Editing Iron oxide

{{Short description|Class of chemical compounds composed of iron and oxygen}}
[[File:Almindeligt rust - jernoxid.jpg|thumb|260px|Electrochemically oxidized iron (rust)]]

An '''iron oxide''' is a [[chemical compound]] composed of [[iron]] and [[oxygen]]. Several iron [[oxide]]s are recognized.  Often they are [[nonstoichiometric|non-stoichiometric]]. [[Ferric oxyhydroxide]]s are a related class of compounds, perhaps the best known of which is [[rust]].<ref name="cor">{{cite book
 | last = Cornell.
 | first = RM.
 |author2=Schwertmann, U
 | title = The iron oxides: structure, properties, reactions, occurrences and 
 | year = 2003
 | publisher = Wiley VCH
 | isbn = 978-3-527-30274-1
}}</ref>

Iron oxides and oxyhydroxides are widespread in nature and play an important role in many geological and biological processes.  They are used as [[iron ore]]s, [[pigment]]s, [[Catalysis|catalysts]], and in [[thermite]], and occur in [[hemoglobin]]. Iron oxides are inexpensive and durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "[[earth tone|earthy]]" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has [[E number]] E172.

==Stoichiometries== 
[[File:IronOxidePigmentUSGOV.jpg|thumb|Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3.|261px]]
[[File:Red and green iron oxides.jpg|thumb|upright|Green and reddish brown stains on a limestone core sample, respectively corresponding to oxides/hydroxides of Fe<sup>2+</sup> and Fe<sup>3+</sup>.]]

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

* Oxides of Fe<sup>II</sup>
** FeO:  [[iron(II) oxide]], [[wüstite]] 
* Mixed oxides of Fe<sup>II</sup> and Fe<sup>III</sup>
** Fe<sub>3</sub>O<sub>4</sub>: [[Iron(II,III) oxide]], [[magnetite]] 
** Fe<sub>4</sub>O<sub>5</sub><ref>{{cite journal|title=Discovery of the recoverable high-pressure iron oxide Fe4O5|date=Oct 2011 | doi=10.1073/pnas.1107573108 |pmid=21969537 | volume=108|issue=42|journal=Proceedings of the National Academy of Sciences|pages=17281–17285|bibcode=2011PNAS..10817281L|pmc=3198347|last1=Lavina |first1=B. |last2=Dera |first2=P. |last3=Kim |first3=E. |last4=Meng |first4=Y. |last5=Downs |first5=R. T. |last6=Weck |first6=P. F. |last7=Sutton |first7=S. R. |last8=Zhao |first8=Y. |doi-access=free }}</ref>
** Fe<sub>5</sub>O<sub>6</sub><ref>{{cite journal|title = Synthesis of Fe5O6|journal = Science Advances|volume = 1|issue = 5|pages = e1400260|doi = 10.1126/sciadv.1400260|pmid = 26601196|year = 2015|last1 = Lavina|first1 = Barbara|last2 = Meng|first2 = Yue|pmc = 4640612}}</ref>
** Fe<sub>5</sub>O<sub>7</sub><ref name = "oxides">{{cite journal| title = Structural complexity of simple Fe2O3 at high pressures and temperatures| journal = Nature Communications| volume = 7| pages = 10661| doi = 10.1038/ncomms10661| pmid = 26864300| pmc = 4753252| year = 2016| last1 = Bykova| first1 = E.| last2 = Dubrovinsky| first2 = L.| last3 = Dubrovinskaia| first3 = N.| last4 = Bykov| first4 = M.| last5 = McCammon| first5 = C.| last6 = Ovsyannikov| first6 = S. V.| last7 = Liermann| first7 = H. -P.| last8 = Kupenko| first8 = I.| last9 = Chumakov| first9 = A. I.| last10 = Rüffer| first10 = R.| last11 = Hanfland| first11 = M.| last12 = Prakapenka| first12 = V.| bibcode = 2016NatCo...710661B}}</ref>
** Fe<sub>25</sub>O<sub>32</sub><ref name="oxides" />
**Fe<sub>13</sub>O<sub>19</sub><ref>{{cite journal| title = The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions| journal = American Mineralogist| volume = 100| issue = 8–9| pages = 2001–2004| doi = 10.2138/am-2015-5369| year = 2015| last1 = Merlini| first1 = Marco| last2 = Hanfland| first2 = Michael| last3 = Salamat| first3 = Ashkan| last4 = Petitgirard| first4 = Sylvain| last5 = Müller| first5 = Harald| bibcode = 2015AmMin.100.2001M| s2cid = 54496448}}</ref>
* Oxides of Fe<sup>III</sup>
** Fe<sub>2</sub>O<sub>3</sub>: [[iron(III) oxide]] 
*** α-Fe<sub>2</sub>O<sub>3</sub>: [[iron(III) oxide#Alpha phase|alpha phase]], [[hematite]] 
*** β-Fe<sub>2</sub>O<sub>3</sub>: [[iron(III) oxide#Other solid phases|beta phase]]
*** γ-Fe<sub>2</sub>O<sub>3</sub>: [[iron(III) oxide#Gamma phase|gamma phase]], [[maghemite]] 
*** ε-Fe<sub>2</sub>O<sub>3</sub>: [[iron(III) oxide#Other solid phases|epsilon phase]]

