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Magnetite

From Wikipedia, the free encyclopedia
This article is about the mineral magnetite as found in natural deposits. For other uses, seeIron(II,III) oxide.
Not to be confused withmagnesiteormanganite.
Magnetite
Magnetite from Bolivia
General
Category
Formula
(repeating unit)		iron(II,III) oxide,Fe2+Fe3+2O4
IMA symbolMag[1]
Strunz classification4.BB.05
Crystal systemIsometric
Crystal classHexoctahedral (m3m)
H-M symbol:(4/m32/m)
Space groupFd3m(no. 227)
Unit cella = 8.397 Å; Z = 8
Identification
ColorBlack, gray with brownish tint in reflected sun
Crystal habitOctahedral,fine granular to massive
TwinningOn {Ill} as both twin and composition plane, the spinel law, as contact twins
CleavageIndistinct, parting on {Ill}, very good
FractureUneven
TenacityBrittle
Mohs scalehardness5.5–6.5
LusterMetallic
StreakBlack
DiaphaneityOpaque
Specific gravity5.17–5.18
SolubilityDissolves slowly inhydrochloric acid
References[2][3][4][5]
Major varieties
LodestoneMagnetic with definite north and south poles
Magnetite is one of the very few minerals that isferrimagnetic;it is attracted by amagnetas shown here
Unit cell of magnetite. The gray spheres are oxygen, green are divalent iron, blue are trivalent iron. Also shown are an iron atom in an octahedral space (light blue) and another in a tetrahedral space (gray).

Magnetiteis amineraland one of the mainiron ores,with the chemical formulaFe2+Fe3+2O4.It is one of theoxides of iron,and isferrimagnetic;[6]it is attracted to amagnetand can bemagnetizedto become a permanent magnet itself.[7][8]With the exception of extremely rarenative irondeposits, it is the most magnetic of all the naturally occurring minerals on Earth.[7][9]Naturally magnetized pieces of magnetite, calledlodestone,will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism.[10]

Magnetite is black or brownish-black with a metallic luster, has aMohs hardnessof 5–6 and leaves a blackstreak.[7]Small grains of magnetite are very common inigneousandmetamorphic rocks.[11]

The chemicalIUPACname isiron(II,III) oxideand the common chemical name isferrous-ferric oxide.[12]

Properties[edit]

In addition to igneous rocks, magnetite also occurs insedimentary rocks,includingbanded iron formationsand in lake and marine sediments as both detrital grains and asmagnetofossils.Magnetite nanoparticles are also thought to form in soils, where they probably oxidize rapidly tomaghemite.[13]

Crystal structure[edit]

The chemical composition of magnetite is Fe2+(Fe3+)2(O2-)4.This indicates that magnetite contains bothferrous(divalent) andferric(trivalent) iron, suggesting crystallization in an environment containing intermediate levels of oxygen.[14][15]The main details of its structure were established in 1915. It was one of the first crystal structures to be obtained usingX-ray diffraction.The structure is inversespinel,with O2-ions forming aface-centered cubiclattice and iron cations occupyinginterstitial sites.Half of the Fe3+cations occupy tetrahedral sites while the other half, along with Fe2+cations, occupy octahedral sites. The unit cell consists of thirty-twoO2-ions and unit cell length isa= 0.839 nm.[15][16]

As a member of the inverse spinel group, magnetite can formsolid solutionswith similarly structured minerals, includingulvospinel(Fe2TiO4) andmagnesioferrite(MgFe2O4).[17]

Titanomagnetite, also known as titaniferous magnetite, is a solid solution between magnetite and ulvospinel that crystallizes in manymaficigneous rocks. Titanomagnetite may undergooxy-exsolutionduring cooling, resulting in ingrowths of magnetite and ilmenite.[17]

Crystal morphology and size[edit]

Natural and synthetic magnetite occurs most commonly asoctahedralcrystals bounded by {111} planes and asrhombic-dodecahedra.[15]Twinning occurs on the {111} plane.[3]

