Iron oxide

Electrochemically oxidized iron (rust)

Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.^[1]

Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely used by humans, e.g., as iron ores, pigments, catalysts, in thermite (see the diagram) and hemoglobin. Common rust is a form of iron(III) oxide. Iron oxides are widely used as inexpensive, 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.

Oxides

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 Fe2+ and Fe3+.

Oxide of Fe^II
- FeO: iron(II) oxide, wüstite
Mixed oxides of Fe^II and Fe^III
- Fe₃O₄: Iron(II,III) oxide, magnetite
- Fe₄O₅^[2]
- Fe₅O₆^[3]
- Fe₅O₇^[4]
- Fe₂₅O₃₂^[4]
- Fe₁₃O₁₉^[5]
Oxide 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

Hydroxides

iron(II) hydroxide (Fe(OH)₂)
iron(III) hydroxide (Fe(OH)₃), (bernalite)

Role of iron oxides

Oxide/hydroxides

goethite (α-FeOOH),
akaganéite (β-FeOOH),
lepidocrocite (γ-FeOOH),
feroxyhyte (δ-FeOOH),
ferrihydrite ( ${\ce {Fe5HO8.4H2O}}$ approx.), or ${\ce {5Fe2O3.9H2O}}$ , better recast as ${\ce {FeOOH.}}0.4{\ce {H2O}}$
high-pressure FeOOH
schwertmannite (ideally ${\ce {Fe8O8(OH)6(SO).{\mathit {n}}H2O}}$ or ${\ce {Fe^{3+}16O16(OH,SO4)}}_{\text{12-13}}\cdot {\text{10-12}}{\ce {H2O}}$ )^[6]
green rust ( ${\ce {Fe_{\mathit {x}}^{III}Fe_{\mathit {y}}^{II}(OH)}}_{3x+2y-z}{\ce {(A^{-})}}_{z}$ where A⁻ is Cl⁻ or 0.5SO₄²⁻)

Microbial degradation

Several species of bacteria, including Shewanella oneidensis, Geobacter sulfurreducens and Geobacter metallireducens metabolically utilize solid iron oxides as a terminal electron acceptor, reducing Fe(III) oxides to Fe(II) containing oxides.^[7]

References

↑ Cornell, RM; Schwertmann, U (2003). The iron oxides: structure, properties, reactions, occurrences and uses. Wiley VCH. ISBN 3-527-30274-3.
↑ "Discovery of the recoverable high-pressure iron oxide Fe4O5". Proceedings of the National Academy of Sciences. 108 (42): 17281–17285. Oct 2011. doi:10.1073/pnas.1107573108.
↑ "Synthesis of Fe5O6".
1 2 "Structural complexity of simple Fe2O3 at high pressures and temperatures".
↑ "The crystal structures of Mg2Fe2C4O13, with tetrahedrally coordinated carbon, and Fe13O19, synthesized at deep mantle conditions".
↑ http://www.mindat.org/min-7281.html Mindat
↑ Bretschger, O.; Obraztsova, A.; Sturm, C. A.; Chang, I. S.; Gorby, Y. A.; Reed, S. B.; Culley, D. E.; Reardon, C. L.; Barua, S.; Romine, M. F.; Zhou, J.; Beliaev, A. S.; Bouhenni, R.; Saffarini, D.; Mansfeld, F.; Kim, B.-H.; Fredrickson, J. K.; Nealson, K. H. (20 July 2007). "Current Production and Metal Oxide Reduction by Shewanella oneidensis MR-1 Wild Type and Mutants". Applied and Environmental Microbiology. 73 (21): 7003–7012. doi:10.1128/AEM.01087-07.

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