A generic term for a group of hydrous iron oxides, mostly amorphous. Goethite and haematite are important constituents, together with colloidal silica, clays, and manganese oxides. Limonite is formed by direct precipitation from marine or fresh water in shallow seas, lagoons, and bogs (thus it is often called bog iron ore) and by oxidation of iron-rich minerals. It is used as an ore of iron and as a pigment.