Limonite
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LimoniteUSGOV.jpg
Limonite is a hydrated iron(III) oxide hydroxide 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, it has been mined as ore for the production of iron. Limonite is heavy and yellowish-brown. It is not a true mineral, but rather 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 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 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_bog_iron_cm01.jpg
Limonite has been known to form pseudomorphs after other minerals such as pyrite, meaning it replaces a crystal of pyrite with limonite but keeps the external 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 pyrite or siderite.
The hardness is variable, but generally in the 4 - 5.5 range. The specific gravity varies from 2.9 to 4.3.
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. These gossans were used by prospectors as guides to buried ore. 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. Similar deposits were mined in Rio Tinto, Spain and Mt. Morgan, 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.
Limonite from occurrences with consistent color is used as the yellow-brown natural earth pigment ochre.
External links
- Mineral galleries (http://mineral.galleries.com/minerals/oxides/limonite/limonite.htm)
- Mindat (http://www.mindat.org/min-2402.html)
- Gold and limonite (http://www.minelinks.com/alluvial/deposits5.html)de:Limonit