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Limonite
Limonite is an ore consisting in a mixture of 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 mining|mined as iron ore|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 iron|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.
It is named from the Greek language|Greek word for meadow (λειμών), in allusion to its occurrence as "Bog iron|bog-ore" in meadows and marshes.
The Mohs scale of mineral hardness|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 iron|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 near Rio Tinto (River)|Rio Tinto in Spain and Mount Morgan, Queensland|Mount Morgan in Australia. In the Dahlonega gold belt in Lumpkin County, Georgia gold was mined from limonite rich laterite|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''.
See also
- Bog iron
- Iron ore
- Ore genesis
External links
- Abandoned Mine Research & History.
- Mineral galleries
- Mindat
- Gold and limonite
Category:Oxide minerals
Category:Hydroxide minerals
Category:Iron minerals
simple:Limonite
Related Images- Limonite
- Limonite pseudomorphs after Garnet
- Limonite "rind" on goethite?
Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL
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