Magnetite
Magnetite from Bolivia
General
Category
Formula
(repeating unit)		iron(II,III) oxide, Fe2+Fe3+2O4
Strunz classification4.BB.05
Crystal systemIsometric
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m
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 scale hardness5.5–6.5
LusterMetallic
StreakBlack
DiaphaneityOpaque
Specific gravity5.17–5.18
SolubilityDissolves slowly in hydrochloric acid
References[1][2][3][4]
Major varieties
LodestoneMagnetic with definite north and south poles

Magnetite is a mineral and one of the main iron ores. With the chemical formula Fe3O4, it is one of the oxides of iron. Magnetite is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself.[5][6] It is the most magnetic of all the naturally-occurring minerals on Earth.[5][7] Naturally-magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism. Today it is mined as iron ore.

Small grains of magnetite occur in almost all igneous and metamorphic rocks. Magnetite is black or brownish-black with a metallic luster, has a Mohs hardness of 5–6 and leaves a black streak.[5]

The chemical IUPAC name is iron(II,III) oxide and the common chemical name is ferrous-ferric oxide.

Properties

In addition to igneous rocks, magnetite also occurs in sedimentary rocks, including banded iron formations and in lake and marine sediments as both detrital grains and as magnetofossils. Magnetite nanoparticles are also thought to form in soils, where they probably oxidize rapidly to maghemite.[8]

Crystal structure

The chemical composition of magnetite is Fe2+Fe23+O42−. The main details of its structure were established in 1915 one of the first obtained using X-ray diffraction. The structure is inverse spinel, with O2− ions forming a face centered cubic lattice and iron cations occupying interstitial 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 32 O2− ions and unit cell length is a = 0.839 nm.[9]

Magnetite differs from most other iron oxides in that it contains both divalent and trivalent iron.[9]

As a member of the spinel group, magnetite can form solid solutions with similarly structured minerals, including ulvospinel (Fe2TiO4), hercynite (FeAl2O4) and chromite (FeCr2O4). Titanomagnetite, also known as titaniferous magnetite, is a solid solution between magnetite and ulvospinel that crystallizes in many mafic igneous rocks. Titanomagnetite may undergo oxyexsolution during cooling, resulting in ingrowths of magnetite and ilmenite.

Crystal morphology and size

Natural and synthetic magnetite occurs most commonly as octahedral crystals bounded by {111} planes and as rhombic-dodecahedra.[9] Twinning occurs on the {111} plane.

Hydrothermal synthesis usually produce single octahedral crystals which can be as large as 10mm across.[9] In the presence of mineralizers such as 0.1M HI or 2M NH4Cl and at 0.207 MPa at 416-800 °C, magnetite grew as crytals whose shapes were a combination of rhombic-dodechahedra forms.[9] 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.[9]

Reactions

Magnetite has been important in understanding the conditions under which rocks form. Magnetite reacts with oxygen to produce hematite, and the mineral pair forms a buffer that can control oxygen fugacity. Commonly, igneous rocks contain solid solutions of both titanomagnetite and hemoilmenite or titanohematite. Compositions of the mineral pairs are used to calculate how oxidizing was the magma (i.e., the oxygen fugacity of the magma): a range of oxidizing conditions are found in magmas and the oxidation state helps to determine how the magmas might evolve by fractional crystallization. Magnetite also is produced from peridotites and dunites by serpentinization.

Magnetic properties

Lodestones were used as an early form of magnetic compass. Magnetite typically carries the dominant magnetic signature in rocks, and so it has been a critical tool in paleomagnetism, a science important in understanding plate tectonics and as historic data for magnetohydrodynamics and other scientific fields.

The relationships between magnetite and other iron-rich oxide minerals such as ilmenite, hematite, and ulvospinel have been much studied; the reactions between these minerals and oxygen influence how and when magnetite preserves a record of the Earth's magnetic field.