Hydrothermal synthesis usually produces single octahedral crystals which can be as large as 10 mm (0.39 in) across.[15]In the presence of mineralizers such as 0.1M HI or 2MNH4Cland at 0.207MPaat 416–800 °C, magnetite grew as crystals whose shapes were a combination of rhombic-dodechahedra forms.[15]The crystals were more rounded than usual. The appearance of higher forms was considered as a result from a decrease in the surface energies caused by the lower surface to volume ratio in the rounded crystals.[15]

Reactions[edit]

Magnetite has been important in understanding the conditions under which rocks form. Magnetite reacts with oxygen to producehematite,and the mineral pair forms abufferthat can control how oxidizing its environment is (theoxygenfugacity). This buffer is known as the hematite-magnetite or HM buffer. At lower oxygen levels, magnetite can form a buffer withquartzandfayaliteknown as the QFM buffer. At still lower oxygen levels, magnetite forms a buffer withwüstiteknown as the MW buffer. The QFM and MW buffers have been used extensively in laboratory experiments on rock chemistry. The QFM buffer, in particular, produces an oxygen fugacity close to that of most igneous rocks.[18][19]

Commonly,igneous rockscontain solid solutions of both titanomagnetite and hemoilmenite or titanohematite. Compositions of the mineral pairs are used to calculate oxygen fugacity: a range ofoxidizing conditionsare found in magmas and the oxidation state helps to determine how the magmas might evolve byfractional crystallization.[20]Magnetite also is produced fromperidotitesanddunitesbyserpentinization.[21]

Magnetic properties[edit]

Lodestones were used as an early form ofmagnetic compass.Magnetite has been a critical tool inpaleomagnetism,a science important in understandingplate tectonicsand as historic data formagnetohydrodynamicsand otherscientific fields.[22]

The relationships between magnetite and other iron oxide minerals such asilmenite,hematite, andulvospinelhave been much studied; thereactionsbetween these minerals and oxygen influence how and when magnetite preserves a record of theEarth's magnetic field.[23]

At low temperatures, magnetite undergoes a crystal structurephase transitionfrom a monoclinic structure to a cubic structure known as theVerwey transition.Optical studies show that this metal to insulator transition is sharp and occurs around 120K.[24]The Verwey transition is dependent on grain size, domain state, pressure,[25]and the iron-oxygenstoichiometry.[26]An isotropic point also occurs near the Verwey transition around 130K, at which point the sign of the magnetocrystalline anisotropy constant changes from positive to negative.[27]TheCurie temperatureof magnetite is 580 °C (853 K; 1,076 °F).[28]

If magnetite is in a large enough quantity it can be found inaeromagnetic surveysusing amagnetometerwhich measures magnetic intensities.[29]

Melting point[edit]

See also:Iron(II,III) oxide § Properties

Solid magnetite particles melt at about 1,583–1,597 °C (2,881–2,907 °F).[30][31]: 794 

Distribution of deposits[edit]

Magnetite and other heavy minerals (dark) in a quartzbeachsand(Chennai,India).

Magnetite is sometimes found in large quantities in beach sand. Suchblack sands(mineral sands oriron sands) are found in various places, such asLung Kwu Tanin Hong Kong;California,United States; and the west coast of theNorth Islandof New Zealand.[32]The magnetite, eroded from rocks, is carried to the beach by rivers and concentrated by wave action and currents. Huge deposits have been found in banded iron formations.[33][34]These sedimentary rocks have been used to infer changes in the oxygen content of the atmosphere of the Earth.[35]

Large deposits of magnetite are also found in theAtacamaregion of Chile (Chilean Iron Belt);[36]theValentinesregion of Uruguay;[37]Kiruna,Sweden;[38]theTallawang regionof New South Wales;[39]and in theAdirondack MountainsofNew Yorkin the United States.[40]Kediet ej Jill,the highest mountain ofMauritania,is made entirely of the mineral.[41]In the municipalities of Molinaseca, Albares, and Rabanal del Camino, in the province of León (Spain), there is a magnetite deposit in Ordovician terrain, considered one of the largest in Europe. It was exploited between 1955 and 1982.[42]Deposits are also found inNorway,Romania,andUkraine.[43]Magnetite-rich sand dunes are found in southern Peru.[44]In 2005, an exploration company, Cardero Resources, discovered a vast deposit of magnetite-bearing sand dunes inPeru.The dune field covers 250 square kilometers (100 sq mi), with the highest dune at over 2,000 meters (6,560 ft) above the desert floor. The sand contains 10% magnetite.[45]

