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Concept
Iron(III) oxide-hydroxide
Summary
Iron(III) oxide-hydroxide or ferric oxyhydroxide is the chemical compound of iron, oxygen, and hydrogen with formula FeO(OH).
The compound is often encountered as one of its hydrates, FeO(OH)·nH2O [rust]. The monohydrate FeO(OH)·H2O is often referred to as iron(III) hydroxide Fe(OH)3, hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42.
Anhydrous ferric hydroxide occurs in the nature as the exceedingly rare mineral bernalite, Fe(OH)3·nH2O (n = 0.0–0.25). Iron oxyhydroxides, FeOOH, are much more common and occur naturally as structurally different minerals (polymorphs) denoted by the Greek letters α, β, γ and δ.
Goethite, α-FeO(OH), has been used as an ochre pigment since prehistoric times.
Akaganeite is the β polymorph, formed by weathering and noted for its presence in some meteorites and the lunar surface. However, recently it has been determined that it must contain some chloride ions to stabilize its structure, so that its more accurate formula is FeO0.833(OH)1.167Cl0.167 or Fe6O5(OH)7Cl.
Lepidocrocite, the γ polymorph, 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.
Siderogel is a naturally occurring colloidal form of iron(III) oxide-hydroxide.
Goethite and lepidocrocite, both crystallizing in orthorhombic system, are the most common forms of iron(III) oxyhydroxide and the most important mineral carriers of iron in soils.
Iron(III) oxyhydroxide is the main component of other minerals and mineraloids:
Limonite is a commonly occurring mixture of mainly goethite, lepidocrocite, quartz and clay minerals.
Ferrihydrite is an amorphous or nanocrystalline hydrated mineral, officially FeOOH·1.8H2O but with widely variable hydration.
The color of iron(III) oxyhydroxide ranges from yellow through dark-brown to black, depending on the degree of hydration, particle size and shape, and crystal structure.
Official source
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