Magnetite
Template:Short description Template:About Template:Distinguish Template:Infobox mineral
Magnetite is a mineral and one of the main iron ores, with the chemical formula Template:Chem2. It is one of the oxides of iron, and is ferrimagnetic;<ref name="Ferrimag2005">Template:Cite book</ref> it is attracted to a magnet and can be magnetized to become a permanent magnet itself.<ref name="Dana">Template:Cite book</ref><ref name="Wasilewski">Template:Cite journal</ref> With the exception of extremely rare native iron deposits, it is the most magnetic of all the naturally occurring minerals on Earth.<ref name="Dana" /><ref>Template:Cite journal</ref> Naturally magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism.<ref name="Tremolet">Template:Cite book</ref>
Magnetite is black or brownish-black with a metallic luster, has a Mohs hardness of 5–6 and leaves a black streak.<ref name="Dana" /> Small grains of magnetite are very common in igneous and metamorphic rocks.<ref>Template:Cite book</ref>
The chemical IUPAC name is iron(II,III) oxide and the common chemical name is ferrous-ferric oxide.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>
Crystal structure
The chemical composition of magnetite is Fe2+(Fe3+)2(O2-)4. This indicates that magnetite contains both ferrous (divalent) and ferric (trivalent) iron, suggesting crystallization in an environment containing intermediate levels of oxygen.<ref>Template:Cite book</ref><ref name=Schwertmann /> The main details of its structure were established in 1915. It was one of the first crystal structures to be obtained using X-ray diffraction. It has an inverse spinel structure, 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 thirty-twoTemplate:NbspO2- ions and unit cell length is a = 0.839 nm.<ref name=Schwertmann>Template:Cite book</ref><ref>an alternative visualisation of the crystal structure of Magnetite using JSMol is found here.</ref>
As a member of the inverse spinel group, magnetite can form solid solutions with similarly structured minerals, including ulvospinel (Template:Chem2) and magnesioferrite (Template:Chem2).Template:Sfn
Titanomagnetite, also known as titaniferous magnetite, is a solid solution between magnetite and ulvospinel that crystallizes in many mafic igneous rocks. Titanomagnetite may undergo oxy-exsolution during cooling, resulting in ingrowths of magnetite and ilmenite.Template:Sfn
Crystal morphology and size
Natural and synthetic magnetite occurs most commonly as octahedral crystals bounded by {111} planes and as rhombic-dodecahedra.<ref name=Schwertmann /> Twinning occurs on the {111} plane.<ref name=Mindat/>
Hydrothermal synthesis usually produces single octahedral crystals which can be as large as Template:Convert across.<ref name=Schwertmann /> In the presence of mineralizers such as 0.1Template:NbspM HI or 2Template:NbspM NH4Cl and at 0.207Template:NbspMPa at 416–800 °C, magnetite grew as crystals whose shapes were a combination of rhombic-dodechahedra forms.<ref name=Schwertmann /> 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.<ref name=Schwertmann />
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 how oxidizing its environment is (the oxygen fugacity). This buffer is known as the hematite-magnetite or HM buffer. At lower oxygen levels, magnetite can form a buffer with quartz and fayalite known as the QFM buffer. At still lower oxygen levels, magnetite forms a buffer with wüstite known 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.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref>
Commonly, igneous rocks contain solid solutions of both titanomagnetite and hemoilmenite or titanohematite. Compositions of the mineral pairs are used to calculate oxygen fugacity: a range of oxidizing conditions are found in magmas and the oxidation state helps to determine how the magmas might evolve by fractional crystallization.<ref>Template:Cite book</ref> Magnetite also is produced from peridotites and dunites by serpentinization.<ref>Template:Cite book</ref>
Magnetic properties
Lodestones were used as an early form of magnetic compass. Magnetite has been a critical tool in paleomagnetism, a science important in understanding plate tectonics and as historic data for magnetohydrodynamics and other scientific fields.Template:Sfn
The relationships between magnetite and other iron 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.<ref>Template:Cite book</ref>
At low temperatures, magnetite undergoes a crystal structure phase transition from a monoclinic structure to a cubic structure known as the Verwey transition. Optical studies show that this metal to insulator transition is sharp and occurs around 120Template:NbspK.<ref>Template:Cite journal</ref> The Verwey transition is dependent on grain size, domain state, pressure,<ref>Template:Cite journal</ref> and the iron-oxygen stoichiometry.<ref>Template:Cite journal</ref> An isotropic point also occurs near the Verwey transition around 130Template:NbspK, at which point the sign of the magnetocrystalline anisotropy constant changes from positive to negative.<ref>Template:Cite book</ref> The Curie temperature of magnetite is Template:Convert.<ref>Template:Cite journal</ref>
