View source for Limonite

{{Short description|Hydrated iron oxide mineral}}
{{Infobox mineral
| name        = Limonite
| boxwidth    = 
| boxbgcolor  = 
| image       = LimoniteUSGOV.jpg
| imagesize   = 260px
| alt         = 
| caption     = 
| category    = Amorphous, mineraloid
| formula     = {{chem2|FeO(OH)*''n''H2O}}
| strunz      = Unclassified
| dana        = 
| symmetry    = 
| unit cell   = 
| molweight   = 
| color       = Various shades of brown and yellow
| colour      = 
| habit       = Fine grained aggregate, powdery coating
| system      = 
| twinning    = 
| cleavage    = Absent
| fracture    = Uneven
| tenacity    = 
| mohs        = 4–5.5
| luster      = Earthy
| streak      = Yellowish brown
| diaphaneity = Opaque
| gravity     = 2.9–4.3
| density     = 2.7–4.3&nbsp;g/cm<sup>3</sup>
| polish      = 
| opticalprop = 
| refractive  = 
| birefringence = 
| pleochroism = 
| 2V          = 
| dispersion  = 
| extinction  = 
| length fast/slow =
| fluorescence = 
| absorption  = 
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| impurities  = 
| alteration  = 
| other       = 
| prop1       = 
| prop1text   = 
| references  = <ref>{{Mindat |title=Limonite |id=2402 |access-date=2011-10-16}}</ref><ref>{{cite web |title=Mineral 1.0: Limonite |url=http://www.spaceman.ca/mineral/index.php?ViewMineral=47 |access-date=2011-10-16}}</ref><ref>{{cite web |title=Limonite (hydrated iron oxide) |url=http://www.galleries.com/minerals/oxides/limonite/limonite.htm |access-date=2011-10-16}}</ref>
}}

'''Limonite''' ({{IPAc-en||ˈ|l|aɪ|m|ə|ˌ|n|aɪ|t|}}) is an [[iron ore]] consisting of a mixture of [[hydrate]]d [[iron(III) oxide-hydroxide]]s in varying composition. The generic formula is frequently written as {{chem2|FeO(OH)*''n''H2O}}, although this is not entirely accurate as the ratio of [[oxide]] to [[hydroxide]] can vary quite widely. Limonite is one of the three principal [[iron ore]]s, the others being [[hematite]] and [[magnetite]], and has been [[mining|mined]] for the production of [[iron]] since at least 400 BC.<ref>MacEachern, Scott (1996) [http://www.panafprehistory.org/images/papers/IRON_AGE_BEGINNINGS_NORTH_OF_THE_MANDARA_MOUNTAINS_CAMEROON_AND_NIGERIA_Scott_MacEachern.pdf "Iron Age beginnings north of the Mandara Mountains, Cameroon and Nigeria"]  pp. 489&ndash;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, {{ISBN|978-0-908307-55-5}}; archived [https://web.archive.org/web/20120311184136/http://www.panafprehistory.org/images/papers/IRON_AGE_BEGINNINGS_NORTH_OF_THE_MANDARA_MOUNTAINS_CAMEROON_AND_NIGERIA_Scott_MacEachern.pdf here] by [[Internet Archive]] on 11 March 2012</ref><ref>Diop-Maes, Louise Marie (1996) [http://www.ankhonline.com/revue/diop_lm_metallurgie_fer_afrique.htm "La question de l'Âge du fer en Afrique" ("The question of the Iron Age in Africa")] ''Ankh'' 4/5: pp. 278&ndash;303, in French; archived [https://web.archive.org/web/20080125103929/http://www.ankhonline.com/revue/diop_lm_metallurgie_fer_afrique.htm here] by [[Internet Archive]] on 25 January 2008</ref>

==Names==
[[File:Limonite bog iron cm01.jpg|thumb|left|[[Bog iron]] ore]]

Limonite is named for the [[Ancient Greek]] word {{wikt-lang|grc|λειμών}} ({{grc-transl|λειμών}} {{IPA|grc|leː.mɔ̌ːn|}}), meaning "[[wet meadow]]", or {{wikt-lang|grc|λίμνη}} ({{grc-transl|λίμνη}} {{IPA|grc|lím.nɛː|}}), meaning "marshy lake", as an allusion to its occurrence as {{em|[[bog iron]] ore}} in [[meadow]]s and [[marsh]]es.<ref name=mindat>{{Mindat |id=2402 |name=Limonite}}</ref> In its brown form, it is sometimes called '''brown hematite'''<ref>{{cite book |editor1-last=Jackson|editor1-first=Julia A. |title=Glossary of Geology |date=1997 |publisher=[[American Geological Institute]] |location=[[Alexandria, Virginia]] |isbn=0-922152-34-9 |edition=4th |chapter=brown hematite}}</ref> or '''brown iron ore'''.{{sfn|Jackson|1997|loc="brown iron ore"}}

