Iron oxide

Class of chemical compounds composed of iron and oxygen

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Iron oxides are compounds that are made up of iron and oxygen. A number of these oxides are well-known, as they are often non-stoichiometric. Closely related to these are compounds known as oxyhydroxides, of which rust is a familiar example.[1]

Iron oxides and oxyhydroxides are ubiquitous in nature and have significant roles in various geological and biological processes. They can be utilized as iron ores, pigments, catalysts, and are even used in thermite. These oxides also occur in hemoglobin. For pigments, iron oxides are cost-effective and durable, frequently used in paints, coatings, and colored concretes. The typical colors available span the "earthy" spectrum ranging from yellow to black. When used in food, they are labeled under the E number E172.

Different Stoichiometries of Iron 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+.

Iron oxides appear in two primary oxidation states: ferrous (Fe(II)) and ferric (Fe(III)), or often as a mix of both. These oxides present either octahedral or tetrahedral coordinate geometries. Among the various types, only a select few are prevalent at the Earth's surface, namely wüstite, magnetite, and hematite.

Thermal Expansion of Iron Oxides

Iron oxide

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

• 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.[7] Once dehydration is triggered, this phase may form FeO2H_x (0 < _x < 1).[8]
• green rust (Fe_IIIx Fe_IIyOH_3x+y−z (A⁻)_z, where A⁻ is Cl⁻ or 0.5 SO₂₋₄)

Reactions of Iron Oxides

In blast furnaces and various factories, iron oxides are reduced to pure metal, typically using different carbon forms as reducing agents. For instance, ferric oxide reacts as follows:[9]

2 Fe2O3 + 3 C → 4 Fe + 3 CO2

The Presence of Iron Oxides in Nature

Iron is stored in many organisms in the form of ferritin, which encases ferrous oxide in a solubilizing protein sheath.[10]

Species of bacteria like Shewanella oneidensis, Geobacter sulfurreducens, and Geobacter metallireducens use iron oxides as terminal electron acceptors.[11]

Applications of Iron Oxides

Almost all iron ores are oxides, making these materials vital precursors to iron metal and its various alloys.

Iron oxides serve as significant pigments, available in a range of colors like black, red, and yellow. They are favored for being economical, intensely colored, and non-toxic.[12]

Moreover, magnetite is an essential component in magnetic recording tapes.

See Also

For more information, please visit iron oxide pigments for concrete.