Iron oxide

Iron oxide pigment

Role of iron oxides

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

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(II) oxide, wüstite (FeO)
• iron(II,III) oxide, magnetite (Fe₃O₄)
• (Fe₄O₅)
• iron(III) oxide (Fe₂O₃)
  • alpha phase, hematite (α-Fe₂O₃)
  • beta phase, (β-Fe₂O₃)
  • gamma phase, maghemite (γ-Fe₂O₃)
  • epsilon phase, (ε-Fe₂O₃)

Hydroxides

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

Oxide/hydroxides

Main article: iron(III) oxide-hydroxide

• goethite (α-FeOOH),
• akaganéite (β-FeOOH),
• lepidocrocite (γ-FeOOH),
• feroxyhyte (δ-FeOOH),
• ferrihydrite (Fe₅HO₈·4H₂O approx.), or 5Fe₂O₃•9H₂O, better recast as FeOOH•0.4H₂O

• high-pressure FeOOH
• schwertmannite (ideally Fe₈O₈(OH)₆(SO)·nH₂O or Fe³⁺₁₆O₁₆(OH,SO₄)_12-13·10-12H₂O)
• green rust (Fe^III_xFe^II_y(OH)_3x+2y-z(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.

See also

• Limonite
• Iron oxide nanoparticles

External links

• Information from Nano-Oxides, Inc. on Fe₂O₃.
• http://chemed.chem.purdue.edu/demos/demosheets/12.3.html
• http://minerals.usgs.gov/minerals/pubs/commodity/iron_oxide/
• CDC - NIOSH Pocket Guide to Chemical Hazards