Hydrated iron oxide
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| Names | |
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IUPAC name
Iron(III) oxide-hydroxide
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| Other names
Ferric acid
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| Identifiers | |
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3D model (Jmol)
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| ChemSpider | |
| ECHA InfoCard | 100.039.754 |
| EC Number | 215-176-6 |
| MeSH | Goethite |
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PubChem CID
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| Properties | |
| Appearance | Vivid, dark orange, opaque crystals |
| Odor | odorless |
| Density | 4.25 g/cm3 |
| insoluble at pH 7 | |
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Solubility product (Ksp)
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2.79×10−39 for Fe(OH)3 |
| Hazards | |
| NFPA 704 | |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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| Infobox references | |
A number of chemicals are dubbed iron(III) oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous (FeO(OH)) or hydrated (FeO(OH)·nH2O) forms. The monohydrate (FeO(OH)·H2O) might otherwise be described as iron(III) hydroxide (Fe(OH)3), and is also known as hydrated iron oxide or yellow iron oxide.
Iron(III) oxide-hydroxide occurs naturally as four minerals, the polymorphs denoted by the Greek letters α, β, γ and δ. Goethite, α-FeO(OH), has been used as a pigment since prehistoric times. Akaganéite is the β polymorph, formed by weathering and noted for its presence in some meteorites and the lunar surface. The γ polymorph lepidocrocite is commonly encountered as rust on the inside of steel water pipes and tanks. Feroxyhyte (δ) is formed under the high pressure conditions of sea and ocean floors, being thermodynamically unstable with respect to the α polymorph (goethite) at surface conditions. It also occurs as siderogel – a colloid – and limonite, which is a commonly found mixture of mainly goethite, lepidocrocite, quartz and clay minerals. Goethite and lepidocrocite, both crystallizing in orthorhombic system, are the most common forms of iron(III) oxide-hydroxide and the most important mineral carriers of iron in soils. The mineral ferrihydrite, also a soil constituent, is a related compound.
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