Dictionary:

magnetite

  (măg'nĭ-tīt')

The mineral form of black iron oxide, Fe₃O₄, that often occurs with magnesium, zinc, and manganese and is an important ore of iron.

A cubic mineral and member of the spinel structure type with composition [Fe³⁺]^IV[Fe²⁺Fe³⁺]^VIO₄. The color is opaque iron-black and streak black, the hardness is 6 (Mohs scale), and the specific gravity is 5.20. The habit is octahedral, but the mineral usually occurs in granular to massive form, sometimes of enormous dimensions. Magnetite is a natural ferrimagnet, but heated above 1072°F (578°C; the Curie temperature) it becomes paramagnetic.

The major magnetic ore of iron, magnetite may be economically important if it occurs in sufficient quantities. The most spectacular ore body occurs at Kiruna in northern Sweden. Other important occurrences are in Norway, Russia, and Canada. See also Iron; Spinel.

For more information on magnetite, visit Britannica.com.

A natural black oxide of iron, containing from about 65 to 72% iron and sometimes a small amount of nickel and titanium; used as an aggregate in high-density concrete.

Environment

Plutonic, pegmatitic, and metamorphic rocks, and sands.

Usually in octahedrons, commonly striated with triangular markings on the octahedron faces. Dodecahedron modifications common. Since these faces are usually built up of heavy octahedron striations, the dodecahedron is striated lengthwise. Cubic habit rare; commonly massive or granular.

Black. Luster metallic; hardness 6; specific gravity 5.2; streak black; fracture subconchoidal to uneven; cleavage none, but sometimes an octahedral parting. Brittle; magnetic; sometimes a natural magnet (lodestone).

Ferrous and ferric iron oxide (72.4% Fe, 27.6% O); also written FeO·Fe ₂ O ₃ . Other elements--magnesium, zinc, and manganese (rarely nickel)--can substitute in part for the first (the FeO, or ferrous) iron, while small amounts of aluminum, chromium, manganic manganese, and vanadium can replace part of the second (the Fe ₂ O ₃ , or ferric) iron. This permits a whole second spinel series of related minerals to which different names have been given, but magnetite is by far the most important.

Naturally magnetic; further tests unnecessary.

The magnetism (and frequent polarity) distinguishes it from most other similar minerals. The streak is blacker than that of ilmenite; it is brittle and much lower in gravity than platinum mixed with iron or the native nickel-iron (josefinite) compounds. Zinc-rich magnetite (franklinite) is less magnetic and unique to New Jersey.

An important ore of iron. A widespread accessory mineral forming small grains in igneous rocks, which, after weathering, are often concentrated into black beach sands (once used as ink-blotting sand). Sometimes magnetite is concentrated by magmatic processes into solid ore deposits rich enough to mine. In schistose metamorphic rocks it may form fine phenocrystlike crystals. Also found well crystallized in pegmatites and high-temperature veins.

A very common mineral, so widespread that only a few U.S. localities need be mentioned. Fine crystals have come from French Creek, Pennsylvania, from Port Henry and Brewster, New York, and from the zinc mines at Franklin, New Jersey (franklinite). Sharp octahedrons (2 cm) lie embedded in chlorite schist at Chester, Vermont, with comparable pyrite crystals. Good irregular lodestone masses are found at Magnet Cove, Arkansas, and good clusters of crystals occur in Millard Co., Utah. Crystals of magnetite may be several inches across, but most are smaller; an inch or so (2-3 cm) is the usual size. Balmat, New York has been the source of a number of inch-sized (2.5 cm) rare cubic crystals. Pseudomorphs of hematite after magnetite are common. Many of the best apparent clusters of magnetite crystals, like those from Pelican Point, Salt Lake, Utah, or Durango, Mexico, are now actually composed of hematite, give a red streak, and are only weakly magnetic. They are called martite.

Early magnets were made by striking the iron with the natural lodestone magnet, whose properties have intrigued men for generations. Like garnet and spinel, magnetite is often found in thin crystals in mica sheets and can be identified by its color (usually black and opaque, sometimes gray) and by regular partings parallel to the crystal outline that produce tiny cracks in the plate. A light viewed through such a crystal held close to the eye will appear to be surrounded by rays, resembling the asterism described under phlogopite (see p. 304).

A black, strongly magnetic iron-oxide mineral; Fe₃O₄.

Magnetite is not to be confused with Magnesite or Magnemite.

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 with chemical formula Fe₃O₄, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron(II,III) oxide and the common chemical name ferrous-ferric oxide. The formula for magnetite may also be written as FeO.Fe₂O₃, which is one part wüstite (FeO) and one part hematite (Fe₂O₃). This refers to the different oxidation states of the iron in one structure, not a solid solution.

The Curie temperature of magnetite is about 580°C. Magnetite is the most magnetic of all the naturally occurring minerals on Earth, and these magnetic properties led to lodestone being 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 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 (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.

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 a valuable source of iron ore. It dissolves slowly in hydrochloric acid.

Distribution of deposits

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, Mexico, and in Oregon, New Jersey, Pennsylvania, North Carolina, Virginia, New Mexico, Utah, and Colorado in the United States. Recently, in June 2005, an exploration company, Cardero 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.

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.

See also

• Ferrite
• Hematite
• Wüstite
• Mineral redox buffer
• Magnesia
• Maghemite
• Bluing (steel) is a process in which steel is partially-protected against rust by a layer of magnetite.

References

Mineralogy related

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
• Webmineral data
• Mineral galleries
• Powder X-Ray Diffraction (XRD) Pattern

Biology related

• Heinz A. Lowenstam and Stephen Weiner, On Biomineralization, Oxford University Press, USA (1989) ISBN 0-19-504977-2
• Shih-Bin Robin Chang' and Joseph Lynn Kirschvink, Magnetofossils, the Magnetization of Sediments, and the Evolution of Magnetite Biomineralization, Ann. Rev. Earth Planet. Sci. 1989. 17:169-95 PDF file
• Bio-magnetics
• Magnetic bacteria (Italian)

Mining related links

• History of Magnetite Mining in the NJ Highlands
• Magnetite mining in New Zealand
• Magnetite mining in Santa Cruz
• Peruvian sand dunes

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