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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 · (CAS 51274-00-1, C.I. 77492) is often referred as iron(III) hydroxide , hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42.
Anhydrous ferric oxyhydroxide occurs naturally as four different minerals (polymorphs) denoted by the Greek letters ?, ?, ? and ?.
Goethite, ?-FeO(OH), has been used as an ocher pigment since prehistoric times.
Akaganeite is the ? 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 .
Lepidocrocite, the ? polymorph, 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.
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.
The crystal structure of ?- (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. 
On heating, ?- decomposes and recrystallizes as ?- (hematite).
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 BCE.
^Safoora Rahimi, Rozita M. Moattari, Laleh Rajabi, Ali Ashraf Derakhshan, and Mohammad Keyhani (2015): "Iron oxide/hydroxide (?,?-FeOOH) nanoparticles as high potential adsorbents for lead removal from polluted aquatic media". Journal of Industrial and Engineering Chemistry, volume 23, pages 33-43. doi:10.1016/j.jiec.2014.07.039
^ abTim Grundl and Jim Delwiche (1993): "Kinetics of ferric oxyhydroxide precipitation". Journal of Contaminant Hydrology, volume 14, issue 1, pages 71-87. doi:10.1016/0169-7722(93)90042-Q
^K. H. Gayer and Leo Woontner (1956): "The Solubility of Ferrous Hydroxide and Ferric Hydroxide in Acidic and Basic Media at 25°". Journal of Physical Chemistry, volume 60, issue 11, pages 1569-1571. doi:10.1021/j150545a021
^ abcEgon Matijevi? and Paul Scheiner (1978): "Ferric hydrous oxide sols: III. Preparation of uniform particles by hydrolysis of Fe(III)-chloride, -nitrate, and -perchlorate solutions". Journal of Colloid and Interface Science, volume 63, issue 3, pages 509-524. doi:10.1016/S0021-9797(78)80011-3
Whittemore and Donald Langmuir (1974): "Ferric Oxyhydroxide Microparticles in Water". Environmental Health Perspective, volume 9, pages 173-176. doi:10.1289/ehp.749173