If magnetite is in a large enough quantity it can be found in aeromagnetic surveys using a magnetometer which measures magnetic intensities.<ref>Template:Cite web</ref>
Melting point
Template:See also Solid magnetite particles melt at about Template:Convert.<ref>Template:Cite web</ref><ref>Template:Cite book</ref>Template:Rp
Distribution of deposits
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 in Hong Kong; California, United States; and the west coast of the North Island of New Zealand.<ref>Template:Cite encyclopedia</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> These sedimentary rocks have been used to infer changes in the oxygen content of the atmosphere of the Earth.<ref name=Klein>Template:Cite journal</ref>
Large deposits of magnetite are also found in the Atacama region of Chile (Chilean Iron Belt);<ref>Template:Cite journal</ref> the Valentines region of Uruguay;<ref>Template:Cite journal</ref> Kiruna, Sweden;<ref>Template:Cite journal</ref> the Tallawang region of New South Wales;<ref>Template:Cite journal</ref> and in the Adirondack Mountains of New York in the United States.<ref>Template:Cite journal</ref> Kediet ej Jill, the highest mountain of Mauritania, is made entirely of the mineral.<ref>European Space Agency, esa.int (access: August 2, 2020)</ref> 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.<ref>Template:Cite book</ref> Deposits are also found in Norway, Romania, and Ukraine.Template:Sfn Magnetite-rich sand dunes are found in southern Peru.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite news</ref>
In large enough quantities magnetite can affect compass navigation. In Tasmania there 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.<ref>Template:Cite web</ref>
Magnetite crystals with a cubic habit are rare but have been found at Balmat, St. Lawrence County, New York,<ref>Template:Cite journal</ref><ref name="minerals">Template:Cite web</ref> and at Långban, Sweden.<ref>Template:Cite journal</ref> This habit may be a result of crystallization in the presence of cations such as zinc.<ref>Template:Cite journal</ref>
Magnetite can also be found in fossils due to biomineralization and are referred to as magnetofossils.<ref>Template:Cite journal</ref> There are also instances of magnetite with origins in space coming from meteorites.<ref>Template:Cite journal</ref>
Biological occurrences
Biomagnetism is usually related to the presence of biogenic crystals of magnetite, which occur widely in organisms.<ref name=Magnetite-based_magnetoreception>Template:Cite journal</ref> These organisms range from magnetotactic 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.<ref name=PMID_25587420/><ref name=Kirschvink_1992>Template:Cite journal</ref> Biomagnetites account for the effects of weak magnetic fields on biological systems.<ref name=Mechanism_for_biological_effects>Template:Cite journal</ref> There is also a chemical basis for cellular sensitivity to electric and magnetic fields (galvanotaxis).<ref name=galvanotaxis>Template:Cite journal</ref>
Pure magnetite particles are biomineralized in magnetosomes, which are produced by several species of magnetotactic bacteria. Magnetosomes consist of long chains of oriented magnetite particles 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 of anaerobic bacteria that are not magnetotactic can also create magnetite in oxygen free sediments by reducing amorphic ferric oxide to magnetite.<ref>Template:Cite web</ref>
Several species of birds are known to incorporate magnetite crystals in the upper beak for magnetoreception,<ref name=Magnetoreception_review>Template:Cite journal</ref> which (in conjunction with cryptochromes in the retina) gives them the ability to sense the direction, polarity, and magnitude of the ambient magnetic field.<ref name=PMID_25587420>Template:Cite journal</ref><ref name=Magnetic_inclination_avian_navigation>Template:Cite journal</ref>
Chitons, a type of mollusk, have a tongue-like structure known as a radula, covered with magnetite-coated teeth, or denticles.<ref name=Chitons>Template:Cite journal</ref> 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.<ref name="Bókkon">Template:Cite journal</ref>
Human brain
