Magnetite

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Magnetite
Magnetite exposed on the ground. The mineral is black and irregularly smooth. Individual chunks jut at angles characteristic of the crystal habit.
Magnetite and pyrite from Piedmont, Italy
General
Category Oxide minerals
Spinel group
Spinel structural group
Formula
(repeating unit)		 iron(II,III) oxide, Fe2+Fe3+2O4
Strunz classification 04.BB.05
Crystal symmetry Isometric 4/m 3 2/m
Unit cell a = 8.397 Å; Z=8
Identification
Color Black, gray with brownish tint in reflected sun
Crystal habit Octahedral, fine granular to massive
Crystal system Isometric Hexoctahedral
Twinning On {Ill} as both twin and composition plane, the spinel law, as contact twins
Cleavage Indistinct, parting on {Ill}, very good
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 5.5–6.5
Luster Metallic
Streak Black
Diaphaneity Opaque
Specific gravity 5.17–5.18
Solubility Dissolves slowly in hydrochloric acid
References [1][2][3][4]
Major varieties
Lodestone Magnetic with definite north and south poles

Magnetite is a mineral and one of the three common naturally-occurring oxides of iron. Its chemical formula is Fe3O4, and it is a member of the spinel group. Magnetite is the most magnetic of all the naturally-occurring minerals on Earth.[5] Naturally-magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism.

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.

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

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.

Magnetite has been very 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 grains of two solid solutions, one of magnetite and ulvospinel and the other of ilmenite and hematite. 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 occurs in many sedimentary rocks, including banded iron formations. In many igneous rocks, magnetite-rich and ilmenite-rich grains occur that precipitated together in magma. Magnetite also is produced from peridotites and dunites by serpentinization.

The Curie temperature of magnetite is 858 K (585 °C; 1,085 °F).

Distribution of deposits

A fine textured sample, ~5cm across
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 California and the west coast of the North Island of New Zealand.[6] The magnetite is carried to the beach via rivers from erosion and is concentrated via 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.

Large deposits of magnetite are also found in the Atacama region of Chile, Valentines region of Uruguay, Kiruna, Sweden, the Pilbara, Midwest and Northern Goldfields regions in Western Australia, New South Wales in the Tallawang Region, 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.[7] Deposits are also found in Norway, Germany, Italy, Switzerland, South Africa, India, Indonesia, Mexico, 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.[8]

Biological occurrences

Biomagnetism is usually related to the presence of biogenic crystals of magnetite, which occur widely in organisms.[9] These organisms range from bacteria (e.g., Magnetospirillum magnetotacticum) to animals and humans, where magnetite crystals (and other magnetically-sensitive compounds) are found in different organs, depending on the species.[10][11] Biomagnetites account for the effects of weak magnetic fields on biological systems.[12] There is also a chemical basis for cellular sensitivity to electric and magnetic fields (galvanotaxis). [13]

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 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,[14] which (in conjunction with cryptochromes in the retina) gives them the ability to sense the direction, polarity, and magnitude of the ambient magnetic field.[10][15]

Chitons, a type of mollusk, have tongue-like structure known as a radula, covered with magnetite-coated teeth, or denticles.[16] 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.[17]

There is also evidence that magnetite exists in the human brain.[11] It is proposed that this could allow certain individuals to use magnetoreception for navigation.[18] More generally, magnetite in the brain is theorized to affect long-term memory.[19]

Applications

Magnetic recording

Audio recording using magnetic acetate tape was developed in the 1930s. The German magnetophon utilized magnetite powder as the recording medium.[20] Following World War II the 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).[20]

Catalysis

Magnetite is the catalyst for the industrial synthesis of ammonia.[21]

Arsenic sorbent

Magnetite powder efficiently removes arsenic(III) and arsenic(V) from water, the efficiency of which increases ~200 times when the magnetite particle size decreases from 300 to 12 nm.[22] Arsenic-contaminated drinking water is a major problem around the world, which can be solved using magnetite as a sorbent.

Other

Because of its stability at high temperatures, it is used for coating industrial watertube steam boilers. The magnetite layer is formed after a chemical treatment (e.g. by using hydrazine).

Iron-metabolizing bacteria can trigger redox reactions in microscopic magnetite particles. Using light, magnetite can reduce chromium (VI) (its toxic form), converting it to less toxic chromium (III), which can then be incorporated into a harmless magnetite crystal. Phototrophic Rhodopseudomonas palustris oxidized the magnetite, while Geobacter sulfurreducens reduced it, readying it for another cycle.[23]

Gallery of magnetite mineral specimens

  • Octahedral crystals of magnetite up to 1.8 cm across, on cream colored feldspar crystals, locality: Cerro Huañaquino, Potosí Department, Bolivia (size: 8.4 x 5.2 x 3.2 cm)

  • Red gem-like crystals of chondrodite with magnetite, Tilly Foster mine, Brewster, New York (size 2.8 x 2.6 x 2.1 cm)

  • Metallic, jet black, complex cubes of magnetite, from the Balmat-Edwards district, St. Lawrence County, New York (field of view, about 4 cm)

See also

References

  1. ↑ Handbook of Mineralogy
  2. ↑ Mindat.org Mindat.org
  3. ↑ Webmineral data
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ Kediet ej Jill
  8. ↑ Ferrous Nonsnotus
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ 10.0 10.1 Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ 11.0 11.1 Lua error in package.lua at line 80: module 'strict' not found.
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  15. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  16. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  17. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  18. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  19. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  20. ↑ 20.0 20.1 Schoenherr, Steven, 2002, The History of Magnetic Recording, Audio Engineering Society
  21. ↑ Max Appl "Ammonia, 2. Production Processes" in Ullmann's Encyclopedia of Industrial Chemistry 2011, Wiley-VCH. doi:10.1002/14356007.o02_o11
  22. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  23. ↑ Lua error in package.lua at line 80: module 'strict' not found.

Further reading

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External links

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