Magnetite

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Not to be confused with Magnesite, Maghemite, or Magnemite.
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[1]
Diaphaneity Opaque
Specific gravity 5.17–5.18
Solubility Dissolves slowly in hydrochloric acid
References [2][3][4][5]
Major varieties
Lodestone Magnetic with definite north and south poles

Magnetite is a mineral, one of the three common naturally occurring iron oxides (chemical formula Fe3O4) and a member of the spinel group. Magnetite is the most magnetic of all the naturally occurring minerals on Earth.[6] Naturally magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, and this was how ancient people first noticed the property of magnetism.

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

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

Properties[edit]

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[edit]

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.[7] 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.[8] Deposits are also found in Norway, Germany, Italy, Switzerland, South Africa, India, Indonesia, Mexico, and in Oregon, New Jersey, Pennsylvania, North Carolina, 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.[9]

Transformation of ferrous hydroxide into magnetite[edit]

Under anaerobic conditions, the ferrous hydroxide (Fe(OH)2) can be oxidized by the protons of water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:

3 Fe(OH)2 → Fe3O4 + H2 + 2 H2O
ferrous hydroxide → magnetite + hydrogen + water

The well-crystallized magnetite (Fe3O4) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)2 ).[10]

Biological occurrences[edit]

Biomagnetism is usually related to the presence of biogenic[11] crystals of magnetite, which occur widely in organisms.[12] These organisms range from bacteria (e.g., Magnetospirillum magnetotacticum) to animals, where these crystals are found in the brain.[13] These crystals are involved in magnetoreception, the ability to sense the polarity or the inclination of the Earth's magnetic field, and aid in navigation. [11]

Chitons have teeth made of magnetite on their radula.[11]

Synthetic magnetite[edit]

Crystal structure of magnetite. Red atoms are oxygens.

Magnetite can be prepared in the laboratory as a ferrofluid in the Massart method by mixing iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide.[14] Magnetite can also be prepared by the chemical co-precipitation in presence of ammonia, which consist in a mixture of a solution 0.1 M of FeCl3·6H2O and FeCl2·4H2O with mechanic agitation of about 2000 rpm. The molar ratio of FeCl3:FeCl2 can be 2:1; heating this solution at 70 °C, and immediately the speed is elevated to 7500 rpm and adding quickly a solution of NH4OH (10 volume %), immediately a dark precipitate will be formed, which consists of nanoparticles of magnetite.[15] In both cases, the precipitation reaction rely on a quick transformation of acidic hydrolyzed iron ions into the spinel iron oxide structure, by hydrolysis at elevated pH values (above ca. 10).

Considerable efforts has been devoted towards controlling the particle formation process of magnetite nanoparticles due to the challenging and complex chemistry reactions involved in the phase transformations prior to the formation of the magnetite spinel structure.[16] Magnetite particles are of interests in bioscience applications such as in magnetic resonance imaging (MRI) since iron oxide magnetite nanoparticles represent a non-toxic alternative to currently employed gadolinium-based contrast agents. However, due to lack of control over the specific transformations involved in the formation of the particles, truly superparamagnetic particles have not yet been prepared from magnetite, i.e. magnetite nanoparticles that completely lose their permanent magnetic characteristic in the absence of an external magnetic field (which by definition show a coercivity of 0 A/m). The smallest values currently reported for nanosized magnetite particles is Hc = 8.5 A m−1,[17] whereas the largest reported magnetization value is 87 Am2 kg−1 for synthetic magnetite.[18][19]

Applications[edit]

Magnetic recording[edit]

Magnetic iron oxides are often used in magnetic storage,[20] for example in the magnetic layer of hard disks, floppy disks and cassette tapes. These consist of a thin sheet of plastic material, with embedded magnetic particles. The particles can be magnetized to represent binary or analog data. Magnetic ink character recognition (MICR) also uses magnetic particles suspended in an ink which can be read by special scanning hardware.

Most newly generated information, such as text, photographs, and audiovisual recordings, is now stored in magnetic media, and much of the world's legacy of information in other media has been transcribed to magnetic form, because it is cheap, compact, and computer-accessible.

Catalysis[edit]

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

As a sorbent[edit]

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[edit]

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).

