Limonite is an ore consisting in a mixture of hydrated
iron(III) oxide-hydroxideof 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.
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 oxideminerals, mostly goethitewith 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 sideriteand iron rich silicates like some garnets.
The hardness is variable, but generally in the 4 - 5.5 range. The
specific gravityvaries 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 oxidationof sulfideore deposits. These gossans were used by prospectors as guides to buried ore. In addition the oxidation of sulfide deposits which contained goldmineralization 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, Californiamining district. Similar deposits were mined near Rio Tinto in Spainand Mount Morgan in Australia. In the Dahlonegagold belt in Lumpkin County, Georgiagold was mined from limonite rich lateritic or saprolitesoil. 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 Brazilserved to concentrate gold with the limonite of the resulting soils.
Limonite from occurrences with consistent color is used as the yellow-brown natural "earth"
* [http://www.ironminers.com Abandoned Mine Research & History] .
* [http://mineral.galleries.com/minerals/oxides/limonite/limonite.htm Mineral galleries]
* [http://www.mindat.org/min-2402.html Mindat]
* [http://www.minelinks.com/alluvial/deposits5.html Gold and limonite]
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