHI Learning Pvt. Ltd.|isbn=81-203-2455-2 |date=2004}}</ref>
[[File:Iron-alpha-pV.svg|thumb|right|240px|Molar volume vs. pressure for α iron at room temperature]]
Because of its significance for planetary cores, the physical properties of iron at high pressures and temperatures have also been studied extensively. The form of iron that is stable under standard conditions can be subjected to pressures up to ca. 15&nbsp;GPa before transforming into a high-pressure form, as described in the next section.

===Phase diagram and allotropes===
{{Main|Allotropes of iron}}
Iron represents an example of [[allotropy]] in a metal. There are at least four allotropic forms of iron, known as α, γ, δ, and ε; at very high pressures, some controversial experimental evidence exists for a phase β stable at very high pressures and temperatures.<ref name="beta-iron">{{Cite journal| first = Reinhard| last = Boehler|title = High-pressure experiments and the phase diagram of lower mantle and core materials| journal = Review of Geophysics| volume = 38| pages = 221–245| publisher = American Geophysical Union| date = 2000| doi=10.1029/1998RG000053| issue = 2| bibcode=2000RvGeo..38..221B}}</ref>
[[File:Pure iron phase diagram (EN).png|thumb|left|240px|Low-pressure [[phase diagram]] of pure iron]]
As molten iron cools it crystallizes at 1538&nbsp;°C into its δ allotrope, which has a [[body-centered cubic]] (bcc) [[crystal structure]]. As it cools further to 1394&nbsp;°C, it changes to its γ-iron allotrope, a [[face-centered cubic]] (fcc) crystal structure, or [[austenite]]. At 912&nbsp;°C and below, the crystal structure again becomes the bcc α-iron allotrope, or [[Ferrite (iron)|ferrite]]. Finally, at 770&nbsp;°C (the [[Curie point]], T<sub>c</sub>) iron becomes [[magnetic]]. As the iron passes through the Curie temperature there is no change in crystalline structure, but there is a change in "domain structure", where each domain contains iron atoms with a particular electronic spin. In unmagnetized iron, all the electronic spins of the atoms within one domain are in the same direction, however, the neighboring domains point in various other directions and thus over all they cancel each other out. As a result, the iron is unmagnetized. In magnetized iron, the electronic spins of all the domains are aligned, so that the magnetic effects of neighboring domains reinforce each other. Although each domain contains billions of atoms, they are very small, about 10&nbsp;micrometres across.<ref name="Metallo">{{Cite book| url = http://books.google.com/?id=hoM8VJHTt24C&pg=PA24|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}}</ref><!--http://books.google.com/books?id=brpx-LtdCLYC--> At pressures above approximately 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into a [[hexagonal close-packed]] (hcp) structure, which is also known as [[Hexaferrum|ε-iron]]; the higher-temperature γ-phase also changes into ε-iron, but does so at higher pressure. The [[Beta ferrite|β-phase]], if it exists, would appear at pressures of at least 50 GPa and temperatures of at least 1500 K; it has been thought to have an orthorhombic or a double hcp structure.<ref name="beta-iron" />

Iron is of greatest importance when mixed with certain other metals and with carbon to form steels. There are many types of steels, all with different properties, and an understanding of the properties of the [[allotropes of iron]] is key to the manufacture of good quality steels.

α-iron, also known as ferrite, is the most stable form of iron at normal temperatures. It is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910&nbsp;°C).<ref>{{Cite book|url=http://books.google.com/?id=xv420pEC2qMC&pg=PA183| page=183| title=Concise encyclopedia of the structure of materials| first=John Wilson|last = Martin| publisher=Elsevier| date= 2007|isbn=0-08-045127-6}}</ref>

Above 912&nbsp;°C and up to 1400&nbsp;°C α-iron undergoes a [[phase transition]] from bcc to the fcc configuration of γ-iron, also called [[austenite]]. This is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146&nbsp;°C). This form of iron is used in the type of [[stainless steel]] used for making cutlery, and hospital and food-service equipment.<ref name="Metallo" />

The high-pressure phases of iron are important as endmember models for the solid parts of planetary cores. The [[inner core]] of the [[Earth]] is generally assumed to consist essentially of an iron-[[nickel]] [[alloy]] with ε (or β) structure.

The melting point of iron is experimentally well defined for pressures up to approximately 50 GPa. For higher pressures, different studies placed the γ-ε-liquid [[triple point]] at pressures differing by tens of gigapascals and yielded differences of more than 1000 K for the melting point. Generally speaking, [[molecular dynamics]] computer simulations of iron melting and shock wave experiments suggest higher melting points and a much steeper slope of the melting curve than static experiments carried out in [[diamond anvil cell]]s.<ref name="melting">{{Cite book| pages=527–541|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}}</ref>

===Isotopes===
{{Main|Isotopes of iron}}

Naturally occurring iron consists of four stable [[isotope]]s: 5.845% of <sup>54</sup>Fe, 91.754% of [[Iron-56|<sup>56</sup>Fe]], 2.119% of <sup>57</sup>Fe and 0.282% of <sup>58</sup>Fe. Of these stable isotopes, only <sup>57</sup>Fe has a nuclear [[spin (physics)|spin]] (−{{frac|1|2}}). The [[nuclide]] <sup>54</sup>Fe is predicted to undergo [[double beta decay]], but this process had never been observed experimentally for these nuclei, and only the lower limit on the half-life was established: t<sub>1/2</sub>>3.1×10<sup>22</sup> years.

<sup>60</sup>Fe is an [[extinct radionuclide]] of long [[half-life]] (2.6&nbsp;million years).<ref name="RugelFaestermann2009">{{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 <sup>60</sup>Fe Half-Life|journal=Physical Review Letters|volume=103|issue=7|date=2009|issn=0031-9007|doi=10.1103/PhysRevLett.103.072502}}</ref> It is not found on Earth, but its ultimate decay product is the stable nuclide [[nickel-60]].

Much of the past work on measuring the isotopic composition of Fe has focused on determining <sup>60</sup>Fe variations due to processes accompanying [[nucleosynthesis]] (i.e., [[meteorite]] studies) and ore formation. In the last decade however, advances in [[mass spectrometry]] technology have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the [[stable isotope]]s of iron. Much of this work has been driven by the [[Earth science|Earth]] and [[planetary science]] communities, although applications to biological and industrial systems are beginning to emerge.<ref>{{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| pages=515–550|url=http://geosci.uchicago.edu/~dauphas/OLwebsite/PDFfiles/Dauphas_Rouxel_MSR06.pdf |doi=10.1002/mas.20078 |pmid=16463281 |issue=4}}</ref>

The most abundant iron isotope <sup>56</sup>Fe is of particular interest to nuclear scientists as it represents the most common endpoint of [[iron#nucleosynthesis|nucleosynthesis]]. It is often cited, falsely, as the isotope of highest binding energy, a distinction which actually belongs to [[nickel-62]].<ref>{{cite journal|last1=Fewell|first1=M. P.|title=The atomic nuclide with the highest mean binding energy|journal=American Journal of Physics|volume=63|page=653|date=1995|doi=10.1119/1.17828|bibcode=1995AmJPh..63..653F|issue=7}}</ref> Since <sup>56</sup>Ni is easily produced from lighter nuclei in the [[alpha process]] in [[nuclear reaction]]s in supernovae (see [[silicon burning process]]), nickel-56 (14 [[alpha particle]]s) is the endpoint of fusion chains inside [[Population III stars|extremely massive stars]], since addition of another alpha particle would result in zinc-60, which requires a great deal more energy. This nickel-56, which has a half-life of about 6&nbsp;days, is therefore made 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 cobalt-56, and then stable iron-56. This last nuclide is therefore common in the universe, relative to other stable [[Metallicity|metals]] of approximately the same [[Atomic mass|atomic weight]].

