Magnetite

From New World Encyclopedia
Magnetite
Magnetite Russia.jpg
Magnetite from the Kola Peninsula, Russia
General
CategoryMineral
Chemical formulairon(II,III) oxide, Fe3O4
Identification
ColorBlack, greyish
Crystal habitOctahedral, fine granular to massive
Crystal systemIsometric
CleavageIndistinct
FractureUneven
Mohs Scale hardness5.5 - 6.5
LusterMetallic
Refractive indexOpaque
StreakBlack
Specific gravity5.17 - 5.18
Major varieties
LodestoneMagnetic 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 the most magnetic of all naturally occurring minerals on Earth. It is a valuable iron ore. Lodestone, used as an early form of magnetic compass, was a naturally magnetized form of magnetite.

Occurrence

Small grains of magnetite occur in almost all igneous rocks and metamorphic rocks. 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 from magma. Magnetite also is produced from peridotites and dunites by serpentinization.

Magnetite is sometimes found in large quantities in beach sand. Such mineral sands or iron sands or black sands are found in various places such as California and the west coast of New Zealand. 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 also are found in Kiruna, Sweden, the Pilbara region in Western Australia, and in the Adirondack region of New York in the United States. Deposits are also found in Norway, Germany, Italy, Switzerland, South Africa, India, and Mexico. In the United States, it is found in Oregon, New Jersey, Pennsylvania, North Carolina, Virginia, New Mexico, Utah, and Colorado. Recently, in June 2005, an exploration company, Candero Resources, discovered a vast deposit of magnetite-bearing sand dunes in Peru. The dune field covers 250 km², with the highest dune at over 2000 m above the desert floor. The sand contains 10% magnetite[1].

Biological occurrences

Crystals of magnetite have been found in some bacteria (e.g., Magnetospirillum magnetotacticum) and in the brains of bees, of termites, of some birds (e.g., the pigeon), and of humans. These crystals are thought to be involved in magnetoreception, the ability to sense the polarity or the inclination of the earth's magnetic field, and to be involved 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.

Characteristics

The Curie temperature of magnetite is about 580°C. It dissolves slowly in hydrochloric acid.

Preparation as a ferrofluid

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.

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 much studied, as the complicated reactions between these minerals and oxygen influence how and when magnetite preserves records of the Earth's magnetic field.

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

References
ISBN links support NWE through referral fees

  • 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.

External links

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