Iron oxide

File:Almindeligt rust - jernoxid.jpg

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.

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

File:IronOxidePigmentUSGOV.jpg

Iron oxide pigment. The brown color indicates that iron is at the oxidation state +3.

File:Red and green iron oxides.jpg

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
  • Fe₅O₆
  • Fe₅O₇
  • Fe₂₅O₃₂
  • Fe₁₃O₁₉
• 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

Iron oxide	CTE (× 10−6 °C−1)
Fe2O3	14.9
Fe3O4	>9.2
FeO	12.1

Oxide-hydroxides

• 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 FeO₂H_x (0 < x < 1).
• green rust (Fe^III
  _xFe^II
  _yOH_{3x + y − z} (A⁻)_z where A⁻ is Cl⁻ or 0.5 SO2−4)

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:

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.

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.

See also

• Great Oxidation Event
• Iron cycle
• Iron oxide nanoparticle
• Limonite
• List of inorganic pigments
• Iron(II) hydroxide

External links

• Information from 4thNano-Oxides, Inc. on Fe₂O₃.
• The Iron One-Pot Reaction
• Iron Oxide Pigments Statistics and Information
• CDC – NIOSH Pocket Guide to Chemical Hazards