In phases of the meteorites ''Semarkona'' and ''Chervony Kut'' a correlation between the concentration of <sup>60</sup>Ni, the [[daughter product]] of <sup>60</sup>Fe, and the abundance of the stable iron isotopes could be found which is evidence for the existence of <sup>60</sup>Fe at the time of [[Formation and evolution of the Solar System|formation of the Solar System]]. Possibly the energy released by the decay of <sup>60</sup>Fe contributed, together with the energy released by decay of the radionuclide <sup>26</sup>Al, to the remelting and [[planetary differentiation|differentiation]] of [[asteroid]]s after their formation 4.6&nbsp;billion years ago. The abundance of <sup>60</sup>Ni present in [[wikt:extraterrestrial|extraterrestrial]] material may also provide further insight into the origin of the [[Solar System]] and its early history.<ref>{{cite journal|doi=10.1016/j.newar.2003.11.022|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|pages=155|bibcode = 2004NewAR..48..155M }}</ref>

Nuclei of iron atoms have some of the highest binding energies per nucleon, surpassed only by the [[Isotopes of nickel|nickel isotope]] <sup>62</sup>Ni. This is formed by [[nuclear fusion]] in stars. Although a further tiny energy gain could be extracted by synthesizing <sup>62</sup>Ni, conditions in stars are unsuitable for this process to be favored. Elemental distribution on Earth greatly favors iron over nickel, and also presumably in supernova element production.<ref>{{cite journal|title = Iron and Nickel Abundances in H~II Regions and Supernova Remnants|date = 1995|bibcode=1995AAS...186.3707B|author=Bautista, Manuel A.|author2=Pradhan, Anil K.|journal=Bulletin of the American Astronomical Society|volume=27|page=865}}</ref>

[[Iron-56]] is the heaviest stable isotope produced by the alpha process in [[stellar nucleosynthesis]]; elements heavier than iron and nickel require a [[supernova]] for their formation. Iron is the most abundant element in the core of [[red giant]]s, and is the most abundant metal in [[iron meteorite]]s and in the dense metal [[Planetary core|cores of planets]] such as [[Earth]].

===Nucleosynthesis===
Iron is created by extremely large stars with extremely hot (over 2.5 billion kelvin) cores through the [[silicon burning process]]. It is the heaviest stable element to be produced in this manner. The process starts with the second largest stable nucleus created by silicon burning, which is calcium. One stable nucleus of calcium fuses with one helium nucleus, creating unstable titanium. Before the titanium decays, it can fuse with another helium nucleus, creating unstable chromium. Before the chromium decays, it can fuse with another helium nucleus, creating unstable iron. Before the iron decays, it can fuse with another helium nucleus, creating unstable nickel-56. Any further fusion of nickel-56 consumes energy instead of producing energy, so after the production of nickel-56, the star does not produce the energy necessary to keep the core from collapsing. Eventually, the nickel-56 decays to unstable cobalt-56, which in turn decays to stable [[iron-56]].
When the core of the star collapses, it creates a [[supernova]]. Supernovas also create additional forms of stable iron via the [[r-process]].

===Occurrence===
====Planetary occurrence====
[[File:Widmanstatten hand.jpg|thumb| right| [[Iron meteorite]]s, similar in composition to the Earth's inner- and outer core]]
Iron is the sixth most [[Abundance of the chemical elements|abundant element]] in the [[Universe]], and the most common [[refractory]] element.<ref name=apjl717_2_L92>{{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 }}</ref> It is formed as the final [[exothermic]] stage of [[stellar nucleosynthesis]], by [[Silicon burning process|silicon fusion]] in massive stars.

Metallic or [[native iron]] is rarely found on the surface of the Earth because it tends to oxidize, but its oxides are pervasive and represent the primary ores. While it makes up about 5% of the [[Crust (geology)|Earth's crust]], both the Earth's [[inner core|inner]] and [[outer core]] are believed to consist largely of an iron-[[nickel]] alloy constituting 35% of the mass of the Earth as a whole. Iron is consequently the most abundant element on Earth, but only the fourth most abundant element in the Earth's crust.<ref>{{cite web|title = Iron: geological information|url = http://www.webelements.com/iron/geology.html|accessdate = 23 May 2010|publisher = WebElements}}</ref><ref>{{Cite journal| doi = 10.1073/pnas.77.12.6973|url = http://www.pnas.org/content/77/12/6973| title = Chemical composition of Earth, Venus, and Mercury|author =John W. Morgan |author2=Edward Anders |last-author-amp=yes |journal = [[Proc. Nat. Acad. Sci.]]|volume = 77|issue = 12|pages = 6973–6977|date = 1980|pmid=16592930|pmc=350422|bibcode=1980PNAS...77.6973M}}</ref> Most of the iron in the crust is found combined with oxygen as [[iron oxide]] minerals such as [[hematite]] (Fe<sub>2</sub>O<sub>3</sub>) and [[magnetite]] (Fe<sub>3</sub>O<sub>4</sub>). Large deposits of iron are found in [[banded iron formations]]. These geological formations are 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}}<ref>{{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 = CT|bibcode=2009Natur.461..179L}}</ref><ref>{{Cite journal| first1 = P.|title = Paleoecological Significance of the Banded Iron-Formation|journal = Economic Geology|volume = 68|last1 = Cloud|pages = 1135–1143|date = 1973|doi = 10.2113/gsecongeo.68.7.1135| issue = 7}}</ref>

About 1 in 20 [[meteorite]]s consist of the unique iron-nickel minerals [[taenite]] (35–80% iron) and [[kamacite]] (90–95% iron). Although rare, [[iron meteorite]]s are the main form of natural metallic iron on the Earth's surface.<ref>{{Cite journal| url = http://books.google.com/?id=QDU7AAAAIAAJ&pg=PA152|page =152|title = Planet earth: cosmology, geology, and the evolution of life and environment|first = Cesare|last = Emiliani|publisher = Cambridge University Press|date = 1992|isbn = 978-0-521-40949-0|chapter = Meteorites}}</ref>

The red color of the surface of [[Mars]] is derived from an iron oxide-rich [[regolith]]. This has been proven by [[Mössbauer spectroscopy]].<ref>{{Cite journal| doi = 10.1007/s10751-007-9508-5|title = Two earth years of Mössbauer studies of the surface of Mars with MIMOS II|date = 2007|last1 = Klingelhöfer|first1 = G.|last2 = Morris|first2 = R. V.|last3 = Souza|first3 = P. A.|last4 = Rodionov|first4 = D.|last5 = Schröder|first5 = C.|journal = Hyperfine Interactions|volume = 170|pages = 169–177|bibcode = 2006HyInt.170..169K }}</ref>

====Stocks in use in society====
According to the [[International Resource Panel]]'s [[Metal Stocks in Society report]], the global stock of iron in use in society is 2200&nbsp;kg per capita. Much of this is in more-developed countries (7000–14000&nbsp;kg per capita) rather than less-developed countries (2000&nbsp;kg per capita).

==Chemistry and compounds==
{{category see also|Iron compounds}}

{| class="wikitable" style="float:right; clear:right; margin-left:1em; margin-top:0;"
|-
! Oxidation <br />state !! Representative compound
|-
| −2 || [[Disodium tetracarbonylferrate]] (Collman's reagent)
|-
| −1 ||
|-
| 0 || [[Iron pentacarbonyl]]
|-
| 1 || [[Cyclopentadienyliron dicarbonyl dimer]] ("Fp<sub>2</sub>")
|-
| 2 || [[Ferrous sulfate]], [[ferrocene]]
|-
| 3 || [[Ferric chloride]], [[ferrocenium tetrafluoroborate]]
|-
| 4 || [[Barium ferrate(IV)]]
|-
| 5 ||
|-
| 6 || [[Potassium ferrate]]
|}

Iron forms compounds mainly in the +2 and +3 [[oxidation state]]s. Traditionally, iron(II) compounds are called [[ferrous]], and iron(III) compounds [[ferric]]. Iron also occurs in higher oxidation states, an example being the purple [[potassium ferrate]] (K<sub>2</sub>FeO<sub>4</sub>) which contains iron in its +6 oxidation state. Iron(IV) is a common intermediate in many biochemical oxidation reactions.<ref>{{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}}</ref><ref name="HollemanAF">{{Cite book|publisher = Walter de Gruyter|date = 1985|edition = 91–100|pages = 1125–1146|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 = German}}</ref> Numerous [[organometallic chemistry|organometallic]] 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]].<ref>{{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 = 245–283}}</ref>
There are also many [[mixed valence compound]]s that contain both iron(II) and iron(III) centers, such as [[magnetite]] and [[Prussian blue]] (Fe<sub>4</sub>(Fe[CN]<sub>6</sub>)<sub>3</sub>).<ref name="HollemanAF" /> The latter is used as the traditional "blue" in [[blueprint]]s.<ref>{{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| url = http://books.google.com/?id=C-7I69gFIbMC&pg=PA11}}</ref>
[[File:Iron(III) chloride hexahydrate.jpg|thumb|alt=Some canary-yellow powder sits, mostly in lumps, on a laboratory watch glass.|Hydrated [[iron(III) chloride]], also known as ferric chloride]]

