Limonite (left) from Ironwood, Mich., and (right) from Montgomery, Pa.
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Together with hematite, it has been mined as ore for the production of iron. Limonite is heavy and yellowish-brown. It is a very common amorphous substance though can be tricky to find when mined with hematite and bog ore.
It is not a true mineral 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. Also known as the Lemon Rock.
It is never crystallized into macroscopic crystals, but may have a fibrous or microcrystalline structure, and commonly occurs in concretionary forms or in compact and earthy masses; sometimes mammillary, 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 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; the carbonate siderite and iron rich silicates like some garnets.
Gold bearing limonite gossans were productively mined in the Shasta County, California mining district. Similar deposits were mined near Rio Tinto in Spain and Mount Morgan in Australia. In the Dahlonega gold belt in Lumpkin County, Georgia gold was mined from limonite rich lateritic or saprolite soil. The gold of the primary veins was concentrated into the limonites of the deeply weathered rocks. In another example the deeply weathered iron formations of Brazil served to concentrate gold with the limonite of the resulting soils.
Publication No. WO/2010/060144 Published on June 3, Assigned to BHP Billiton SSM Development for Limonite, Saprolite Separation Process (Australian Inventors)
Jun 03, 2010; GENEVA, June 3 -- Damien Krebs, Graham Andrew Reynolds and Harald Theowald Muller, all from Australia, have developed a process...