==Characteristics==
Limonite is relatively [[density|dense]] with a [[specific gravity]] varying from 2.7 to 4.3.<ref name="Northrop">Northrop, Stuart A. (1959) "Limonite" ''Minerals of New Mexico'' (revised edition) University of New Mexico Press, Albuquerque, New Mexico, pp. 329&ndash;333, {{OCLC|2753195}}</ref> It is usually medium to dark yellowish brown in color. The [[Streak (mineralogy)|streak]] 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. The [[Mohs scale of mineral hardness|hardness]] 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.<ref name=Nesse2000>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=978-0-19-510691-6 |pages=371–372}}</ref>

Although originally defined as a single mineral, limonite is now recognized as a [[Field work|field]] term for a mixture of related [[hydrate]]d [[iron oxide]] minerals,<ref>{{cite book |last1=Klein |first1=Cornelis |last2=Hurlbut |first2=Cornelius S. Jr. |title=Manual of mineralogy: (after James D. Dana) |date=1993 |publisher=Wiley |location=New York |isbn=0-471-57452-X |edition=21st}}</ref> among them [[goethite]], [[lepidocrocite]],<ref name=Nesse2000/> [[akaganeite]],<ref>{{cite journal |last1=Mackay |first1=A. L. |title=β-Ferric oxyhydroxide—akaganéite |journal=Mineralogical Magazine and Journal of the Mineralogical Society |date=December 1962 |volume=33 |issue=259 |pages=270–280 |doi=10.1180/minmag.1962.033.259.02|bibcode=1962MinM...33..270M }}</ref> and [[jarosite]].<ref>{{cite journal |last1=Zuo |first1=Pengfei |last2=Sun |first2=Jiangtao |last3=Liu |first3=Xuefei |last4=Hao |first4=Jinhua |last5=Zheng |first5=Deshun |last6=Li |first6=Yu |title=Two types of jarosite in the early Cambrian sedimentary rocks: Insights for genesis and transformation of jarosite on Mars |journal=Icarus |date=November 2021 |volume=369 |article-number=114651 |doi=10.1016/j.icarus.2021.114651|bibcode=2021Icar..36914651Z }}</ref> Determination of the precise mineral composition is practical only with [[X-ray diffraction]] techniques.<ref name=Nesse2000/> Individual minerals in limonite may form [[crystal]]s, but limonite does not, although specimens may show a fibrous or [[microcrystalline]] structure,<ref>Boswell, P. F. and Blanchard, Roland (1929) "Cellular structure in limonite" ''Economic Geology'' 24(8):&nbsp;pp.&nbsp;791&ndash;796</ref> 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 [[pseudomorph]]s after other minerals such as [[pyrite]].<ref name="Northrop" /> 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|almandine garnets]].

<gallery widths="200px" heights="170px">
File:La Palma Limonit.jpg|Limonite deposited from mine runoff
File:Galena Limonite.jpg|[[Galena]] and limonite
File:GranadaEZ.jpg|Limonite pseudomorphs after garnet
</gallery>

==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]].<ref name=Nesse2000/> It is often the major iron component in [[Laterite|lateritic soils]], and limonite laterite ores are a source of nickel and potentially cobalt and other valuable metals, present as trace elements.<ref>{{cite journal |last1=Rubisov |first1=D.H |last2=Krowinkel |first2=J.M |last3=Papangelakis |first3=V.G |title=Sulphuric acid pressure leaching of laterites — universal kinetics of nickel dissolution for limonites and limonitic/saprolitic blends |journal=Hydrometallurgy |date=November 2000 |volume=58 |issue=1 |pages=1–11 |doi=10.1016/S0304-386X(00)00094-3}}</ref><ref>{{cite journal |last1=Gao |first1=Jian-ming |last2=Cheng |first2=Fangqin |title=Study on the preparation of spinel ferrites with enhanced magnetic properties using limonite laterite ore as raw materials |journal=Journal of Magnetism and Magnetic Materials |date=August 2018 |volume=460 |pages=213–222 |doi=10.1016/j.jmmm.2018.04.010|bibcode=2018JMMM..460..213G |s2cid=125368631 }}</ref> It is often deposited in run-off streams from mining operations.

