A general name for a rock rich in hydrous iron oxides, formed by the tropical weathering of iron ore. It is formed in bogs (bog ore or brown ore).
Limonite is a hydrated iron(III) oxide-hydroxide of varying composition.
Together with hematite, it has been mined as ore for the production of iron.
It is not a true mineral, but a mineraloid, and it is composed by a mixture of similar hydrated iron oxide minerals, mostly goethite with lepidocrocite, jarosite, and others. Limonite forms mostly in or near oxidized iron and other metal ore deposits and as sedimentary beds. Limonite may occur as the cementing material in iron rich sandstones.
It is never crystallized, but may have a fibrous or microcrystalline structure, and commonly occurs in concretionary forms or in compact and earthy masses; It is sometimes called brown hematite or brown iron ore.
Limonite has been known to form pseudomorphs after other minerals such as pyrite, meaning that the chemical weathering transforms the crystal of pyrite into limonite but keeps the external shape of the pyrite crystal. It has also been formed from other iron oxides, hematite and magnetite;
Uses of limonite
In the past bog ore or brown iron ore were mined as a source of iron. Iron caps or gossans of siliceous iron oxide typically forms as the result of intensive oxidation of sulfide ore deposits. In addition the oxidation of sulfide deposits which contained gold mineralization often resulted in the concentration of gold in the iron oxide and quartz of the gossans.
Gold bearing limonite gossans were productively mined in the Shasta County, California mining district. In the Dahlonega gold belt in Lumpkin County, Georgia gold was mined from limonite rich lateritic or saprolite soil. In another example the deeply weathered iron formations of Brazil served to concentrate gold with the limonite of the resulting soils.
Limonite from occurrences with consistent color is used as the yellow-brown natural earth pigment ochre.
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