Limonite () is an
iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
consisting of a mixture of
hydrated
iron(III) oxide-hydroxide
Iron(III) oxide-hydroxide or ferric oxyhydroxideA. L. Mackay (1960): "β-Ferric Oxyhydroxide". ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 32, issue 250, pages 545-557. is the chemical compound of iron, oxygen, ...
s in varying composition. The generic formula is frequently written as FeO(OH)·H
2O, although this is not entirely accurate as the ratio of
oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
to
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...
can vary quite widely. Limonite is one of the three principal
iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
s, the others being
hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
and
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
, and has been
mined for the production of
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
since at least 2500 BP.
Names
![Limonite bog iron cm01](https://upload.wikimedia.org/wikipedia/commons/9/90/Limonite_bog_iron_cm01.jpg)
Limonite is named for the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
word
λειμών (/leː.mɔ̌ːn/), meaning "wet meadow", or
λίμνη (/lím.nɛː/), meaning “marshy lake” as an allusion to its occurrence as ''
bog iron ore'' in
meadow
A meadow ( ) is an open habitat, or field, vegetated by grasses, herbs, and other non-woody plants. Trees or shrubs may sparsely populate meadows, as long as these areas maintain an open character. Meadows may be naturally occurring or artifi ...
s and
marsh
A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found at ...
es.
In its brown form it is sometimes called brown hematite or brown iron ore.
Characteristics
Limonite is relatively
dense
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...
with a
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...
varying from 2.7 to 4.3.
[Northrop, Stuart A. (1959) "Limonite" ''Minerals of New Mexico'' (revised edition) University of New Mexico Press, Albuquerque, New Mexico, pp. 329–333, ] It is usually medium to dark yellowish brown in color. The
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
hardness
In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...
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.
Although originally defined as a single mineral, limonite is now recognized as a
field
Field may refer to:
Expanses of open ground
* Field (agriculture), an area of land used for agricultural purposes
* Airfield, an aerodrome that lacks the infrastructure of an airport
* Battlefield
* Lawn, an area of mowed grass
* Meadow, a grass ...
term for a mixture of related
hydrated
iron oxide minerals, among them
goethite
Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...
,
lepidocrocite
Lepidocrocite (γ-FeO(OH)), also called esmeraldite or hydrohematite, is an iron oxide-hydroxide mineral. Lepidocrocite has an orthorhombic crystal structure, a hardness of 5, specific gravity of 4, a submetallic luster and a yellow-brown stre ...
,
[ ]akaganeite
Akaganeite, also written as the deprecated Akaganéite,Ernst A.J. Burke (2008):Tidying up Mineral Names: an IMA-CNMNC Scheme for Suffixes, Hyphens and Diacritical marks. ''Mineralogical Record'', volume 39, issue 2. is a chloride-containing ir ...
, and jarosite
Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe3(SO4)2(OH)6. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct du ...
. Determination of the precise mineral composition is practical only with X-ray diffraction techniques.[ Individual minerals in limonite may form ]crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
s, but limonite does not, although specimens may show a fibrous or microcrystalline
A microcrystalline material is a crystallized substance or rock that contains small crystals visible only through microscopic examination. There is little agreement on the range of crystal sizes that should be regarded as microcrystalline, but th ...
structure, and limonite often occurs in concretionary forms or in compact and earthy masses; sometimes mammillary, botryoidal
A botryoidal ( ) texture or mineral habit, is one in which the mineral has an external form composed of many rounded segments, named for the Ancient Greek (), meaning "a bunch of grapes".Adjective form: ''botruoeidēs'' This is a common form f ...
, 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
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced b ...
s after other minerals such as pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic Luster (mineralogy), lust ...
. 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
Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...
and from iron rich silicates such as almandine garnets.
File:La Palma Limonit.jpg, Limonite deposited from mine runoff
File:Galena Limonite.jpg, Galena and limonite
File:GranadaEZ.jpg, Limonite pseudomorphs after garnet
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
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
, pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
, amphibole
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...
, and biotite.[ It is often the major iron component in lateritic soils, 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 Lateritic nickel ore deposits are surficial, weathered rinds formed on ultramafic rocks.
They account for 73% of the continental world nickel resources and will be in the future the dominant source for the mining of nickel.
Genesis and types of ni ...
.
One of the first uses was as a pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...
