Limonite is an ore consisting in a mixture of hydrated iron(III) oxide-hydroxide
Iron(III) oxide-hydroxide
A number of species are dubbed iron oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous or hydrated forms...

 of varying composition. The generic formula is frequently written as FeO(OH)·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.

Together with hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

, it has been mined
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

 as ore for the production of iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...