Gallery of magnetite mineral specimens[edit]

See also[edit]

References[edit]

  1. ^ Identifying Magnetite by Streak
  2. ^ Handbook of Mineralogy
  3. ^ Mindat.org Mindat.org
  4. ^ Webmineral data
  5. ^ Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). Wiley. ISBN 0-471-80580-7. 
  6. ^ Harrison, R. J.; Dunin-Borkowski, RE; Putnis, A (2002). "Direct imaging of nanoscale magnetic interactions in minerals" (free-download pdf). Proceedings of the National Academy of Sciences 99 (26): 16556–16561. Bibcode:2002PNAS...9916556H. doi:10.1073/pnas.262514499. PMC 139182. PMID 12482930. 
  7. ^ Templeton, Fleur (15 Jun 3 2010). "1. Iron – an abundant resource - Iron and steel". Te Ara Encyclopedia of New Zealand. Retrieved 4 January 2013. 
  8. ^ Kediet ej Jill
  9. ^ Ferrous Nonsnotus
  10. ^ Ma, Ming; Zhang, Yu; Guo, Zhirui; Gu, Ning (2013). "Facile synthesis of ultrathin magnetic iron oxide nanoplates by Schikorr reaction". Nanoscale Research Letters 8 (1): 16. doi:10.1186/1556-276X-8-16. 
  11. ^ a b c Magnetite in Human Tissues: A Mechanism for the Biological Effects of Weak ELF Magnetic Fields Joseph L. Kirschvink, Atsuko Kobayashi-Kirschvink, Juan C. Diaz- Ricci, and Steven J. Kirschvink
  12. ^ H. A. Lowenstam "Minerals formed by organisms" Science 1981, volume 211,:1126-31.
  13. ^ Baker, R R; J G Mather; J H Kennaugh (1983-01-06). "Magnetic bones in human sinuses". Nature 301 (5895): 79–80. Bibcode:1983Natur.301...78R. doi:10.1038/301078a0. PMID 6823284. 
  14. ^ Massart, R., Preparation of aqueous magnetic liquids in alkaline and acidic media, IEEE transactions on magnetics, 17, 2, 1981. 1247–1248
  15. ^ Keshavarz, Sahar, Yaolin Xu, Spencer Hrdy, Clay Lemley, Tim Mewes, and Yuping Bao. "Relaxation of Polymer Coated Magnetic Nanoparticles in Aqueous Solution", IEEE Transactions on Magnetics, Volume: 46, Issue: 6 pp. 1541-43, June 2010, Tuscaloosa. Retrieved on 3 September 2012.
  16. ^ Jean-Pierre Jolivet, Corinne Chanéac and Elisabeth Tronc, Iron oxide chemistry. From molecular clusters to extended solid networks,Chem. Commun., 2004, 5, 481-483
  17. ^ Valter Ström, Richard T. Olsson, K. V. Rao, Real-time monitoring of the evolution of magnetism during precipitation of superparamagnetic nanoparticles for bioscience applications, J. Mater. Chem., 2010, 20, 4168-4175
  18. ^ Mei Fang, Valter Ström, Richard T. Olsson, Lyubov Belova, K. V. Rao, Rapid mixing: A route to synthesize magnetite nanoparticles with high moment, Appl. Phys. Lett. 99, 222501 (2011)
  19. ^ Mei Fang, Valter Ström, Richard T. Olsson, Lyubov Belova, K. V. Rao, Particle size and magnetic properties dependence on growth temperature for rapid mixed co-precipitated magnetite nanoparticles, Nanotechnology, 2012, 23, 14, 145601
  20. ^ Ullah, Zaka; Atiq, Shahid; Naseem, Shahzad (2013). "Influence of Pb doping on structural, electrical and magnetic properties of Sr-hexaferrites". Journal of Alloys and Compounds 555: 263–267. doi:10.1016/j.jallcom.2012.12.061. 
  21. ^ Max Appl "Ammonia, 2. Production Processes" in Ullmann's Encyclopedia of Industrial Chemistry 2011, Wiley-VCH. doi:10.1002/14356007.o02_o11
  22. ^ J.T. Mayo et al. (2007). "The effect of nanocrystalline magnetite size on arsenic removal". Sci. Technol. Adv. Mater. (free download) 8: 71. Bibcode:2007STAdM...8...71M. doi:10.1016/j.stam.2006.10.005. 

Further reading[edit]

External links[edit]