==Thermal expansion==
{| class="wikitable"
|+
!Iron oxide
! [[Thermal expansion#Coefficients|CTE]] (× 10<sup>−6</sup> °C<sup>−1</sup>)
|-
|Fe<sub>2</sub>O<sub>3</sub>
|14.9<ref name=":0">{{Cite journal|last1=Fakouri Hasanabadi|first1=M.|last2=Kokabi|first2=A.H.|last3=Nemati|first3=A.|last4=Zinatlou Ajabshir|first4=S.|date=February 2017|title=Interactions near the triple-phase boundaries metal/glass/air in planar solid oxide fuel cells|journal=International Journal of Hydrogen Energy|volume=42|issue=8|pages=5306–5314|doi=10.1016/j.ijhydene.2017.01.065|issn=0360-3199}}</ref>
|-
|Fe<sub>3</sub>O<sub>4</sub>
|>9.2<ref name=":0" />
|-
|FeO
|12.1<ref name=":0" />
|}

==Oxide-hydroxides==
{{main|iron(III) oxide-hydroxide}}
* [[goethite]] (α-FeOOH)
* [[akaganéite]] (β-FeOOH)
* [[lepidocrocite]] (γ-FeOOH)
* [[feroxyhyte]] (δ-FeOOH)
* [[ferrihydrite]] (Fe<sub>5</sub>HO<sub>8</sub> · 4 H<sub>2</sub>O approx., or 5 Fe<sub>2</sub>O<sub>3</sub> · 9 H<sub>2</sub>O, better recast as FeOOH · 0.4 H<sub>2</sub>O)
* high-pressure pyrite-structured FeOOH.<ref>{{Cite journal|last1=Nishi|first1=Masayuki|last2=Kuwayama|first2=Yasuhiro|last3=Tsuchiya|first3=Jun|last4=Tsuchiya|first4=Taku|date=2017|title=The pyrite-type high-pressure form of FeOOH|url=https://www.nature.com/articles/nature22823|journal=Nature|language=en|volume=547|issue=7662|pages=205–208|doi=10.1038/nature22823|pmid=28678774|bibcode=2017Natur.547..205N |s2cid=205257075|issn=1476-4687|url-access=subscription}}</ref> Once [[Dehydration reaction|dehydration]] is triggered, this phase may form FeO<sub>2</sub>H<sub>''x''</sub> (0 < ''x'' < 1).<ref>{{cite journal|last1=Hu|first1=Qingyang|last2=Kim|first2=Duckyoung|last3=Liu|first3=Jin|last4=Meng|first4=Yue|last5=Liuxiang|first5=Yang|last6=Zhang|first6=Dongzhou|last7=Mao|first7=Wendy L.|author-link7=Wendy Mao|last8=Mao|first8=Ho-kwang|year=2017|title=Dehydrogenation of goethite in Earth's deep lower mantle|journal=Proceedings of the National Academy of Sciences|volume=114|issue=7|pages=1498–1501|doi=10.1073/pnas.1620644114|pmc=5320987|pmid=28143928|bibcode=2017PNAS..114.1498H |doi-access=free}}</ref>
* [[green rust]] (Fe{{sup sub|III|''x''}}Fe{{sup sub|II|''y''}}OH<sub>3''x'' + ''y'' − ''z''</sub> (A<sup>−</sup>)<sub>''z''</sub> where A<sup>−</sup> is Cl<sup>−</sup> or 0.5 {{chem2|SO4(2-)}})

