| Limonite | |
|---|---|
 | |
| General | |
| Category | Amorphous, mineraloid |
| Formula | FeO(OH)·nH2O |
| Strunz classification | Unclassified |
| Identification | |
| Color | Various shades of brown and yellow |
| Crystal habit | Fine grained aggregate, powdery coating |
| Cleavage | Absent |
| Fracture | Uneven |
| Mohs scale hardness | 4–5.5 |
| Luster | Earthy |
| Streak | Yellowish brown |
| Diaphaneity | Opaque |
| Specific gravity | 2.9–4.3 |
| Density | 2.7–4.3 g/cm3 |
| References | [1][2][3] |
Limonite (/ˈlaɪməˌnaɪt/) is aniron ore consisting of a mixture ofhydratediron(III) oxide-hydroxides in varying composition. The generic formula is frequently written asFeO(OH)·nH2O, although this is not entirely accurate as the ratio ofoxide tohydroxide can vary quite widely. Limonite is one of the three principaliron ores, the others beinghematite andmagnetite, and has beenmined for the production ofiron since at least 400 BC.[4][5]
Limonite is named for theAncient Greek wordλειμών (leimṓn[leː.mɔ̌ːn]), meaning "wet meadow", orλίμνη (límnē[lím.nɛː]), meaning "marshy lake", as an allusion to its occurrence asbog iron ore inmeadows andmarshes.[6] In its brown form, it is sometimes calledbrown hematite[7] orbrown iron ore.[8]
Limonite is relativelydense with aspecific gravity varying from 2.7 to 4.3.[9] It is usually medium to dark yellowish brown in color. Thestreak of limonite on an unglazed porcelain plate is always yellowish brown, a character which distinguishes it from hematite with a red streak, or from magnetite with a black streak. Thehardness is quite variable, ranging from 1 to 5. In thin section it appears as red, yellow, or brown and has a high index of refraction, 2.0–2.4. Limonite minerals are strongly birefringent, but grain sizes are usually too small for this to be detectable.[10]
Although originally defined as a single mineral, limonite is now recognized as afield term for a mixture of relatedhydratediron oxide minerals,[11] among themgoethite,lepidocrocite,[10]akaganeite,[12] andjarosite.[13] Determination of the precise mineral composition is practical only withX-ray diffraction techniques.[10] Individual minerals in limonite may formcrystals, but limonite does not, although specimens may show a fibrous ormicrocrystalline structure,[14] and limonite often occurs in concretionary forms or in compact and earthy masses; sometimes mammillary,botryoidal, reniform or stalactitic. Because of its amorphous nature, and occurrence in hydrated areas limonite often presents as a clay or mudstone. However, there are limonitepseudomorphs after other minerals such aspyrite.[9] This means that chemical weathering transforms the crystals of pyrite into limonite by hydrating the molecules, but the external shape of the pyrite crystal remains. Limonite pseudomorphs have also been formed from other iron oxides, hematite and magnetite; from the carbonatesiderite and from iron rich silicates such asalmandine garnets.
Limonite usually forms from the hydration of hematite and magnetite, from the oxidation and hydration of iron rich sulfide minerals, and chemical weathering of other iron rich minerals such asolivine,pyroxene,amphibole, andbiotite.[10] It is often the major iron component inlateritic soils, and limonite laterite ores are a source of nickel and potentially cobalt and other valuable metals, present as trace elements.[15][16] It is often deposited in run-off streams from mining operations.
Nickel-rich limonite ores represent the largest reserves of nickel. Such minerals are classified aslateritic nickel ore deposits.[17]
One of the first uses was as apigment. The yellow form produced yellowochre for whichCyprus was famous,[18] while the darker forms produced more earthy tones. Roasting the limonite changed it partially to hematite, producing red ochres,burnt umbers andsiennas.[19] Bog iron ore and limonite mudstones are mined as a source of iron.
Iron caps orgossans of siliceous iron oxide typically form as the result of intensiveoxidation ofsulfide ore deposits.[20] These gossans were used by prospectors as guides to buried ore.
Limonite was mined for its ancillary gold content. The oxidation of sulfide deposits which containedgold, often resulted in the concentration of gold in the iron oxide and quartz of the gossans. The gold of the primary veins was concentrated into the limonites of the deeply weathered rocks. In another example the deeply weathered iron formations ofBrazil served to concentrate gold with the limonite of the resulting soils.
Limonite was one of the earliest materials used as a pigment by humans, and can be seen in Neolithiccave paintings andpictographs.[21]
While the first iron ore was likelymeteoric iron, and hematite was far easier tosmelt, in Africa, where the first evidence of iron metallurgy occurs,[dubious –discuss] limonite is the most prevalent iron ore. Before smelting, as the ore was heated and the water driven off, more and more of the limonite was converted to hematite. The ore was then pounded as it was heated above 1250 °C,[22] at which temperature the metallic iron begins sticking together and non-metallic impurities are thrown off as sparks.[dubious –discuss] Complex systems developed, notably in Tanzania, to process limonite.[23] Nonetheless, hematite and magnetite remained the ores of choice when smelting was bybloomeries, and it was only with the development ofblast furnaces in the 1st century BCE in China[24] and about 1150 CE in Europe,[25] that the brown iron ore of limonite could be used to best advantage.
Bog iron ore and limonite were mined in the US, but this ended with the development of advanced mining techniques.
Goldbearing limonite gossans were productively mined in theShasta County, California mining district.[20] Similar deposits were mined nearRio Tinto inSpain andMount Morgan inAustralia. In theDahlonega gold belt inLumpkin County, Georgia gold was mined from limonite-richlateritic orsaprolite soil.
As saprolite deposits have been exhausted in many mining sites, limonite has become the most prominent source of nickel for use in energy dense batteries.
