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Limonite () is an iron ore consisting of a mixture of iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as , although this is not entirely accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the three principal , the others being hematite and magnetite, and has been mining for the production of iron since at least 400 BC.MacEachern, Scott (1996) "Iron Age beginnings north of the Mandara Mountains, Cameroon and Nigeria" pp. 489–496 In Pwiti, Gilbert and Soper, Robert (editors) (1996) Aspects of African Archaeology: Proceedings of the Tenth Pan-African Congress University of Zimbabwe Press, Harare, Zimbabwe, ; archived here by Internet Archive on 11 March 2012Diop-Maes, Louise Marie (1996) "La question de l'Âge du fer en Afrique" ("The question of the Iron Age in Africa") Ankh 4/5: pp. 278–303, in French; archived here by Internet Archive on 25 January 2008
Although originally defined as a single mineral, limonite is now recognized as a Field work term for a mixture of related iron oxide minerals,
One of the first uses was as a pigment. The yellow form produced yellow ochre for which Cyprus was famous,Constantinou, G. and Govett, G. J. S. (1972). "Genesis of sulphide deposits, ochre and umber of Cyprus". Transactions of the Institution of Mining and Metallurgy. 81: pp. 34–46 while the darker forms produced more earthy tones. Roasting the limonite changed it partially to hematite, producing red ochres, and .Heckel, George B. (1910) "Iron Oxide Paints". Paint, oil and drug review. 50(4): pp. 14–21, page 14 Bog iron ore and limonite mudstones are mined as a source of iron.
Iron caps or of siliceous iron oxide typically form as the result of intensive oxidation of sulfide ore deposits.Brown, G. Chester (1915) Mines and mineral resources of Shasta county, Siskiyou county, Trinity county California State Mining Bureau, California State Printing Office, Sacramento, California, pages 15–16, 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 contained gold, 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 of Brazil served to concentrate gold with the limonite of the resulting soils.
While the first iron ore was likely meteoric iron, and hematite was far easier to Smelting, in Africa, where the first evidence of iron metallurgy occurs, 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,Iron oxide becomes metallic iron at roughly 1250°C, almost 300 degrees below iron's melting point of 1538°C. at which temperature the metallic iron begins sticking together and non-metallic impurities are thrown off as sparks. Complex systems developed, notably in Tanzania, to process limonite.Schmidt, Peter and Avery, Donald H. (22 September 1978) "Complex Iron Smelting and Prehistoric Culture in Tanzania" Science201(4361): pp. 1085–1089 Nonetheless, hematite and magnetite remained the ores of choice when smelting was by Bloomery, and it was only with the development of in the 1st century BCE in ChinaWagner, Donald B. (1999) "The earliest use of iron in China" pp. 1–9 In Young, Suzanne M. M. et al. (editors) (1999) Metals in Antiquity Archaeopress, Oxford, England, and about 1150 CE in Europe,Jockenhövel, Albrecht et al. (1997) "Archaeological Investigations on the Beginning of Blast Furnace-Technology in Central Europe" Abteilung für Ur- und Frühgeschichtliche Archäologie, Westfälische Wilhelms-Universität Münster; abstract published as: Jockenhövel, A. (1997) "Archaeological Investigations on the Beginning of Blast Furnace-Technology in Central Europe". In Crew, Peter and Crew, Susan (editors) (1997) Early Ironworking in Europe: Archaeology and Experiment: Abstracts of the International Conference at Plas Tan y Bwlch 19–25 Sept. 1997 (Plas Tan y Bwlch Occasional Papers No 3) Snowdonia National Park Study Centre, Gwynedd, Wales, pp. 56–58. . Archived here by WebCite on 11 March 2012 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 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 laterite or saprolite 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.
(1993). 047157452X, Wiley. 047157452X among them goethite, lepidocrocite, akaganeite, and jarosite. Determination of the precise mineral composition is practical only with X-ray diffraction techniques. Individual minerals in limonite may form , but limonite does not, although specimens may show a fibrous or microcrystalline structure,Boswell, P. F. and Blanchard, Roland (1929) "Cellular structure in limonite" Economic Geology 24(8): pp. 791–796 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 limonite after other minerals such as pyrite. 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 carbonate siderite and from iron rich silicates such as Almandine.
Formation
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 as olivine, pyroxene, amphibole, and biotite. It is often the major iron component in Laterite, and limonite laterite ores are a source of nickel and potentially cobalt and other valuable metals, present as trace elements. It is often deposited in run-off streams from mining operations.
Uses
Nickel-rich limonite ores represent the largest reserves of nickel. Such minerals are classified as lateritic nickel ore deposits.
History
Limonite was one of the earliest materials used as a pigment by humans, and can be seen in Neolithic and pictographs.Wilford, John Noble (13 October 2011) "In African Cave, Signs of an Ancient Paint Factory" The New York Times; hardcopy published 14 October 2011 under title "African Cave, Ancient Paint Factory Pushes Human Symbolic Thought ‘Far Back’" New York edition page A-14; archived by WebCite page 1 and
target="_blank" rel="nofollow"> page 2 on 11 March 2012
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