Magnetite
| Magnetite | |
|---|---|
 Magnetite from the Kola Peninsula, Russia |
|
| General | |
| Category | Mineral |
| Chemical formula | iron(II,III) oxide, Fe3O4 |
| Identification | |
| Color | Black, greyish |
| Crystal habit | Octahedral, fine granular to massive |
| Crystal system | Isometric |
| Cleavage | Indistinct |
| Fracture | Uneven |
| Mohs Scale hardness | 5.5 - 6.5 |
| Luster | Metallic |
| Refractive index | Opaque |
| Streak | Black |
| Specific gravity | 5.17 - 5.18 |
| Major varieties | |
| Lodestone | Magnetic with definite north and south poles |
Magnetite is a ferrimagnetic mineral and one of several types of iron oxide. Its common chemical name is ferrous-ferric oxide, its official (IUPAC) name is iron(II,III) oxide, and its chemical formula is Fe3O4.
This member of the spinel group is a valuable iron ore. It is the most magnetic of all naturally occurring minerals on Earth. Lodestone, used as an early form of magnetic compass, was a naturally magnetized form of magnetite.
Occurrence
Magnetite occurs in many sedimentary rocks, and huge deposits have been found in banded iron formations. In addition, this mineral (especially in the form of small grains) occurs in almost all igneous and metamorphic rocks. Many igneous rocks contain magnetite-rich and ilmenite-rich grains that precipitated together from magma. Magnetite is also produced from peridotites and dunites by serpentinization.
Magnetite is sometimes found in large quantities in beach sand. It is carried to the beach by the erosive action of rivers and is concentrated by waves and currents. Such mineral sands (also called iron sands or black sands) are found in various places, including beaches in California and the west coast of New Zealand. In June 2005, an exploration company (Candero Resources) discovered a vast deposit of magnetite-bearing sand dunes in Peru, where the highest dune is more than 2,000 meters (m) above the desert floor. The dune field covers 250 square kilometers (km²), and 10 percent of the sand is magnetite.[1]
Large deposits of magnetite have been found in Kiruna, Sweden, and the Pilbara region of Western Australia. Additional deposits occur in Norway, Germany, Italy, Switzerland, South Africa, India, and Mexico. In the United States, it is found in the states of New York (Adirondack region), New Jersey, Pennsylvania, North Carolina, Virginia, New Mexico, Colorado, Utah, and Oregon.
Biological occurrences
Crystals of magnetite have been found in some bacteria (such as Magnetospirillum magnetotacticum) and in the brains of bees, termites, some birds (including pigeons), and humans. These crystals are thought to be involved in magnetoreception—the ability to sense the polarity or inclination of the Earth's magnetic field—and to aid in navigation. Also, chitons have teeth made of magnetite on their radula, making them unique among animals. This means they have an exceptionally abrasive tongue with which to scrape food from rocks. The study of biomagnetism began with the discoveries of Caltech paleoecologist Heinz Lowenstam in the 1960s.
Laboratory preparation
Magnetite can be prepared in the laboratory as a ferrofluid using the Massart method. It involves mixing iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide.
Characteristics
This mineral is the most magnetic of all known naturally occurring minerals. Its Curie temperature is about 580°C. It dissolves slowly in hydrochloric acid.
Uses
Magnetite typically carries the dominant magnetic signature in rocks, and so it has been a critical tool in paleomagnetism, a science important in discovering and understanding plate tectonics. The relationships between magnetite and other iron-rich oxide minerals such as ilmenite, hematite, and ulvospinel have been studied extensively, as the complicated reactions between these minerals and oxygen influence how and when magnetite preserves records of the Earth's magnetic field. Also, changes in the oxygen content of the Earth's atmosphere can be inferred by studying sedimentary rocks containing magnetite.
Magnetite has been very important in understanding the conditions under which rocks form and evolve. 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 between magnetite and ulvospinel and the other between ilmenite and hematite. Compositions of the mineral pairs are used to calculate how oxidizing was the magma (that is, 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.
See also
ReferencesISBN links support NWE through referral fees
- Chang', Shih-Bin Robin, and Joseph Lynn Kirschvink. 1989. Magnetofossils, the Magnetization of Sediments, and the Evolution of Magnetite Biomineralization. Ann. Rev. Earth Planet. Sci. 17:169-95. Retrieved April 10, 2007.
- Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.
- Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574.
- Lowenstam, Heinz A., and Stephen Weiner. 2003. On Biomineralization. New York: Oxford University Press. ISBN 0195049772.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
- Mindat.org. 2007. Magnetite. Mindat.org. Retrieved April 10, 2007.
- Mineral Gallery. 2006. The Mineral Magnetite. Amethyst Galleries. Retrieved April 10, 2007.
External links
- Magnetite Mineral Data - Webmineral.com Retrieved April 10, 2007.
- Do animals really use magnetism in any interesting way to navigate? - Astronomycafe.net. Retrieved April 10, 2007.
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