Goethite

Goethite is an iron oxyhydroxide mineral with formula α-FeOOH. It commonly forms in oxidizing environments and gives many soils and iron-rich rocks yellow, brown, or reddish colors.

Iron oxyhydroxide: A mineral compound containing iron, oxygen, and hydroxyl groups. Goethite is one member of this mineral family.
Oxidation: A chemical process in which a substance loses electrons or increases its oxidation state. Oxidation of dissolved iron can produce the ferric iron in goethite.
Iron ore: Rock or mineral material mined as a source of iron. Goethite is an iron-bearing constituent of some mined ores.
Oolitic ironstone: An iron-rich sedimentary rock containing small, rounded grains called ooids. Goethite can cement or compose ooids in iron-rich sedimentary deposits.
Iron(III) oxide: An oxide of iron in which iron has the +3 oxidation state. Goethite can dehydrate into hematite, an iron(III) oxide.
Hydrolysis: A chemical reaction in which water participates in breaking or forming chemical bonds. Hydrolysis of iron-bearing solutions contributes to iron hydroxide precipitation.
Ochre: An earth pigment made chiefly from iron oxides and iron oxyhydroxides. Goethite contributes yellow and brown colors to natural ochres.
Limonite: A broad name for mixtures of hydrated iron oxides rather than a single mineral species. Goethite is a defined mineral that often makes up much of limonite.
Crystal system: A classification of crystals based on their symmetry and unit-cell geometry. Goethite crystallizes in the orthorhombic system.
Weathering: The breakdown and alteration of rocks and minerals at Earth’s surface. Goethite commonly forms as iron-bearing materials weather.

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