Limonite Limonite is a ferric hydrate of varying composition, the generic formula is frequently written as FeO(OH)Â·nH2O, although this is not entirely accurate as Limonite often contains a varying amount of oxide compared to hydroxide.
Together with hematite, it is mined as ore for the production of iron. Limonite is heavy and yellowish-brown. It is not a true mineral, however, but a mixture of similar hydrated iron oxide minerals, mostly Goethite. Limonite forms mostly in or near oxidized iron and other metal ore deposits and as sedimentary beds. It is named from the Greek word for meadow, in allusion to its occurrence as “bog-ore” in meadows and marshes. It is never crystallized, but may have a fibrous or microcrystalline structure, and commonly occurs in concretionary forms or in compact and earthy masses; sometimes mammillated, botryoidal, reniform or stalactitic. The colour presents various shades of brown and yellow, and the streak is always brownish, a character which distinguishes it from hematite with a red, or from magnetite with a black streak. It is sometimes called brown hematite.
Limonite has been known to pseudomorph other minerals such as pyrite, meaning it replaces a crystal of pyrite with limonite but keeps the shape of the pyrite crystal. In many cases it has been formed from other iron oxides, like hematite and magnetite, or by the alteration of pyrites or siderite (chalybite).
Hardness is 4 – 5.5, specific gravity is 2.9 to 4.3.