At low temperatures, magnetite undergoes a crystal structure phase transition from a monoclinic structure to a cubic structure known as the Verwey transition. The Verwey transition occurs around 121 K and is dependent on grain size, domain state, and the iron-oxygen stoichiometry.[10] An isotropic point also occurs near the Verwey transition around 130 K, at which point the sign of the magnetocrystalline anisotropy constant changes from positive to negative.[11] The Curie temperature of magnetite is 858 K (585 °C; 1,085 °F).

Distribution of deposits

Magnetite and other heavy minerals (dark) in a quartz beach sand (Chennai, India).

Magnetite is sometimes found in large quantities in beach sand. Such black sands (mineral sands or iron sands) are found in various places, such as Lung Kwu Tan of Hong Kong; California, United States; and the west coast of the North Island of New Zealand.[12] 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. These sedimentary rocks have been used to infer changes in the oxygen content of the atmosphere of the Earth.[13]

Large deposits of magnetite are also found in the Atacama region of Chile; the Valentines region of Uruguay; Kiruna, Sweden; the Pilbara, Midwest and Northern Goldfields regions in Western Australia; the Eyre Peninsula in South Australia; the Tallawang Region of New South Wales; and in the Adirondack region of New York in the United States. Kediet ej Jill, the highest mountain of Mauritania, is made entirely of the mineral. Deposits are also found in Norway, Germany, Italy, Switzerland, South Africa, India, Indonesia, Mexico, Hong Kong, and in Oregon, New Jersey, Pennsylvania, North Carolina, West Virginia, Virginia, New Mexico, Utah, and Colorado in the United States. In 2005, an exploration company, Cardero Resources, discovered a vast deposit of magnetite-bearing sand dunes in Peru. 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.[14]

Magnetite crystals with a cubic habit have been found in just one location: Balmat, St. Lawrence County, New York.[15]

Biological occurrences

Biomagnetism is usually related to the presence of biogenic crystals of magnetite, which occur widely in organisms.[16] These organisms range from 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.[17][18] Biomagnetites account for the effects of weak magnetic fields on biological systems.[19] There is also a chemical basis for cellular sensitivity to electric and magnetic fields (galvanotaxis).[20]

Magnetite magnetosomes in Gammaproteobacteria

Pure magnetite particles are biomineralized in magnetosomes, which are produced by several species of magnetotactic 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.

Several species of birds are known to incorporate magnetite crystals in the upper beak for magnetoreception,[21] which (in conjunction with cryptochromes in the retina) gives them the ability to sense the direction, polarity, and magnitude of the ambient magnetic field.[17][22]

Chitons, a type of mollusk, have a tongue-like structure known as a radula, covered with magnetite-coated teeth, or denticles.[23] The hardness of the magnetite helps in breaking down food, and its magnetic properties may additionally aid in navigation. It has also been proposed that biological magnetite may store information.[24]

Human brain

There is also evidence that magnetite exists in the human brain,[18] where it is theorized to affect long-term memory.[25] Some researchers also suggest that humans possess a magnetic sense,[26] proposing that this could allow certain people to use magnetoreception for navigation.[27] 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.[28]

Electron microscope scans 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 rounded nanoparticles. 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. A further eight, aged 62 to 92, came from Manchester, and some had died with varying severities of neurodegenerative diseases.[29] According to researchers led by Prof. Barbara Maher at Lancaster University and published in the Proceedings of the National Academy of Sciences, such particles could conceivably contribute to diseases like Alzheimer's disease. Though a causal link has not been established, laboratory studies suggest that iron oxides like magnetite are a component of protein plaques in the brain, linked to Alzheimer's disease.[30]

Applications

Due to its high iron content, magnetite has long been a major iron ore.[31] It is reduced in blast furnaces to pig iron or sponge iron for conversion to steel.