In large enough quantities magnetite can affectcompassnavigation.InTasmaniathere are many areas with highly magnetized rocks that can greatly influence compasses. Extra steps and repeated observations are required when using a compass in Tasmania to keep navigation problems to the minimum.[46]

Magnetite crystals with acubichabit are rare but have been found at Balmat,St. Lawrence County, New York,[47][48]and atLångban, Sweden.[49]This habit may be a result of crystallization in the presence of cations such as zinc.[50]

Magnetite can also be found infossilsdue tobiomineralizationand are referred to asmagnetofossils.[51]There are also instances of magnetite with origins inspacecoming frommeteorites.[52]

Biological occurrences[edit]

Biomagnetismis usually related to the presence of biogenic crystals of magnetite, which occur widely in organisms.[53]These organisms range frommagnetotactic bacteria(e.g.,Magnetospirillum magnetotacticum) to animals, including humans, where magnetite crystals (and other magnetically sensitive compounds) are found in different organs, depending on the species.[54][55]Biomagnetites account for the effects of weak magnetic fields on biological systems.[56]There is also a chemical basis for cellular sensitivity to electric and magnetic fields (galvanotaxis).[57]

Magnetite magnetosomes inGammaproteobacteria

Pure magnetite particles arebiomineralizedinmagnetosomes,which are produced by several species ofmagnetotactic bacteria.Magnetosomes consist of long chains of oriented magnetite particle that are used by bacteria for navigation. After the death of these bacteria, the magnetite particles in magnetosomes may be preserved in sediments as magnetofossils. Some types ofanaerobic bacteriathat are not magnetotactic can also create magnetite in oxygen free sediments by reducing amorphic ferric oxide to magnetite.[58]

Several species of birds are known to incorporate magnetite crystals in the upper beak formagnetoreception,[59]which (in conjunction withcryptochromesin theretina) gives them the ability to sense the direction,polarity,and magnitude of the ambientmagnetic field.[54][60]

Chitons,a type of mollusk, have a tongue-like structure known as aradula,covered with magnetite-coated teeth, ordenticles.[61]The hardness of the magnetite helps in breaking down food.

Biological magnetite may store information about the magnetic fields the organism was exposed to, potentially allowing scientists to learn about the migration of the organism or about changes in the Earth's magnetic field over time.[62]

Human brain[edit]

See also:Particulates § Cognitive hazards and mental health

Living organisms can produce magnetite.[55]In humans, magnetite can be found in various parts of the brain including thefrontal,parietal,occipital,andtemporal lobes,brainstem,cerebellumandbasal ganglia.[55][63]Iron can be found in three forms in the brain – magnetite, hemoglobin (blood) andferritin(protein), and areas of the brain related tomotor functiongenerally contain more iron.[63][64]Magnetite can be found in thehippocampus.The hippocampus is associated with information processing, specifically learning and memory.[63]However, magnetite can have toxic effects due to its charge or magnetic nature and its involvement in oxidative stress or the production offree radicals.[65]Research suggests thatbeta-amyloidplaques andtau proteinsassociated withneurodegenerative diseasefrequently occur after oxidative stress and the build-up of iron.[63]

Some researchers also suggest that humans possess a magnetic sense,[66]proposing that this could allow certain people to use magnetoreception for navigation.[67]The role of magnetite in the brain is still not well understood, and there has been a general lag in applying more modern, interdisciplinary techniques to the study of biomagnetism.[68]

