Iron oxide

{{Short description|Class of chemical compounds composed of iron and oxygen}} thumb|260px|Electrochemically oxidized iron (rust)

An '''iron oxide''' is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides 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 ores, pigments, catalysts, and in thermite, and occur in hemoglobin. Iron oxides are inexpensive and durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172.

The earliest applications of paint served purely ornamental purposes. Consequently, pigment lacking any adhesive agent—composed mainly of iron oxide was employed in prehistoric cave art around the 15,000s BC in parts of Asia.<ref>{{Cite book |last=Hunter |first=Erica C. D |url=https://archive.org/details/funkwagnallsnewe20mors/page/73/mode/1up?q=%2215th+millennium+BC%22 |title=Funk & Wagnalls new encyclopedia p 73|publisher= New York, Funk & Wagnalls |year=1972 |isbn=978-0-8343-0094-1}}</ref>

==Stoichiometries== thumb|Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3.|261px thumb|upright|Green and reddish brown stains on a limestone core sample, respectively corresponding to oxides/hydroxides of Fe²⁺ and Fe³⁺.

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

* Oxides of Fe^II ** FeO: iron(II) oxide, wüstite * Mixed oxides of Fe^II and Fe^III ** Fe₃O₄: Iron(II,III) oxide, magnetite ** Fe₄O₅<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> ** Fe₅O₆<ref>{{cite journal|title = Synthesis of Fe₅O₆|journal = Science Advances|volume = 1|issue = 5|article-number = e1400260|doi = 10.1126/sciadv.1400260|pmid = 26601196|year = 2015|last1 = Lavina|first1 = Barbara|last2 = Meng|first2 = Yue|pmc = 4640612}}</ref> ** Fe₅O₇<ref name = "oxides">{{cite journal| title = Structural complexity of simple Fe2O3 at high pressures and temperatures| journal = Nature Communications| volume = 7| article-number = 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> ** Fe₂₅O₃₂<ref name="oxides" /> **Fe₁₃O₁₉<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| doi-access = free}}</ref> * Oxides of Fe^III ** Fe₂O₃: iron(III) oxide *** α-Fe₂O₃: alpha phase, hematite *** β-Fe₂O₃: beta phase *** γ-Fe₂O₃: gamma phase, maghemite *** ε-Fe₂O₃: epsilon phase

==Thermal expansion== {| class="wikitable" |+ !Iron oxide ! CTE (× 10⁻⁶ °C⁻¹) |- |Fe₂O₃ |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|bibcode=2017IJHE...42.5306F |issn=0360-3199}}</ref> |- |Fe₃O₄ |>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 (Fe₅HO₈ · 4 H₂O approx., or 5 Fe₂O₃ · 9 H₂O, better recast as FeOOH · 0.4 H₂O) * 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 is triggered, this phase may form FeO₂H_''x'' (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_{3''x'' + ''y'' − ''z''} (A⁻)_''z'' where A⁻ is Cl⁻ or 0.5 {{chem2|SO4(2-)}})

==Reactions== In blast furnaces and related factories, iron oxides are converted to the metal. Typical reducing agents are various forms of carbon. A representative reaction starts with ferric oxide:<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 bacteria, including ''Shewanella oneidensis'', ''Geobacter sulfurreducens'' and ''Geobacter metallireducens'', use iron oxides as 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 pigments, 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 Fe₂O₃.] * [http://chemed.chem.purdue.edu/demos/demosheets/12.3.html The Iron One-Pot Reaction] * [https://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}}

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