Iron(III) oxide-hydroxide

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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.

Video Iron(III) oxide-hydroxide

Natural occurrences

Iron(III) oxide-hydroxide occurs naturally as four minerals, the polymorphs denoted by the Greek letters ?, ?, ? and ?. Goethite, ?-FeO(OH), has been used as a pigment since prehistoric times. Akaganéite is the ? polymorph, formed by weathering and noted for its presence in some meteorites and the lunar surface. The ? polymorph lepidocrocite is commonly encountered as rust on the inside of steel water pipes and tanks. Feroxyhyte (?) is formed under the high pressure conditions of sea and ocean floors, being thermodynamically unstable with respect to the ? 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.

Maps Iron(III) oxide-hydroxide

Uses

Yellow iron oxide (CAS 51274-00-1) is used as a pigment, e.g. Pigment Yellow 42 or C.I. 77492. Pigment Yellow 42 is Food and Drug Administration (FDA) approved for use in cosmetics and is used in some tattoo inks. Solid material color ranges from yellow through dark-brown to black. Iron oxide-hydroxide is also used in aquarium water treatment as a phosphate binder. Recently, two forms of Iron oxide-hydroxides nanoparticles were identified as very good adsorbents for lead removal from aquatic media. Note that CAS numbers are incorrect.