Template:See also Living organisms can produce magnetite.<ref name=Kirschvink_1992/> In humans, magnetite can be found in various parts of the brain including the frontal, parietal, occipital, and temporal lobes, brainstem, cerebellum and basal ganglia.<ref name=Kirschvink_1992/><ref name="ReferenceA">Magnetite Nano-Particles in Information Processing: From the Bacteria to the Human Brain Neocortex - Template:ISBN</ref> Iron can be found in three forms in the brain – magnetite, hemoglobin (blood) and ferritin (protein), and areas of the brain related to motor function generally contain more iron.<ref name="ReferenceA"/><ref name="Zecca2004">Template:Cite journal</ref> Magnetite can be found in the hippocampus. The hippocampus is associated with information processing, specifically learning and memory.<ref name="ReferenceA"/> However, magnetite can have toxic effects due to its charge or magnetic nature and its involvement in oxidative stress or the production of free radicals.<ref name="ReferenceB">Template:Cite journal</ref> Research suggests that beta-amyloid plaques and tau proteins associated with neurodegenerative disease frequently occur after oxidative stress and the build-up of iron.<ref name="ReferenceA"/>
Some researchers also suggest that humans possess a magnetic sense,<ref name="Human_magnetoreception">Template:Cite journal</ref> proposing that this could allow certain people to use magnetoreception for navigation.<ref name=human_magnetoreception>Template:Cite journal</ref> 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.<ref name=PMID_20071390>Template:Cite journal</ref>
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. 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.<ref name="ReferenceA"/><ref name="ReferenceB"/> 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.<ref>Template:Cite news</ref> Such particles could conceivably contribute to diseases like Alzheimer's disease.<ref>Template:Cite journal</ref> Though a causal link has not yet been established, laboratory studies suggest that iron oxides such as magnetite are a component of protein plaques in the brain. Such plaques have been linked to Alzheimer's disease.<ref>Template:Cite journal</ref>
Increased iron levels, specifically magnetic iron, have been found in portions of the brain in Alzheimer's patients.<ref name="Qin, Y. 2011">Template:Cite journal</ref> 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<ref name="Zecca2004"/><ref name="Qin, Y. 2011"/> due to the relationship between magnetite and ferritin.<ref name="ReferenceA"/> In tissue, magnetite and ferritin can produce small magnetic fields which will interact with magnetic resonance imaging (MRI) creating contrast.<ref name="Qin, Y. 2011"/> Huntington patients have not shown increased magnetite levels; however, high levels have been found in study mice.<ref name="ReferenceA"/>
Applications
Due to its high iron content, magnetite has long been a major iron ore.<ref>Franz Oeters et al"Iron" in Ullmann's Encyclopedia of Industrial Chemistry, 2006, Wiley-VCH, Weinheim. Template:Doi</ref> It is reduced in blast furnaces to pig iron or sponge iron for conversion to steel.<ref>Template:Cite book</ref>
Magnetic recording
Audio recording using magnetic acetate tape was developed in the 1930s. The German magnetophon first utilized magnetite powder that BASF coated onto cellulose acetate before soon switching to gamma ferric oxide for its superior morphology.<ref name= Schoenherr>Template:Cite web</ref> Following World War II, 3M Company 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 of gamma ferric oxide (γ-Fe2O3).<ref name=Schoenherr/>
Catalysis
Approximately 2–3% of the world's energy budget is allocated to the Haber Process for 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 the oxygen in 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.<ref name="jozwiak">Template:Cite journal</ref><ref name=Appl>Template:Ullmann</ref>
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.<ref name=":0">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
Another application of magnetic nanoparticles is in the creation of ferrofluids. These are used in several ways. Ferrofluids can be used for targeted drug delivery in the human body.<ref name=":0" /> 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.<ref>Template:Cite journal</ref>
Coal mining industry
For the separation of coal from waste, dense medium baths are used. This technique employs the difference in densities between coal (1.3–1.4 tonnes per m3) and shales (2.2–2.4 tonnes per m3). In a medium with intermediate density (water with magnetite), stones sink and coal floats.<ref>Template:Cite journal</ref>
Magnetene
Magnetene is a two-dimensional flat sheet of magnetite noted for its ultra-low-friction properties.<ref>Template:Cite web</ref>
Gallery
-
Octahedral crystals of magnetite up to 1.8 cm across, on cream colored feldspar crystals, locality: Cerro Huañaquino, Potosí Department, Bolivia
-
Magnetite crystals with epitaxial elevations on their faces
-
Magnetite in contrasting chalcopyrite matrix
-
Magnetite with a rare cubic habit from St. Lawrence County, New York
See also
- Bluing (steel), a process in which steel is partially protected against rust by a layer of magnetite
- Buena Vista Iron Ore District
- Corrosion product
- Ferrite
- Greigite
- Magnesia (in natural mixtures with magnetite)
- Mill scale
- Magnes the shepherd
- Rainbow lattice sunstone
References
Further reading
External links
- Mineral galleries Template:Webarchive
- Bio-magnetics
- Magnetite mining in New Zealand Accessed 25-Mar-09