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)·nH2O, although this is not entirely accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the three principal iron ores, the others being hematite and magnetite, and has been mined for the production of iron since at least 2500 BP

Limonite is named for the Greek word λειμών (/leː.mɔ̌ːn/), meaning "wet meadow", or λίμνη (/lím.nɛː/), meaning “marshy lake” as an allusion to its occurrence as bog iron ore in meadows and marshes. In its brown form, it is sometimes called brown hematite or brown iron ore.

Characteristics
Limonite is relatively dense with a specific gravity varying from 2.7 to 4.3. It is usually medium to dark yellowish-brown in color. The streak of limonite on an unglazed porcelain plate is always yellowish brown, a character that distinguishes it from hematite with a red streak, or from magnetite with a black streak. The hardness is quite variable, ranging from 1 to 5. In the thin section, it appears as red, yellow, or brown and has a high index of refraction, 2.0 - 2.4. Limonite minerals are strongly birefringent, but grain sizes are usually too small for this to be detectable.

Although originally defined as a single mineral, limonite is now recognized as a field term for a mixture of related hydrated iron oxide minerals, among them goethite, lepidocrocite, akaganeite, and jarosite. Determination of the precise mineral composition is practical only with X-ray diffraction techniques. Individual minerals in limonite may form crystals, but limonite does not, although specimens may show a fibrous or microcrystalline structure, and limonite often occurs in concretionary forms or in compact and earthy masses; sometimes mammillary, botryoidal, reniform, or stalactitic. Because of its amorphous nature, and occurrence in hydrated areas limonite often presents as a clay or mudstone. However, there are limonite pseudomorphs after other minerals such as pyrite. This means that chemical weathering transforms the crystals of pyrite into limonite by hydrating the molecules, but the external shape of the pyrite crystal remains. Limonite pseudomorphs have also been formed from other iron oxides, hematite, and magnetite; from the carbonate siderite, and from iron-rich silicates such as almandine garnets.


Source
Wikipedia