Electron microscopescans of human brain-tissue samples are able to differentiate between magnetite produced by the body's own cells and magnetite absorbed from airborne pollution, the natural forms being jagged and crystalline, while magnetite pollution occurs as roundednanoparticles.Potentially a human health hazard, airborne magnetite is a result of pollution (specifically combustion). These nanoparticles can travel to the brain via the olfactory nerve, increasing the concentration of magnetite in the brain.[63][65]In some brain samples, the nanoparticle pollution outnumbers the natural particles by as much as 100:1, and such pollution-borne magnetite particles may be linked to abnormal neural deterioration. In one study, the characteristic nanoparticles were found in the brains of 37 people: 29 of these, aged 3 to 85, had lived and died in Mexico City, a significant air pollution hotspot. Some of the further eight, aged 62 to 92, from Manchester, England, had died with varying severities of neurodegenerative diseases.[69]Such particles could conceivably contribute to diseases likeAlzheimer's disease.[70]Though a causal link has not yet been established, laboratory studies suggest that iron oxides such as magnetite are a component ofprotein plaquesin the brain. Such plaques have been linked toAlzheimer's disease.[71]

Increased iron levels, specifically magnetic iron, have been found in portions of the brain in Alzheimer's patients.[72]Monitoring changes in iron concentrations may make it possible to detect the loss of neurons and the development of neurodegenerative diseases prior to the onset of symptoms[64][72]due to the relationship between magnetite andferritin.[63]In tissue, magnetite and ferritin can produce small magnetic fields which will interact withmagnetic resonance imaging(MRI) creating contrast.[72]Huntington patients have not shown increased magnetite levels; however, high levels have been found in study mice.[63]

Applications[edit]

Due to its high iron content, magnetite has long been a majoriron ore.[73]It is reduced inblast furnacestopig ironorsponge ironfor conversion tosteel.[74]

Magnetic recording[edit]

Audio recordingusing magnetic acetate tape was developed in the 1930s. The Germanmagnetophonfirst utilized magnetite powder that BASF coated onto cellulose acetate before soon switching to gamma ferric oxide for its superior morphology.[75]FollowingWorld War II,3MCompany continued work on the German design. In 1946, the 3M researchers found they could also improve their own magnetite-based paper tape, which utilized powders of cubic crystals, by replacing the magnetite with needle-shaped particles ofgamma ferric oxide(γ-Fe2O3).[75]

Catalysis[edit]

Approximately 2–3% of the world's energy budget is allocated to theHaber Processfor nitrogen fixation, which relies on magnetite-derived catalysts. The industrial catalyst is obtained from finely ground iron powder, which is usually obtained by reduction of high-purity magnetite. The pulverized iron metal is burnt (oxidized) to give magnetite or wüstite of a defined particle size. The magnetite (or wüstite) particles are then partially reduced, removing some of theoxygenin the process. The resulting catalyst particles consist of a core of magnetite, encased in a shell of wüstite, which in turn is surrounded by an outer shell of iron metal. The catalyst maintains most of its bulk volume during the reduction, resulting in a highly porous high-surface-area material, which enhances its effectiveness as a catalyst.[76][77]

Magnetite nanoparticles[edit]

Magnetite micro- and nanoparticles are used in a variety of applications, from biomedical to environmental. One use is in water purification: in high gradient magnetic separation, magnetite nanoparticles introduced into contaminated water will bind to the suspended particles (solids, bacteria, or plankton, for example) and settle to the bottom of the fluid, allowing the contaminants to be removed and the magnetite particles to be recycled and reused.[78]This method works with radioactive and carcinogenic particles as well, making it an important cleanup tool in the case of heavy metals introduced into water systems.[79]

Another application of magnetic nanoparticles is in the creation offerrofluids.These are used in several ways. Ferrofluids can be used for targeteddrug deliveryin the human body.[78]The magnetization of the particles bound with drug molecules allows "magnetic dragging" of the solution to the desired area of the body. This would allow the treatment of only a small area of the body, rather than the body as a whole, and could be highly useful in cancer treatment, among other things. Ferrofluids are also used inmagnetic resonance imaging(MRI) technology.[80]

Coal mining industry[edit]

For theseparation of coal from waste,dense medium baths were used. This technique employed the difference in densities betweencoal(1.3–1.4 tonnes per m3) and shales (2.2–2.4 tonnes per m3). In a medium with intermediatedensity(water with magnetite), stones sank and coal floated.[81]

Magnetene[edit]

Magnetene is a two-dimensional flat sheet of magnetite noted for its ultra-low-friction properties.[82]

Gallery[edit]

See also[edit]

References[edit]

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External links[edit]

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