Iron oxide

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Electrochemically oxidized iron (rust)

An iron oxide is a

Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust.[1]

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+.

Iron oxides feature as

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.

Thermal expansion

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

Oxide-hydroxides

Main article: iron(III) oxide-hydroxide
  • 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 FeO2Hx (0 < x < 1).[8]
  • green rust (FeIII
    x    FeII
    y    OH3x + 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:[9]

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.[10]

Species of

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

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.[12]

Magnetite is a component of magnetic recording tapes.

See also

References

  1. ISBN 978-3-527-30274-1
    .
  2. PMID 21969537
    .
  3. PMID 26601196
    .
  4. ^
    PMID 26864300
    .
  5. S2CID 54496448
    .
  6. ^
    ISSN 0360-3199
    .
  7. S2CID 205257075
    .
  8. PMID 28143928
    .
  9. ISBN 978-0-08-037941-8
    .
  10. PMID 25418839
    .
  11. PMID 17644630
    .
  12. ISBN 978-3527306732
    .
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