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Iron oxide

Class of chemical compounds composed of iron and oxygen

Iron oxide

Class of chemical compounds composed of iron and oxygen

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.{{cite book

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.

Stoichiometries

Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3.
Green and reddish brown stains on a limestone core sample, respectively corresponding to oxides/hydroxides of Fe<sup>2+</sup> and Fe<sup>3+</sup>.

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 FeII
    • FeO: iron(II) oxide, wüstite
  • Mixed oxides of FeII and FeIII
    • Fe3O4: Iron(II,III) oxide, magnetite
    • Fe4O5
    • Fe5O6
    • Fe5O7
    • Fe25O32
    • Fe13O19
  • Oxides of FeIII
    • Fe2O3: iron(III) oxide
      • α-Fe2O3: alpha phase, hematite
      • β-Fe2O3: beta phase
      • γ-Fe2O3: gamma phase, maghemite
      • ε-Fe2O3: epsilon phase

Thermal expansion

Iron oxideCTE (× 10−6 °C−1)
Fe2O3last1=Fakouri Hasanabadifirst1=M.last2=Kokabifirst2=A.H.last3=Nematifirst3=A.last4=Zinatlou Ajabshirfirst4=S.date=February 2017title=Interactions near the triple-phase boundaries metal/glass/air in planar solid oxide fuel cellsjournal=International Journal of Hydrogen Energyvolume=42issue=8pages=5306–5314doi=10.1016/j.ijhydene.2017.01.065bibcode=2017IJHE...42.5306Fissn=0360-3199}}
Fe3O49.2
FeO12.1

Oxide-hydroxides

Main article: 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. Once dehydration is triggered, this phase may form FeO2Hx (0
  • green rust (FeFeOH3x + y − z (A−)z where A− is Cl− or 0.5 )

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: :

In nature

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

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

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.

Magnetite is a component of magnetic recording tapes.

References

References

  1. Hunter, Erica C. D. (1972). "Funk & Wagnalls new encyclopedia p 73". New York, Funk & Wagnalls.
  2. (Oct 2011). "Discovery of the recoverable high-pressure iron oxide Fe4O5". Proceedings of the National Academy of Sciences.
  3. (2015). "Synthesis of Fe5O6". Science Advances.
  4. (2016). "Structural complexity of simple Fe2O3 at high pressures and temperatures". Nature Communications.
  5. (2015). "The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions". American Mineralogist.
  6. (February 2017). "Interactions near the triple-phase boundaries metal/glass/air in planar solid oxide fuel cells". International Journal of Hydrogen Energy.
  7. (2017). "The pyrite-type high-pressure form of FeOOH". Nature.
  8. (2017). "Dehydrogenation of goethite in Earth's deep lower mantle". Proceedings of the National Academy of Sciences.
  9. {{Greenwood&Earnshaw2nd
  10. (2015). "Unity in the Biochemistry of the Iron-Storage Proteins Ferritin and Bacterioferritin". Chemical Reviews.
  11. (20 July 2007). "Current Production and Metal Oxide Reduction by Shewanella oneidensis MR-1 Wild Type and Mutants". Applied and Environmental Microbiology.
  12. (2009). "Pigments, Inorganic, 3. Colored Pigments".
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iron-compounds iron-oxide-pigments transition-metal-oxides
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