Magnetic recording

Audio recording using magnetic acetate tape was developed in the 1930s. The German magnetophon utilized magnetite powder as the recording medium.[32] Following World War II, 3M Company continued work on the German design. In 1946, the 3M researchers found they could improve the magnetite-based tape, which utilized powders of cubic crystals, by replacing the magnetite with needle-shaped particles of gamma ferric oxide (γ-Fe2O3).[32]

Catalysis

Magnetite is the catalyst for the industrial synthesis of ammonia.[33] It is also used to catalyze the breakdown of hydrogen peroxide into hydroxyl free radicals and to decompose organic contaminants such as p-nitrophenol (p-NP), which results from chemical processes such as oil refining, petrochemical manufacturing, pulp and paper mills, and the production of many paints, plastics, and pesticides.[34] The removal of such contaminants is an important environmental application.

Magnetite Nanoparticles

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.[35] 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.[36][37] These heavy metals can enter watersheds due to a variety of industrial processes that produce them, which are in use across the country. Being able to remove contaminants from potential drinking water for citizens is an important application, as it greatly reduces the health risks associated with drinking contaminated water.

Another application of magnetic nanoparticles is in the creation of ferrofluids. These are used in several ways, in addition to being fun to play with. Ferrofluids can be used for targeted drug delivery in the human body.[35] 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 in magnetic resonance imaging (MRI) technology.[38]

See also

References

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  19. ^ Kirschvink, J L; Kobayashi-Kirschvink, A; Diaz-Ricci, J C; Kirschvink, S J (1992). "Magnetite in human tissues: a mechanism for the biological effects of weak ELF magnetic fields". Bioelectromagnetics. Suppl 1: 101–13. PMID 1285705. A simple calculation shows that magnetosomes moving in response to earth-strength ELF fields are capable of opening trans-membrane ion channels, in a fashion similar to those predicted by ionic resonance models. Hence, the presence of trace levels of biogenic magnetite in virtually all human tissues examined suggests that similar biophysical processes may explain a variety of weak field ELF bioeffects. ((cite journal)): Unknown parameter |laysummary= ignored (help)
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  21. ^ Kishkinev, D A; Chernetsov, N S (2014). "[Magnetoreception systems in birds: a review of current research]". Zhurnal obshcheĭ biologii. 75 (2): 104–23. There are good reasons to believe that this visual magnetoreceptor processes compass magnetic information which is necessary for migratory orientation. ((cite journal)): Unknown parameter |laysummary= ignored (help)
  22. ^ Wiltschko, Roswitha; Stapput, Katrin; Thalau, Peter; Wiltschko, Wolfgang (2010). "Directional orientation of birds by the magnetic field under different light conditions". Journal of the Royal Society, Interface / the Royal Society. 7 (Suppl 2): S163–77. doi:10.1098/rsif.2009.0367.focus. PMC 2843996. PMID 19864263. ((cite journal)): Unknown parameter |laysource= ignored (help); Unknown parameter |laysummary= ignored (help)
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  24. ^ Bókkon, Istvan; Salari, Vahid (2010). "Information storing by biomagnetites". Journal of biological physics. 36 (1): 109–20. doi:10.1007/s10867-009-9173-9. PMC 2791810. PMID 19728122.
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  34. ^ HE, Hongping; ZHONG, Yuanhong; LIANG, Xiaoliang; TAN, Wei; ZHU, Jianxi; Yan WANG, Christina (2015-05-11). "Natural Magnetite: an efficient catalyst for the degradation of organic contaminant". Scientific Reports. 5. doi:10.1038/srep10139. ISSN 2045-2322. PMC 4426601. PMID 25958854.
  35. ^ a b Lee, Blaney, (2007). "Magnetite (Fe3O4): Properties, Synthesis, and Applications". ((cite journal)): Cite journal requires |journal= (help)CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
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  38. ^ Stephen, Zachary R.; Kievit, Forrest M.; Zhang, Miqin. "Magnetite nanoparticles for medical MR imaging". Materials Today. 14 (7–8): 330–338. doi:10.1016/s1369-7021(11)70163-8.

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