The iron compounds produced on the largest scale in industry are [[iron(II) sulfate]] (FeSO<sub>4</sub>·7[[Water of crystallization|H<sub>2</sub>O]]) and [[iron(III) chloride]] (FeCl<sub>3</sub>). The former is one of the most readily available sources of iron(II), but is less stable to aerial oxidation than [[Mohr's salt]] ((NH<sub>4</sub>)<sub>2</sub>Fe(SO<sub>4</sub>)<sub>2</sub>·6H<sub>2</sub>O). Iron(II) compounds tend to be oxidized to iron(III) compounds in the air.<ref name="HollemanAF" />

Unlike many other metals, iron does not form amalgams with mercury. As a result, mercury is traded in standardized 76 pound flasks (34&nbsp;kg) made of iron.<ref>{{Cite book|title = Hand-book of chemistry|volume = 6| first = Leopold|last1 = Gmelin|authorlink = Leopold Gmelin|pages = 128–129| chapter = Mercury and Iron|url = http://books.google.com/?id=nosMAAAAYAAJ&pg=PA128|publisher = Cavendish Society|date = 1852}}</ref>

===Binary compounds===
Iron reacts with oxygen in the air to form various [[iron oxide|oxide and hydroxide compounds]]; the most common are [[iron(II,III) oxide]] (Fe<sub>3</sub>O<sub>4</sub>), and [[iron(III) oxide]] (Fe<sub>2</sub>O<sub>3</sub>). [[Iron(II) oxide]] also exists, though it is unstable at room temperature. These oxides are the principal ores for the production of iron (see [[bloomery]] and blast furnace). They are also used in the production of [[Ferrite (magnet)|ferrites]], useful [[magnetic storage]] media in computers, and pigments. The best known sulfide is [[iron pyrite]] (FeS<sub>2</sub>), also known as fool's gold owing to its golden luster.<ref name="HollemanAF" />

The binary ferrous and ferric halides are well known, with the exception of ferric iodide. The ferrous halides typically arise from treating iron metal with the corresponding binary halogen acid to give the corresponding hydrated salts.<ref name="HollemanAF" />
:Fe + 2 HX → FeX<sub>2</sub> + H<sub>2</sub>
Iron reacts with fluorine, chlorine, and bromine to give the corresponding ferric halides, [[ferric chloride]] being the most common:
:2 Fe + 3 X<sub>2</sub> → 2 FeX<sub>3</sub> (X = F, Cl, Br)

===Coordination and organometallic compounds===
{{See also|Organoiron chemistry}}
[[File:Prussian blue.jpg|thumb|right|[[Prussian blue]]]]
Several cyanide complexes are known. The most famous example is [[Prussian blue]], (Fe<sub>4</sub>(Fe[CN]<sub>6</sub>)<sub>3</sub>). [[Potassium ferricyanide]] and [[potassium ferrocyanide]] are also known; the formation of Prussian blue upon reaction with iron(II) and iron(III) respectively forms the basis of a "wet" chemical test.<ref name="HollemanAF" /> Prussian blue is also used as an antidote for [[thallium]] and radioactive [[caesium]] poisoning.<ref>{{cite web| url =http://www.fda.gov/Drugs/EmergencyPreparedness/BioterrorismandDrugPreparedness/ucm130337.htm| title = Questions and Answers on Prussian Blue| accessdate = 6 June 2009}}</ref><ref>{{Cite journal| doi =10.1345/aph.1E024|pages = 1509–1514|pmid =15252192|title =Soluble or Insoluble Prussian Blue for Radiocesium and Thallium Poisoning?|first2 =ED|date =2004|last2 =Callen|last1 =Thompson|issue =9|first1 =D. F|journal =Annals of Pharmacotherapy|volume =38}}</ref> Prussian blue can be used in laundry bluing to correct the yellowish tint left by ferrous salts in water.

[[File:Ferrocene-2D.png|thumb|80px|left|[[Ferrocene]]]]
Several carbonyl compounds of iron are known. The premier iron(0) compound is [[iron pentacarbonyl]], Fe(CO)<sub>5</sub>, which is used to produce [[carbonyl iron]] powder, a highly reactive form of metallic iron. Thermolysis of iron pentacarbonyl gives the trinuclear cluster, [[triiron dodecacarbonyl]]. 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.<ref>{{Greenwood&Earnshaw1st|pages=1282–86}}.</ref>

[[Ferrocene]] is an extremely stable complex. The first [[sandwich compound]], it contains an iron(II) center with two [[cyclopentadienyl]] ligands bonded through all ten carbon atoms. This arrangement was a shocking novelty when it was first discovered,<ref>{{cite journal|title=Ferrocene: Ironclad History of Rashomon Tale?|pages =123–124|pmid=10649350|url=http://www.roaldhoffmann.com/sites/all/files/ferrocene.pdf|date=2000|last1=Laszlo|first1=P|last2=Hoffmann|first2=R|volume=39|issue=1|doi=10.1002/(SICI)1521-3773(20000103)39:1<123::AID-ANIE123>3.0.CO;2-Z|journal=Angewandte Chemie (International ed. in English)}}</ref> but the discovery of ferrocene has led to a new branch of organometallic chemistry. Ferrocene itself can be used as the backbone of a ligand, e.g. [[dppf]]. Ferrocene can itself be oxidized to the [[ferrocenium]] cation (Fc<sup>+</sup>); the ferrocene/ferrocenium couple is often used as a reference in electrochemistry.<ref>{{Cite journal|doi = 10.1002/chin.200443242|title = Ferrocene: 50 Years of Transition Metal Organometallic Chemistry—From Organic and Inorganic to Supramolecular Chemistry|date = 2004|last1 = Federman Neto|first1 = Alberto|last2 = Pelegrino|first2 = Alessandra Caramori|last3 = Darin|first3 = Vitor Andre|journal = ChemInform|volume = 35|issue = 43}}</ref>

==History==
{{Main|History of ferrous metallurgy}}

===Wrought iron===
{{further|Ancient iron production}}
[[File:Mars symbol.svg|left|thumb|80px|alt=A circle, with a short, simple arrow shape extending diagonally upwards and rightwards from its edge|The symbol for Mars has been used since antiquity to represent iron.]] [[File:QtubIronPillar.JPG|thumb|alt=An pillar, slightly fluted, with some ornamentation at its top. It is black, slightly weathered to a dark brown near the base. It is around {{convert|7|m|ft|abbr=off|sp=us}} tall. It stands upon a raised circular base of stone, and is surrounded by a short, square fence.|The [[Iron pillar of Delhi|Delhi iron pillar]] is an example of the iron extraction and processing methodologies of early India. The iron pillar at Delhi has withstood corrosion for the last 1600&nbsp;years.]]
Iron objects of great age are much rarer than objects made of gold or silver due to the ease of corrosion of iron.{{sfn|Weeks|1968|p=29}} Beads made from [[meteoric iron]] in 3500&nbsp;BCE or earlier were found in Gerzah, Egypt by G. A. Wainwright.{{sfn|Weeks|1968|p=31}}<!-- Typo on page 31 refers to "G. W." but reference at end of chapter says "G. A." --> The beads contain 7.5% nickel, which is a signature of meteoric origin since iron found in the Earth's crust has very little to no nickel content. Meteoric iron was highly regarded due to its origin in the heavens and was often used to forge weapons and tools or whole specimens placed in churches.{{sfn|Weeks|1968|p=31}} Items that were likely made of iron by Egyptians date from 2500 to 3000&nbsp;BCE.{{sfn|Weeks|1968|p=29}} Iron had a distinct advantage over bronze in warfare implements. It was much harder and more durable than bronze, although susceptible to rust. However, this is contested. [[Hittites|Hittitologist]] [[Trevor Bryce]] argues that before advanced iron-working techniques were developed in [[India]], meteoritic iron weapons used by early [[Mesopotamia]]n armies had a tendency to shatter in combat, due to their high carbon content.<ref>{{cite book|author=Bryce, Trevor|title=Hittite Warrior|url=http://books.google.com/books?id=0_oi1CLayh8C&pg=PA22|date=2007|publisher=Osprey Publishing|isbn=978-1-84603-081-9|pages=22–23}}</ref>

