Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are nonstoichiometric. 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.
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 Fe2+ and Fe3+.
Ir oxides feature ferrous (Fe(II) or ferric (Fe(III)) or both. Fe adopts an octahedral or tetrahedral coordination geometry. Oxide adopts tetrahedral or octahedral coordination. 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
- Oxide 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
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 ({\displaystyle {\ce {Fe5HO8.4H2O}}} approx.), or {\displaystyle {\ce {5Fe2O3.9H2O}}}, better recast as {\displaystyle {\ce {FeOOH.}}0.4{\ce {H2O}}}
- high-pressure pyrite-structured FeOOH. Once dehydration is triggered, this phase may form {\displaystyle {\ce {FeO2Hx (0<x<1)}}}.
- green rust ({\displaystyle {\ce {Fe_{\mathit {x}}^{III}Fe_{\mathit {y}}^{II}(OH)}}_{3x+2y-z}{\ce {(A^{-})}}_{z}} where A− is Cl− or 0.5SO42−)
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.