. The yellow form produced yellow ochre
Ochre ( ; , ), or ocher in American English, is a natural clay earth pigment, a mixture of ferric oxide and varying amounts of clay and sand. It ranges in colour from yellow to deep orange or brown. It is also the name of the colours produced ...
for which Cyprus
Cyprus ; tr, Kıbrıs (), officially the Republic of Cyprus,, , lit: Republic of Cyprus is an island country located south of the Anatolian Peninsula in the eastern Mediterranean Sea. Its continental position is disputed; while it is geo ...
was famous, while the darker forms produced more earthy tones. Roasting the limonite changed it partially to hematite, producing red ochres, burnt umber
Umber is a natural brown earth pigment that contains iron oxide and manganese oxide. In its natural form, it is called raw umber. When calcined, the color becomes warmer and it becomes known as burnt umber.
Its name derives from ''terra d'ombr ...
s and sienna
Sienna (from it, terra di Siena, meaning "Siena earth") is an earth pigment containing iron oxide and manganese oxide. In its natural state, it is yellowish brown and is called raw sienna. When heated, it becomes a reddish brown and is call ...
s.
Bog iron ore and limonite mudstones are mined as a source of iron, although commercial mining of them has ceased in the United States.
''Iron caps'' or gossan
Gossan (eiserner hut or eisenhut) is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the ''classic'' gossan or iron cap all that remains is iron oxides and quartz, often ...
s of siliceous iron oxide typically form as the result of intensive oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
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. In addition the oxidation of those sulfide deposits which contained gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
, often resulted in the concentration of gold in the iron oxide and quartz of the gossans. Goldbearing limonite gossans were productively mined in the Shasta County, California
Shasta County (), officially the County of Shasta, is a county in the northern portion of the U.S. state of California. Its population is 182,155 as of the 2020 census, up from 177,223 from the 2010 census. The county seat is Redding.
Shasta ...
mining district. Similar deposits were mined near Rio Tinto Rio Tinto, meaning "red river", may refer to:
Businesses
* Rio Tinto (corporation), an Anglo-Australian multinational mining and resources corporation
** Rio Tinto Alcan, based in Canada
** Rio Tinto Borax in America
*** Rio Tinto Borax Mine, ...
in Spain
, image_flag = Bandera de España.svg
, image_coat = Escudo de España (mazonado).svg
, national_motto = ''Plus ultra'' (Latin)(English: "Further Beyond")
, national_anthem = (English: "Royal March")
, i ...
and Mount Morgan in Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
. In the Dahlonega
The city of Dahlonega () is the county seat of Lumpkin County, Georgia, United States. As of the 2010 census, the city had a population of 5,242, and in 2018 the population was estimated to be 6,884.
Dahlonega is located at the north end of ...
gold belt in Lumpkin County, Georgia
Lumpkin County is a county located in the north central portion of the U.S. state of Georgia. As of the 2020 census, the population was 33,488. Its county seat is Dahlonega.
History
This area was settled by the Cherokee, who also occupied a ...
gold was mined from limonite-rich lateritic
Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by ...
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
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
served to concentrate gold with the limonite of the resulting soils.
History
While the first iron ore was likely meteoric iron
Meteoric iron, sometimes meteoritic iron, is a native metal and early-universe protoplanetary-disk remnant found in meteorites and made from the elements iron and nickel, mainly in the form of the mineral phases kamacite and taenite. Meteoric ir ...
, and hematite was far easier to smelt
Smelt may refer to:
* Smelting, chemical process
* The common name of various fish:
** Smelt (fish), a family of small fish, Osmeridae
** Australian smelt in the family Retropinnidae and species ''Retropinna semoni''
** Big-scale sand smelt ''Ath ...
, 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, 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. Nonetheless, hematite and magnetite remained the ores of choice when smelting was by bloomeries, and it was only with the development of blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric ...
s in the 1st century BCE in China and about 1150 CE in Europe, that the brown iron ore of limonite could be used to best advantage.
Limonite was one of the earliest materials used as a pigment by humans, and can be seen in Neolithic cave painting
In archaeology, Cave paintings are a type of parietal art (which category also includes petroglyphs, or engravings), found on the wall or ceilings of caves. The term usually implies prehistoric origin, and the oldest known are more than 40,000 ye ...
s and pictographs
A pictogram, also called a pictogramme, pictograph, or simply picto, and in computer usage an icon, is a graphic symbol that conveys its meaning through its pictorial resemblance to a physical object. Pictographs are often used in writing and g ...
.[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
WebCite was an on-demand archive site, designed to digitally preserve scientific and educationally important material on the web by taking snapshots of Internet contents as they existed at the time when a blogger or a scholar cited or quoted ...
br>page 1
an
on 11 March 2012
See also
* Ore genesis
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.
Ore-genesis theories generally involve three components: sou ...
Notes
External links
{{Commons, Limonite
Mineral galleries
Iron ores
Rocks