The first iron production started in the [[Middle Bronze Age]] but it took several centuries before iron displaced bronze. Samples of [[smelting|smelted]] iron from [[Asmar]], Mesopotamia and Tall Chagar Bazaar in northern Syria were made sometime between 2700 and 3000&nbsp;BCE.{{sfn|Weeks|1968|p=32}} The [[Hittites]] appear to be the first to understand the production of iron from its ores and regard it highly in their society. They began to smelt iron between 1500 and 1200&nbsp;BCE and the practice spread to the rest of the Near East after their empire fell in 1180&nbsp;BCE.{{sfn|Weeks|1968|p=32}} The subsequent period is called the [[Iron Age]]. Iron smelting, and thus the Iron Age, reached Europe two hundred years later and arrived in [[Zimbabwe]], Africa by the 8th century.{{sfn|Weeks|1968|p=32}} In China, iron only appears circa 700–500&nbsp;BCE.<ref>Sawyer, Ralph D. and Mei-chün Sawyer. ''The Seven Military Classics of Ancient China.'' Boulder: Westview, (1993), p. 10.</ref> Iron smelting may have been introduced into China through Central Asia.<ref name="pigott2">Pigott, Vincent C. (1999). p. 8.</ref> The earliest evidence of the use of a [[blast furnace]] in China dates to the 1st century AD,<ref name="Golas1999">{{cite book|author=Peter J. Golas|title=Science and Civilisation in China: Volume 5, Chemistry and Chemical Technology, Part 13, Mining|url=http://books.google.com/books?id=TSiII7s2wLkC&pg=PA152|date=25 February 1999|publisher=Cambridge University Press|isbn=978-0-521-58000-7|page=152|quote=earlist blast furnace discovered in China from about the first century AD}}</ref> and cupola furnaces were used as early as the [[Warring States period]] (403–221 BCE).<ref name="pigott">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.</ref> Usage of the blast and cupola furnace remained widespread during the [[Song dynasty|Song]] and [[Tang dynasty|Tang Dynasties]].<ref name="The Coming of the Ages of Steel">{{cite book|title=The Coming of the Ages of Steel|url=http://books.google.com/books?id=uMwUAAAAIAAJ&pg=PA54|publisher=Brill Archive|page=54|id=GGKEY:DN6SZTCNQ3G|date=1961}}</ref>

Artifacts of smelted iron are found in [[History of metallurgy in the Indian subcontinent|India]] dating from 1800 to 1200&nbsp;BCE,<ref name=Tewari>{{cite web| url = http://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|accessdate = 23 May 2010}}</ref> and in the [[Levant]] from about 1500&nbsp;BCE (suggesting smelting in [[Anatolia]] or the [[Caucasus]]).<ref>{{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=403–421|publisher=Taylor & Francis, Ltd.|jstor = 124562}}</ref><ref>{{Cite book| last = Muhly|first = James D.|chapter = Metalworking/Mining in the Levant|pages = 174–183|title =Near Eastern Archaeology IN: Eisenbrauns |editor = Lake, Richard Winona |date = 2003|volume = 180}}</ref>

The [[Book of Genesis]], fourth chapter, verse 22 contains the first mention of iron in the [[Old Testament]] of the [[Bible]]; "Tubal-cain, an instructor of every artificer in brass and iron."{{sfn|Weeks|1968|p=29}} Other verses allude to iron mining (Job 28:2), iron used as a stylus (Job 19:24), furnace (Deuteronomy 4:20), chariots (Joshua 17:16), nails (I Chron. 22:3), saws and axes (II Sam. 12:31), and cooking utensils (Ezekiel 4:3).{{sfn|Weeks|1968|pp=29–30}} The metal is also mentioned in the [[New Testament]], for example in Acts chapter 12 verse 10, "[Peter passed through] the iron gate that leadeth unto the city" of Antioch.{{sfn|Weeks|1968|p=30}}

Iron working was introduced to [[Ancient Greece|Greece]] in the late 11th century&nbsp;BCE.<ref>Riederer, Josef; Wartke, Ralf-B.: "Iron", Cancik, Hubert; Schneider, Helmuth (eds.): [[Brill's New Pauly]], Brill 2009</ref> The spread of ironworking in Central and Western Europe is associated with [[Celts|Celtic]] expansion. According to [[Pliny the Elder]], iron use was common in the [[Ancient Rome|Roman]] era.{{sfn|Weeks|1968|p=31}} The annual iron output of the [[Roman Empire]] is estimated at 84,750 [[Tonnes|t]],<ref>Craddock, Paul T. (2008): "Mining and Metallurgy", in: [[John Peter Oleson|Oleson, John Peter]] (ed.): ''The Oxford Handbook of Engineering and Technology in the Classical World'', Oxford University Press, ISBN 978-0-19-518731-1, p. 108</ref> while the similarly populous Han China produced around 5,000 t.<ref>Wagner, Donald B.: "The State and the Iron Industry in Han China", NIAS Publishing, Copenhagen 2001, ISBN 87-87062-77-1, p. 73</ref>

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 (metallurgy)|puddling process]] for refining iron ore. It was later improved by others, including [[Joseph Hall (metallurgist)|Joseph Hall]].

===Cast iron===
[[Cast iron]] was first produced in [[China]] during 5th century BCE,<ref>{{Cite journal|author=Wagner, Donald B. |title=Chinese blast furnaces from the 10th to the 14th century|journal=Historical Metallurgy|volume=37|issue=1|date=2003|pages=25–37}} 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}}</ref> but was hardly in Europe until the medieval period.<ref>Giannichedda, Enrico (2007): [http://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.</ref><ref name="Biddle">{{Cite book| title = Chemistry, Precision and Design|publisher = A Beka Book, Inc.|first1 = Verne|last1 =Biddle|first2= Gregory|last2 =Parker}}</ref><!--Missing page numbers (how would you add them?)--> The earliest [[cast iron]] artifacts were discovered by archaeologists in what is now modern [[Luhe County]], [[Jiangsu]] in China. Cast iron was used in [[History of China#Ancient China|ancient China]] for warfare, agriculture, and architecture.<ref name="Wagner">{{cite book|author=Donald B. Wagner|title=Iron and Steel in Ancient China|date=1993|publisher=BRILL|isbn=978-90-04-09632-5|pages=335–340}}</ref> 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 forge]]s. For all these processes, [[charcoal]] was required as fuel.
[[File:Philipp Jakob Loutherbourg d. J. 002.jpg|thumb|''[[Coalbrookdale by Night]]'', 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.<ref name="Biddle" /> Modern blast furnaces have grown much bigger.

In 1709, [[Abraham Darby I]] established a [[Coke (fuel)|coke]]-fired blast furnace to produce cast iron. 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 wasn't 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 [[The Iron Bridge|first iron bridge]] in 1778.