==Reactions==
In [[Blast furnace|blast furnaces]] and related factories, iron oxides are converted to the metal.  Typical [[Reducing agent|reducing agents]] are various forms of carbon.  A representative reaction starts with ferric oxide:<ref>{{Greenwood&Earnshaw2nd|page=1072}}</ref> 
:{{chem2|2 Fe2O3  +  3 C  ->  4 Fe  +  3 CO2}}

===In nature ===
Iron is stored in many organisms in the form of [[ferritin]], which is a ferrous oxide encased in a solubilizing protein sheath.<ref>{{cite journal|doi=10.1021/cr5004908|title=Unity in the Biochemistry of the Iron-Storage Proteins Ferritin and Bacterioferritin |year=2015 |last1=Honarmand Ebrahimi |first1=Kourosh |last2=Hagedoorn |first2=Peter-Leon |last3=Hagen |first3=Wilfred R. |journal=Chemical Reviews |volume=115 |issue=1 |pages=295–326 |pmid=25418839 |doi-access=free }}</ref>

Species of [[Dissimilatory metal-reducing bacteria|bacteria]], including ''[[Shewanella oneidensis]]'', ''[[Geobacter sulfurreducens]]'' and ''[[Geobacter metallireducens]]'', use iron oxides as [[Electron acceptor|terminal electron acceptors]].<ref>{{cite journal|last1=Bretschger|first1=O.|last2=Obraztsova|first2=A.|last3=Sturm|first3=C. A.|last4=Chang|first4=I. S.|last5=Gorby|first5=Y. A.|last6=Reed|first6=S. B.|last7=Culley|first7=D. E.|last8=Reardon|first8=C. L.|last9=Barua|first9=S.|last10=Romine|first10=M. F.|last11=Zhou|first11=J.|last12=Beliaev|first12=A. S.|last13=Bouhenni|first13=R.|last14=Saffarini|first14=D.|last15=Mansfeld|first15=F.|last16=Kim|first16=B.-H.|last17=Fredrickson|first17=J. K.|last18=Nealson|first18=K. H.|title=Current Production and Metal Oxide Reduction by Shewanella oneidensis MR-1 Wild Type and Mutants|journal=Applied and Environmental Microbiology|date=20 July 2007|volume=73|issue=21|pages=7003–7012|doi=10.1128/AEM.01087-07|pmid=17644630|pmc=2223255|bibcode=2007ApEnM..73.7003B }}</ref>

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

Iron oxides are important [[pigment]]s, coming in a variety of colors (black, red, yellow).  Among their many advantages, they are inexpensive, strongly colored, and nontoxic.<ref>{{Ullmann|doi=10.1002/14356007.n20_n02|title=Pigments, Inorganic, 3. Colored Pigments |year=2009 |last1=Buxbaum |first1=Gunter |last2=Printzen |first2=Helmut |last3=Mansmann |first3=Manfred |last4=Räde |first4=Dieter |last5=Trenczek |first5=Gerhard |last6=Wilhelm |first6=Volker |last7=Schwarz |first7=Stefanie |last8=Wienand |first8=Henning |last9=Adel |first9=Jörg |last10=Adrian |first10=Gerhard |last11=Brandt |first11=Karl |last12=Cork |first12=William B. |last13=Winkeler |first13=Heinrich |last14=Mayer |first14=Wielfried |last15=Schneider |first15=Klaus |isbn=978-3527306732 }}</ref>

[[Magnetite]] is a component of magnetic recording tapes.

== See also ==
*[[Great Oxidation Event]]
*[[Iron cycle]]
*[[Iron oxide nanoparticle]]
*[[Limonite]]
*[[List of inorganic pigments]]
*[[Iron(II) hydroxide]]

==References==
{{reflist|30em}}

==External links==
{{commons category|Iron oxides}}
* [https://web.archive.org/web/20171118055115/http://www.nano-oxides.com/pdf/Fe2O3_Brochure.pdf Information from 4thNano-Oxides, Inc. on Fe<sub>2</sub>O<sub>3</sub>.]
* [http://chemed.chem.purdue.edu/demos/demosheets/12.3.html The Iron One-Pot Reaction]
* [http://minerals.usgs.gov/minerals/pubs/commodity/iron_oxide/ Iron Oxide Pigments Statistics and Information]
* [https://www.cdc.gov/niosh/npg/npgd0344.html CDC – NIOSH Pocket Guide to Chemical Hazards]

{{Iron compounds}}
{{Oxides}}

{{Authority control}}

[[Category:Iron compounds]]
[[Category:Iron oxide pigments]]
[[Category:Transition metal oxides]]