Limonite is an
iron ore consisting of a mixture of hydrated
iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·nH
2O, although this is not entirely accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the two principal iron ores, the other being
hematite, and has been
mined for the production of
iron since at least 2500 BCE.