===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 Iran were making good steel by 1000&nbsp;BCE.{{sfn|Weeks|1968|p=32}} Then improved versions, [[Wootz steel]] by India and [[Damascus steel]] were developed around 300&nbsp;BCE and 500&nbsp;CE respectively. These methods were specialized, and so steel did not become a major commodity until the 1850s.<ref>Spoerl, Joseph S. [http://www.anselm.edu/homepage/dbanach/h-carnegie-steel.htm A Brief History of Iron and Steel Production]. Saint Anselm College</ref>

New methods of producing it by [[carburizing]] bars of iron in the [[cementation process]] were devised in the 17th century AD. 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.<ref>{{cite book | url = http://books.google.de/books?id=fUmTX8yKU4gC&pg=PA190 | pages = 190–191 | title = Encyclopedia of the Elements: Technical Data - History - Processing - Applications | isbn = 9783527612345 | author1 = Enghag | first1 = Per | date = 8 January 2008}}</ref>

===Foundations of modern chemistry===
[[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 mass conservation. Anaerobic oxidation of iron at high temperature can be schematically represented by the following reactions:

:Fe + H<sub>2</sub>O → FeO + H<sub>2</sub>

:2 Fe + 3 H<sub>2</sub>O → Fe<sub>2</sub>O<sub>3</sub> + 3 H<sub>2</sub>

:3 Fe + 4 H<sub>2</sub>O → Fe<sub>3</sub>O<sub>4</sub> + 4 H<sub>2</sub>
<!--
===Recent discoveries===
* discovery of [[Mössbauer effect]]
* many enzymes use iron in the catalytic center
* Nickel-56 is the natural end product of silicon burning in massive stars. However, nickel-56 decays to cobalt-56 and then to stable iron-56, ultimately making iron the most abundant heavy element produced by that nucleosynthesis.
* superconductivity?
* magnetic effect
* [[ferrocene]] -->

==Production of metallic iron==

===Industrial routes===
{{See also|Iron ore}}
The production of iron or steel is a process consisting of two main stages, unless the desired product is cast iron. In the first stage pig iron is produced in a blast furnace. Alternatively, it may be directly reduced. The second stage, pig iron is converted to wrought iron or steel.<!--http://books.google.com/books?id=xkVPNtRagDkC-->
[[File:Chinese Fining and Blast Furnace.jpg|thumb|The fining process of smelting [[iron ore]] to make wrought iron from pig iron, with the right illustration displaying men working a blast furnace, from the ''Tiangong Kaiwu'' encyclopedia, published in 1637 by [[Song Yingxing]].]] [[File:Iron-Making.jpg|thumb|How iron was extracted in the 19th century]]

For a few limited purposes like electromagnet cores, pure iron is produced by electrolysis of a [[ferrous sulfate]] solution

====Blast furnace processing====
{{Main|Blast furnace}}
Industrial iron production starts with iron ores, principally [[hematite]], which has a nominal formula Fe<sub>2</sub>O<sub>3</sub>, and [[magnetite]], with the formula Fe<sub>3</sub>O<sub>4</sub>.  These ores are reduced to the metal in a [[carbothermic]] reaction, i.e. by treatment with carbon.  The conversion is typically conducted in in a blast furnace at temperatures of about 2000&nbsp;°C. Carbon is provided in the form of coke.  The process also contains a ''flux'' such as [[limestone]], which is used to remove silicaceous minerals in the ore, which would otherwise clog the furnace.  The coke and limestone are fed into the top of the furnace, while a massive blast of heated [[Earth's atmosphere|air]], about 4 tons per ton of iron,<ref name="Biddle" /> is forced into the furnace at the bottom.

In the furnace, the coke reacts with oxygen in the air blast to produce [[carbon monoxide]]:
:2 C + O<sub>2</sub> → 2 CO

The carbon monoxide reduces the iron ore (in the [[chemical equation]] below, hematite) to molten iron, becoming [[carbon dioxide]] in the process:
:Fe<sub>2</sub>O<sub>3</sub> + 3 CO → 2 Fe + 3 CO<sub>2</sub>

Some iron in the high-temperature lower region of the furnace reacts directly with the coke:
:2 Fe<sub>2</sub>O<sub>3</sub> + 3 C → 4 Fe + 3 CO<sub>2</sub>

The flux present to melt impurities in the ore is principally [[limestone]] ([[calcium carbonate]]) and [[dolomite]] (calcium-magnesium carbonate). Other specialized fluxes are used depending on the details of the ore. In the heat of the furnace the limestone flux decomposes to [[calcium oxide]] (also known as quicklime):

:CaCO<sub>3</sub> → CaO + CO<sub>2</sub>

Then calcium oxide combines with silicon dioxide to form a liquid [[slag]].
:CaO + SiO<sub>2</sub> → CaSiO<sub>3</sub>

The slag melts in the heat of the furnace. In the bottom of the furnace, the molten slag floats on top of the denser molten iron, and apertures in the side of the furnace are opened to run off the iron and the slag separately. The iron, once cooled, is called pig iron, while the slag can be used as a material in [[road]] construction or to improve mineral-poor soils for [[agriculture]]<ref name="Biddle" />
[[File:LightningVolt Iron Ore Pellets.jpg|thumb|This heap of iron ore pellets will be used in steel production.]]

====Direct iron reduction====
Owing to environmental concerns, alternative methods of processing iron have been developed. "Direct iron reduction" reduces iron ore to a powder called "sponge" iron or "direct" iron that is suitable for steelmaking.<ref name="Biddle" /> Two main reactions comprise the direct reduction process:

Natural gas is partially oxidized (with heat and a catalyst):
:2 CH<sub>4</sub> + O<sub>2</sub> → 2 CO + 4 H<sub>2</sub>

These gases are then treated with iron ore in a furnace, producing solid sponge iron:
:Fe<sub>2</sub>O<sub>3</sub> + CO + 2 H<sub>2</sub> → 2 Fe + CO<sub>2</sub> + 2 H<sub>2</sub>O

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

====Further processes====
{{Main|Steelmaking|Ironworks}}
[[File:Steel pd.svg|thumb|right|340px|Iron-carbon phase diagram, various stable [[solid solution]] forms]]
<!-- Several other articles cover the material that might go into this section: please do not expand it excessively. This article concerns all aspects of the element iron, and should thus NOT be overburdened with details of metallurgy... Agree. Should be a summary. -->
Pig iron is not pure iron, but has 4–5% carbon dissolved in it with small amounts of other impurities like sulfur, magnesium, phosphorus and manganese. As the carbon is the major impurity, the iron (pig iron) becomes brittle and hard. This form of iron, also known as cast iron, is used to cast articles in [[foundry|foundries]] such as stoves, pipes, radiators, lamp-posts and rails.

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 forge]]s, [[Puddling (metallurgy)|puddling]] furnaces, [[Bessemer converter]]s, [[open hearth furnace]]s, [[basic oxygen furnace]]s, and [[electric arc furnace]]s. 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.

<!-- why is this in production? The hardness of the steel depends upon its carbon content: the higher the percentage of carbon, the greater the hardness and the lesser the malleability. The properties of the steel can also be changed by several methods.-->

[[Annealing (metallurgy)|Annealing]] involves the heating of a piece of steel to 700–800&nbsp;°C for several hours and then gradual cooling. It makes the steel softer and more workable.
<!--why is this in production of iron?Steel may be hardened by [[cold working]]. The metal is bent or hammered into its final shape at a relatively cool temperature. Cold forging is the stamping of a piece of steel into shape by a heavy press. Wrenches are commonly made by cold forging. Cold rolling, which involves making a thinner but harder sheet, and cold drawing, which makes a thinner but stronger wire, are two other methods of cold working. To harden the steel, it is heated to red-hot and then cooled by quenching it in the water. It becomes harder and more brittle. If it is too hardened, it is then heated to a required temperature and allowed to cool. The steel thus formed is less brittle.

[[Heat treatment]] is another way to harden steel. The steel is heated red-hot, then cooled quickly. The iron carbide molecules are decomposed by the heat, but do not have time to reform. Since the free carbon atoms are stuck, it makes the steel much harder and stronger than before.<ref name="Biddle" />

Sometimes both toughness and hardness are desired. A process called [[case hardening]] may be used. Steel is heated to about 900&nbsp;°C then plunged into oil or water. Carbon from the oil can diffuse into the steel, making the surface very hard. The surface cools quickly, but the inside cools slowly, making an extremely hard surface and a durable, resistant inner layer.