==Uses==
[[File:Limonite concretion.jpg|thumb|upright|Limonite concretion from the [[spoil bank]] of a uranium mine]]
Nickel-rich limonite ores represent the largest reserves of nickel.  Such minerals are classified as [[lateritic nickel ore deposits]].<ref name="ullmann-1">{{Ullmann|author=Kerfoot, Derek G. E. |title=Nickel|doi=10.1002/14356007.a17_157|year=2005}}</ref>

One of the first uses was as a [[pigment]].  The yellow form produced yellow [[ochre]] for which [[Cyprus]] was famous,<ref>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</ref> while the darker forms produced more earthy tones. Roasting the limonite changed it partially to hematite, producing red ochres, [[burnt umber]]s and [[sienna]]s.<ref>Heckel, George B. (1910) "Iron Oxide Paints". ''Paint, oil and drug review''. 50(4): pp. 14–21,  [https://books.google.com/books?id=7P9YAAAAYAAJ&pg=PA52 page 14]</ref> Bog iron ore and limonite mudstones are mined as a source of iron. 

''Iron caps'' or [[gossan]]s of siliceous iron oxide typically form as the result of intensive [[oxidation]] of [[sulfide]] ore deposits.<ref name="Brown">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, [https://books.google.com/books?id=WbpIAAAAMAAJ&pg=PA15 pages 15&ndash;16], {{OCLC|5458708}}</ref> 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.

==History==
{{main|History of ferrous metallurgy}}{{See|Ochre#History}}
Limonite was one of the earliest materials used as a pigment by humans, and can be seen in Neolithic [[cave painting]]s and [[pictographs]].<ref>Wilford, John Noble (13 October 2011) [https://www.nytimes.com/2011/10/14/science/14paint.html?_r=2&hp "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]] [https://web.archive.org/web/20120103231857/http://www.nytimes.com/2011/10/14/science/14paint.html?_r=1 page 1] and [https://web.archive.org/web/20220617104805/https://www.nytimes.com/2011/10/14/science/14paint.html?_r=3&pagewanted=2 page 2] on 11 March 2012</ref>

While the first iron ore was likely [[meteoric iron]], and hematite was far easier to [[Smelting|smelt]], in Africa, where the first evidence of iron metallurgy occurs,{{dubious|date=April 2022|Ferrous metallurgy in Africa}} 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&nbsp;°C,<ref>Iron oxide becomes metallic iron at roughly 1250°C, almost 300 degrees below iron's melting point of 1538°C.</ref> at which temperature the metallic iron begins sticking together and non-metallic impurities are thrown off as sparks.{{dubious|date=April 2022|Ferrous metallurgy in Africa}} Complex systems developed, notably in Tanzania, to process limonite.<ref>Schmidt, Peter and Avery, Donald H. (22 September 1978) [http://www.sciencemag.org/cgi/content/abstract/201/4361/1085 "Complex Iron Smelting and Prehistoric Culture in Tanzania"] ''Science''201(4361):&nbsp;pp.&nbsp;1085&ndash;1089</ref> Nonetheless, hematite and magnetite remained the ores of choice when smelting was by [[Bloomery|bloomeries]], and it was only with the development of [[blast furnace]]s in the 1st century BCE in China<ref>Wagner, Donald B. (1999) [http://www.staff.hum.ku.dk/dbwagner/EARFE/EARFE.html "The earliest use of iron in China"] {{webarchive|url=https://web.archive.org/web/20060718053841/http://www.staff.hum.ku.dk/dbwagner/EARFE/EARFE.html |date=2006-07-18 }} pp.&nbsp;1&ndash;9 ''In'' Young, Suzanne M. M. ''et al.'' (editors) (1999) ''Metals in Antiquity'' Archaeopress, Oxford, England, {{ISBN|978-1-84171-008-2}}</ref> and about 1150 CE in Europe,<ref>Jockenhövel, Albrecht ''et al.'' (1997) [https://www.uni-muenster.de/UrFruehGeschichte/forschen/maerkischessauerland_engl.html "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. {{OCLC|470699473}}. Archived [https://web.archive.org/web/20130224111115/http://www.uni-muenster.de/UrFruehGeschichte/forschen/maerkischessauerland_engl.html here] by [[WebCite]] on 11 March 2012</ref> 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.<ref name="Brown" /> 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. 

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.

==See also==
* [[Ore genesis]]

==Notes==
{{Reflist}}

==External links==
{{Commons}}
*[https://web.archive.org/web/20050622082227/http://mineral.galleries.com/minerals/oxides/limonite/limonite.htm Mineral galleries]
*[http://www.mindat.org/min-2402.html Mindat]
*[https://web.archive.org/web/20100926205333/http://minelinks.com/alluvial/deposits5.html Gold and limonite]

[[Category:Iron ores]]
[[Category:Rocks]]