Iron oxide
Class of chemical compounds composed of iron and oxygen
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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. 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 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
- Fe2O3: iron(III) oxide
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Thermal expansion
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 FeO2Hx (0 < x < 1).
- green rust (FeIII
xFeII
yOH3x + 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.
Iron oxide is also a component of iron-air batteries.