Iron may be [[Passivation (chemistry)|passivated]] by dipping it into a concentrated [[nitric acid]] solution. This forms a protective layer of oxide on the metal, protecting it from further corrosion.<ref>{{cite book|url=http://www.euro-inox.org/pdf/map/Passivating_Pickling_EN.pdf |title=Picking and passivating stainless steel, Materials and Application Series, Volume 4 |publisher=Euro Inox |year=2007 |edition=2nd |isbn=978-2-87997-224-4}}</ref>-->

===Laboratory methods===
Metallic iron is generally produced in the laboratory by two methods.  One route is electrolysis of ferrous chloride onto an iron cathode.  The second method involves reduction of iron oxides with hydrogen gas at about 500&nbsp;°C.<ref>H. Lux "Metallic Iron" in in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 2. p. 1490-1..</ref>

==Applications==
[[File:Iron powder.JPG|thumb|right|250px|[[Iron powder]]]]

===Metallurgical===
{|class="wikitable" style="float:left; clear:left; margin-right:1em; margin-top:0;"
|+Iron production 2009 (million [[tonnes]])<ref>[http://www.worldsteel.org/statistics/statistics-archive/yearbook-archive.html Steel Statistical Yearbook 2010]. World Steel Association</ref>
!Country!![[Iron ore]]!![[Pig iron]]!![[Direct reduced iron|Direct iron]]!![[Steel]]
|-
|China|| 1,114.9||549.4 || || 573.6
|-
|Australia||393.9|| 4.4|| ||5.2
|-
|Brazil||305.0||25.1 ||0.011 ||26.5
|-
|Japan|| || 66.9|| || 87.5
|-
|India||257.4||38.2 || 23.4||63.5
|-
|Russia||92.1|| 43.9|| 4.7||60.0
|-
|Ukraine||65.8|| 25.7|| ||29.9
|-
|South Korea|| 0.1|| 27.3|| ||48.6
|-
|Germany||0.4 || 20.1||0.38 ||32.7
|-
!World!! 1,594.9!!914.0!! 64.5!! 1,232.4
|}

Iron is the most widely used of all the metals, accounting for 95% of worldwide metal production.{{Citation needed|date=May 2010}}<!-- The UGSG gives a production of iron including recycling with 998Mt, while aluminium (39Mt), copper (18Mt), zinc (11Mt) and lead (8.6Mt) add up to 77 Mt, all including recycling. This more like 8% than 5.--> Its low cost and high strength make it indispensable in engineering applications such as the construction of machinery and [[machine tool]]s, [[automobile]]s, the [[hull (ship)|hulls]] of large [[ship]]s, and structural components for [[building]]s. Since pure iron is quite soft, it is most commonly combined with alloying elements to make steel.

Commercially available iron is classified based on purity and the abundance of additives. [[Pig iron]] has 3.5–4.5% carbon<ref name="msts">{{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 = 173–174|url = http://books.google.com/?id=P9MxAAAAMAAJ|isbn = 1-147-64423-3}}</ref> 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]]. It has a [[melting point]] in the range of 1420–1470&nbsp;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. Its mechanical properties vary greatly and depend on the form the carbon takes in the alloy.

"White" cast irons contain their carbon in the form of [[cementite]], or iron-carbide. 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.

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. 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.<ref name="msts" /> It is a tough, malleable product, but not as fusible as pig iron. If honed to an edge, it loses it quickly. Wrought iron is characterized by the presence of fine fibers of [[slag]] entrapped within the metal. Wrought iron is more corrosion resistant than steel. It has been almost completely replaced by [[mild steel]] for traditional "wrought iron" products and [[blacksmith]]ing.

Mild steel corrodes more readily than wrought iron, but is cheaper and more widely available. [[Carbon steel]] contains 2.0% carbon or less,<ref name="kts">{{cite web|title = Classification of Carbon and Low-Alloy Steels|url = http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=62|accessdate = 5 January 2008}}</ref> with small amounts of [[manganese]], [[sulfur]], [[phosphorus]], and silicon. [[Alloy steel]]s 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 steel|HSLA]]' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.

[[File:Ironattenuation.PNG|thumb|alt=A graph of attenuation coefficient vs. energy between 1 meV and 100 keV for several photon scattering mechanisms.|[[Photon]] [[mass attenuation coefficient]] for iron.]]
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. The attenuation of radiation as a function of energy is shown in the graph.

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. [[Paint]]ing, [[galvanization]], [[Passivation (chemistry)|passivation]], plastic coating and [[bluing (steel)|bluing]] are all used to protect iron from rust by excluding [[water]] and oxygen or by [[cathodic protection]].

===Iron compounds===
Although its metallurgical role is dominant in terms of amounts, iron compounds are pervasive in industry as well being used in many niche uses. 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 [[hydrocarbon]]s for fuels and lubricants.<ref>{{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|url = http://books.google.com/?id=OA7L1l6oHAYC&pg=PR15|chapter = Where are Heterogenous Reactions Important|pages = 15–16}}</ref> Powdered iron in an acidic solvent was used in the [[Bechamp reduction]] the reduction of [[nitrobenzene]] to [[aniline]].<ref>{{Cite book| url = http://books.google.com/?id=BiywGdlot9kC&pg=PA167|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 = 166–167}}</ref>

[[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 [[Industrial etching|etchant]] for [[copper]] in the manufacture of [[printed circuit board]]s.<ref>{{Cite journal| doi = 10.1002/14356007.a14_591| title = 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| chapter = Iron Compounds| isbn = 3527306730}}</ref> It can also be dissolved in alcohol to form tincture of iron. The other halides tend to be limited to laboratory uses.

[[Iron(II) sulfate]] is used as a precursor to other iron compounds. It is also used to [[redox|reduce]] chromate in cement. It is used to fortify foods and treat [[iron deficiency anemia]]. These are its main uses. [[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.

==Biological role==
Iron is abundant in biology.<ref>{{cite book 
|last1=Dlouhy |first1=Adrienne C. |last2=Outten|first2=Caryn E. 
|editor1-first=Lucia |editor1-last=Banci |series=Metal Ions in Life Sciences |volume=12 
|chapter= Chapter 8 The Iron Metallome in Eukaryotic Organisms
|title=Metallomics and the Cell |date=2013 |publisher=Springer |isbn=978-94-007-5560-4|doi=10.1007/978-94-007-5561-1_8}} electronic-book ISBN 978-94-007-5561-1 {{ISSN|1559-0836}} electronic-{{ISSN|1868-0402}}</ref><ref>
{{cite book
|first1=Gereon M.
|last1=Yee 
|first2=William B. 
|last2=Tolman 
|editor=Peter M.H. Kroneck and Martha E. Sosa Torres
|title=Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases
|series=Metal Ions in Life Sciences
|volume=15
|year=2015
|publisher=Springer
|chapter=Chapter 5 ''Transition Metal Complexes and the Activation of Dioxygen''
|pages=131–204
|doi=10.1007/978-3-319-12415-5_5
}}
</ref> Iron-proteins are found in all living organisms, ranging from the evolutionarily primitive [[archaea]] to humans. The color of blood is due to the hemoglobin, an iron-containing protein. As illustrated by hemoglobin, iron is often bound to [[Cofactor (biochemistry)|cofactors]], e.g. in [[heme]]s. The [[iron-sulfur cluster]]s are pervasive and include [[nitrogenase]], the enzymes responsible for biological [[nitrogen fixation]]. Influential theories of evolution have invoked a role for iron sulfides in the [[iron-sulfur world theory]].
[[File:Heme b.png|thumb|Structure of [[Heme|Heme b]], in the protein additional [[ligand]](s) would be attached to Fe.]]

Iron is a necessary [[trace element]] found in nearly all living organisms. <!--Probably incorrect: The only exceptions are several organisms that live in iron-poor environments and have evolved to use different elements in their metabolic processes, such as manganese instead of iron for catalysis, or [[hemocyanin]] instead of hemoglobin.{{Citation needed|date=September 2010}} 2010}}--> Iron-containing enzymes and proteins, often containing [[heme]] [[prosthetic group]]s, participate in many biological oxidations and in transport. Examples of proteins found in higher organisms include hemoglobin, [[cytochrome]] (see [[high-valent iron]]), and [[catalase]].<ref>{{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}}</ref>

===Bioinorganic compounds===
The most commonly known and studied "[[bioinorganic chemistry|bioinorganic]]" compounds of iron (i.e., iron compounds used in biology) are the [[heme proteins]]: examples are [[hemoglobin]], [[myoglobin]], and [[cytochrome P450]]. These compounds can transport gases, build [[enzymes]], and be used in transferring [[electrons]]. [[Metalloproteins]] are a group of proteins with metal ion [[cofactor (biochemistry)|cofactors]]. Some examples of iron metalloproteins are [[ferritin]] and [[rubredoxin]]. Many enzymes vital to life contain iron, such as [[catalase]], [[lipoxygenases]], and [[IRE-BP]].

