Iron(III) oxide-hydroxide or ferric oxyhydroxide is the chemical compound of iron, oxygen, and hydrogen with formula . The compound is often encountered as one of its hydrates, ·n [rust]. The monohydrate · is often referred to as iron(III) hydroxide , hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42. Natural occurrences Minerals Anhydrous ferric hydroxide occurs in the nature as the exceedingly rare mineral bernalite, Fe(OH)3·nH2O (n=0.0-0.25). Iron oxyhydroxides, , 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 or . 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. Non-mineral 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. Mineraloids 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 •1.8 but with widely variable hydration. Properties 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. Structure The crystal structure of β- (akaganeite) is that of hollandite or . The unit cell is tetragonal with a=1.048 and c=0.3023 nm, and contains eight formula units of FeOOH. Its dimensions are about 500 × 50 × 50 nm. Twinning often produces particles with the shape of hexagonal stars. Chemistry On heating, β- decomposes and recrystallizes as α- (hematite). Uses Limonite, a mixture of various hydrates and polymorphs of ferric oxyhydroxide, is one of the three major iron ores, having been used since at least 2500 BC. Yellow iron oxide, or Pigment Yellow 42, is Food and Drug Administration