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| Magnetite | |
|---|---|
 Magnetite from the Kola Peninsula, Russia |
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| 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 858 K. 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.
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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.
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