===Health and diet===
{{Main|Iron deficiency|Human iron metabolism}}
Iron is pervasive, but particularly rich sources of dietary iron include [[red meat]], [[lentil]]s, [[bean]]s, [[poultry]], [[fish]], [[leaf vegetable]]s, [[watercress]], [[tofu]], [[chickpea]]s, [[black-eyed peas]], [[blackstrap molasses]], fortified [[bread]], and fortified [[breakfast cereal]]s. Iron in low amounts is found in [[molasses]], [[teff]], and [[farina (food)|farina]]. Iron in meat ([[heme]] iron) is more easily absorbed than iron in vegetables.<ref>[http://www.eatwell.gov.uk/healthissues/irondeficiency/ Food Standards Agency – Eat well, be well – Iron deficiency]. Eatwell.gov.uk (5 March 2012). Retrieved on 27 June 2012.</ref> Although some studies suggest that heme/hemoglobin from red meat has effects which may increase the likelihood of [[colorectal cancer]],<ref name="pmid10582688">{{Cite journal|title=Red meat and colon cancer: the cytotoxic and hyperproliferative effects of dietary heme |journal=Cancer Research|volume=59 |issue=22 |date=1999 |pmid=10582688 |last=Sesink |first= Aloys L. A. |author2=T |author3=K |author4=V|pages=5704–9}}</ref><ref name="pmid16226281">{{Cite journal|title=Hemoglobin and hemin induce DNA damage in human colon tumor cells HT29 clone 19A and in primary human colonocytes |journal=[[Mutat. Res.]] |volume=594 |issue=1–2 |pages=162–171 |date=2006 |pmid=16226281 |doi=10.1016/j.mrfmmm.2005.08.006 |last1=Glei |first1=M. |last2=Klenow |first2=S. |last3=Sauer |first3=J. |last4=Wegewitz |first4=U. |last5=Richter |first5=K. |last6=Pool-Zobel |first6=B. L.}}</ref> there is still some controversy,<ref>{{Cite journal|url=http://cebp.aacrjournals.org/content/10/5/439.full|title=Systematic Review of the Prospective Cohort Studies on Meat Consumption and Colorectal Cancer Risk: A Meta-Analytical Approach |journal=Cancer Epidemiology, Biomarkers & Prevention |date=2001 |volume=10 |pmid=11352852 |issue=5|last1=Sandhu|first1=M. S.|last2=White|first2=I. R.|last3=McPherson|first3=K.|pages=439–46}}</ref> and even a few studies suggesting that there is not enough evidence to support such claims.<ref>{{cite web| url = http://www.sciencedaily.com/releases/2007/06/070611113729.htm|title = Eating Red Meat Will Not Increase Colorectal Cancer Risk, Study Suggests|publisher = ScienceDaily|date = 13 June 2007| accessdate = 23 May 2010}}</ref>

Iron provided by [[dietary supplement]]s is often found as [[iron(II) fumarate]], although iron 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),<ref>{{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}}</ref> is often added to foods such as breakfast cereals or enriched wheat flour. Iron is most available to the body when [[Chelation|chelated]] to amino acids<ref name="pmid11377130">{{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.}}</ref> and is also available for use as a common [[iron supplement]]. Often the amino acid chosen for this purpose is the cheapest and most common amino acid, [[glycine]], leading to "iron glycinate" supplements.<ref name="Ashmead">{{Cite book|last = Ashmead |first = H. DeWayne |date = 1989 |title = ''Conversations on Chelation and Mineral Nutrition'' |publisher = Keats Publishing |isbn = 0-87983-501-X}}</ref> The [[Recommended Dietary Allowance]] (RDA) for iron varies considerably based on age, gender, and source of dietary iron (heme-based iron has higher [[bioavailability]]).<ref>{{cite web|url=http://www.iom.edu/Object.File/Master/7/294/0.pdf |title=Dietary Reference Intakes: Elements |publisher = The National Academies |date = 2001 |format=PDF |accessdate = 21 May 2008}}</ref> Infants may require iron supplements if they are bottle-fed cow's milk.<ref>{{cite web|url=http://bodyandhealth.canada.com/condition_info_details.asp?disease_id=274 |title=Iron Deficiency Anemia |publisher=MediResource |accessdate =17 December 2008}}</ref> [[Blood donation|Blood donors]] and pregnant women are at special risk of low iron levels and are often advised to supplement their iron intake.<ref>{{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]]}}</ref>

===Uptake and storage===
Iron acquisition poses a problem for aerobic organisms, because ferric iron is poorly soluble near neutral pH. Thus, bacteria have evolved high-affinity [[wikt:sequester|sequestering]] agents called [[siderophore]]s.<ref>{{Cite journal| url = http://www.jbc.org/content/270/45/26723.short|pmid = 7592901|doi = 10.1074/jbc.270.45.26723|date = 1995|last1 = Neilands|first1 = JB|title = Siderophores: structure and function of microbial iron transport compounds|volume = 270|issue = 45|pages = 26723–6|journal = The Journal of Biological Chemistry}}</ref><ref>{{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}}</ref><ref>{{Cite journal| doi = 10.1023/A:1020218608266|date = 2002|last1 = Boukhalfa|first1 = Hakim|last2 = Crumbliss|first2 = Alvin L.|journal = BioMetals|volume = 15|pages = 325–39|pmid = 12405526|title = Chemical aspects of siderophore mediated iron transport|issue = 4}}</ref>

After uptake, in [[cell (biology)|cells]], iron storage is carefully regulated; "free" iron ions do not exist as such. A major component of this regulation is the protein [[transferrin]], which binds iron ions absorbed from the [[duodenum]] and carries it in the [[bloodstream|blood]] to cells.<ref>{{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.|journal=PLoS Biology|volume=1|pages=e9 |pmid=14551907 |issue=3 |pmc=212690}}</ref> In animals, plants, and fungi, iron is often the metal ion incorporated into the heme complex. Heme is an essential component of [[cytochrome]] proteins, which mediate [[redox]] reactions, and of oxygen [[carrier protein]]s such as [[hemoglobin]], [[myoglobin]], and [[leghemoglobin]].

Inorganic iron contributes to redox reactions in the [[iron-sulfur cluster]]s of many [[enzyme]]s, such as [[nitrogenase]] (involved in the synthesis of [[ammonia]] from [[nitrogen]] and [[hydrogen]]) and [[hydrogenase]]. Non-heme iron proteins include the [[enzymes]] [[methane monooxygenase]] (oxidizes [[methane]] to [[methanol]]), [[ribonucleotide reductase]] (reduces [[ribose]] to [[deoxyribose]]; [[DNA replication|DNA biosynthesis]]), [[hemerythrin]]s ([[oxygen]] transport and fixation in [[marine invertebrates]]) and purple [[acid phosphatase]] ([[hydrolysis]] of [[phosphate]] [[ester]]s).

Iron distribution is heavily regulated in [[mammal]]s, partly because iron ions have a high potential for biological toxicity.<ref>{{Cite journal|title=Tumor necrosis factor-α-induced iron sequestration and oxidative stress in human endothelial cells |url=http://cat.inist.fr/?aModele=afficheN&cpsidt=17328512 |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|journal=Arteriosclerosis, thrombosis, and vascular biology |first2=T |date=2005 |volume=25 |last2=Ookawara|issue=12 |pmid=16224057 |pages=2495–2501 |doi=10.1161/01.ATV.0000190610.63878.20 |last1=Nanami |first1=M.}}</ref>

===Regulation of uptake===
{{Main|Hepcidin}}

[[Human iron metabolism|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 mostly by regulating uptake.<ref>{{cite book|author1=Ramzi S. Cotran|author2=Vinay Kumar|author3=Tucker Collins|author4=Stanley Leonard Robbins|title=Robbins pathologic basis of disease|url=http://books.google.com/books?id=kdhrAAAAMAAJ|accessdate= 27 June 2012|date=1999|publisher=Saunders|isbn=978-0-7216-7335-6}}</ref>
Regulation of iron uptake is impaired in some people as a result of a [[Genetic disorder|genetic defect]] that maps to the HLA-H gene region on chromosome 6. In these people, excessive iron intake can result in [[iron overload disorder]]s, such as [[hemochromatosis]]. Many people have a genetic susceptibility to iron overload without realizing it or being aware of a family history of the problem. For this reason, it is advised that people do not take iron supplements unless they suffer from [[iron deficiency (medicine)|iron deficiency]] and have consulted a doctor. [[Hemochromatosis]] is estimated to cause disease in between 0.3 and 0.8% of Caucasians.<ref>{{Cite journal|title=Hereditary hemochromatosis|journal=Rev Med 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}}</ref>

[[MRI]] finds that iron accumulates in the [[hippocampus]] of the brains of those with [[Alzheimer's disease]] and in the [[substantia nigra]] of those with [[Parkinson disease]].<ref>{{Cite journal| url = http://archneur.highwire.org/cgi/content/abstract/66/3/371 |pmid= 19273756|doi = 10.1001/archneurol.2008.586|date = 2009|last1 = Brar|first1 = S|last2 = Henderson|first2 = D|last3 = Schenck|first3 = J|last4 = Zimmerman|first4 = EA|title = Iron accumulation in the substantia nigra of patients with Alzheimer disease and parkinsonism|volume = 66|issue = 3|pages = 371–4|journal = Archives of neurology}}</ref>

===Bioremediation===

Iron-eating bacteria live in the hulls of [[sunken ship]]s such as the ''[[Titanic]]''.<ref>{{cite book
  | last = Ward
  | first = Greg
  | title = The Rough Guide to the ''Titanic''
  | date = 2012
  | publisher = Rough Guides Ltd
  | location = London
  | page=171
  | isbn = 978-1-4053-8699-9
  | ref = harv
  }}</ref> The acidophile bacteria ''[[Acidithiobacillus|Acidithiobacillus ferrooxidans]]'', ''[[Leptospirillum ferrooxidans]]'', ''[[Sulfolobus]]'' spp., ''[[Acidianus|Acidianus brierleyi]]'' and ''[[Sulfobacillus thermosulfidooxidans]]'' can oxidize ferrous iron enzymically.<ref>{{cite journal|url=http://mic.sgmjournals.org/content/156/3/609.full|title=Metals, minerals and microbes: geomicrobiology and bioremediation|journal=Microbiology|author=Geoffrey Michael Gadd|volume=156|date=March 2010|pages=609–643|doi=10.1099/mic.0.037143-0|pmid=20019082|issue=3}}</ref> A sample of the fungus ''[[Aspergillus niger]]'' was found growing from gold mining solution, and was found to contain cyano metal complexes such as gold, silver, copper iron and zinc. The fungus also plays a role in the solubilization of heavy metal sulfides.<ref>{{cite book|url=http://books.google.com/books?id=WY3YvfNoouMC&pg=PA533&cad=4#v=onepage&q&f=false|title=Mycoremediation: Fungal Bioremediation|author=Harbhajan Singh|page=509}}</ref>

===Permeable reactive barriers===
[[Zerovalent iron]] is the main reactive material for [[permeable reactive barriers]].<ref>{{cite book | url = http://books.google.de/books?id=0MJap7ncvx8C&pg=PA5 | page = 5 | title = Long-Term Performance of Permeable Reactive Barriers | isbn = 9780080535616 | author1 = Roehl | first1 = K.E. | last2 = Meggyes | first2 = T | last3 = Simon | first3 = F.G. | last4 = Stewart | first4 = D.I. | date = 27 April 2005}}</ref>

==Toxicity==
{{NFPA 704|Health = 0|Flammability = 1|Reactivity = 1|S=|caption=Fire diamond for powdered iron metal}}

{{Main|Iron poisoning}}
Large amounts of ingested iron can cause excessive levels of iron in the blood. High blood levels of free ferrous iron react with [[peroxide]]s to produce [[free radical]]s, which are highly reactive and can damage [[DNA]], [[proteins]], [[lipids]], and other cellular components. Thus, iron toxicity occurs when there is free iron in the cell, which generally occurs when iron levels exceed the capacity 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, which can cause significant adverse effects, including [[coma]], [[metabolic acidosis]], [[Shock (circulatory)|shock]], [[liver failure]], [[coagulopathy]], [[adult respiratory distress syndrome]], long-term organ damage, and even death.<ref name="Cheney" /> Humans experience iron toxicity above 20&nbsp;milligrams of iron for every kilogram of mass, and 60&nbsp;milligrams per kilogram is considered a [[lethal dose]].<ref name="emed-topic285">{{cite web|url=http://www.emedicine.com/emerg/topic285.htm|title=Toxicity, Iron
| publisher = Medscape|accessdate=23 May 2010}}</ref> 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.<ref name="emed-topic285" /> The [[Dietary Reference Intake]] (DRI) lists the Tolerable Upper Intake Level (UL) for adults as 45&nbsp;mg/day. For children under fourteen years old the UL is 40&nbsp;mg/day.

The medical management of iron toxicity is complicated, and can include use of a specific [[chelation|chelating]] agent called [[deferoxamine]] to bind and expel excess iron from the body.<ref name="Cheney">{{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=61–6 |date=1995 |pmid=7837315 |doi=10.3109/15563659509020217}}</ref><ref>{{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=485–489 |date=1996 |pmid=8800185 |doi=10.3109/15563659609028005}}</ref><ref name="pmid21102602">{{cite journal | author = 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}}</ref>

==See also==
{{Portal|Chemistry}}
<!-- Please keep this list tidy and in alphabetical order. Avoid links prominently featured in article. -->
* [[El Mutún]] in [[Bolivia]], where 10% of the world's accessible iron ore is located.
* [[Iron fertilization]] – proposed fertilization of oceans to stimulate [[phytoplankton]] growth
* [[Iron (metaphor)]]
* [[Iron in folklore]]
* [[List of countries by iron production]]
* [[Pelletizing|Pelletising]] – process of creation of iron ore pellets
* [[Rustproof iron]]
* [[Steel]]

==References==
{{Reflist|colwidth=30em}}

==Bibliography==
* <!-- We -->{{Cite book
|last = Weeks
|first = Mary Elvira
|authorlink=Mary Elvira Weeks|author2=Leichester, Henry M.
|year = 1968
|title = Discovery of the Elements
|publisher = Journal of Chemical Education
|location = Easton, PA
|chapter = Elements Known to the Ancients
|pages = 29–40
|lccn = 68-15217
|ref = CITEREFWeeks1968
|isbn = 0-7661-3872-0
}}

==Further reading==
* 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.

==External links==
{{Wiktionary|iron}}
{{Commons|Iron}}
* [http://education.jlab.org/itselemental/ele026.html It's Elemental – Iron]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html The Most Tightly Bound Nuclei]
* [http://www.rsc.org/chemistryworld/podcast/element.asp Chemistry in its element podcast] (MP3) from the [[Royal Society of Chemistry]]'s [[Chemistry World]]: [http://www.rsc.org/images/CIIE_iron_48kbps_tcm18-120046.mp3 Iron]
* [http://www.periodicvideos.com/videos/026.htm Iron] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://books.google.com/books?id=brpx-LtdCLYC&pg=frontcover&d#v=onepage&q&f=true Metallurgy for the non-Metallurgist]

{{Compact periodic table}}
{{Iron compounds}}

{{Authority control}}
[[Category:Iron| ]]
[[Category:Biology and pharmacology of chemical elements]]
[[Category:Building materials]]
[[Category:Chemical elements|Iron]]
[[Category:Cubic minerals]]
[[Category:Dietary minerals]]
[[Category:Ferromagnetic materials]]
[[Category:Pyrotechnic fuels]]
[[Category:Transition metals]]