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Magnetite is a ferrimagnetic mineral with IUPAC name iron(II,III) oxide and with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group.

Magnetite is the most magnetic of all the minerals on Earth, and these magnetic properties led to lodestone being used as an early form of magnetic compass.

Small grains of magnetite occur in almost all igneous rocks and metamorphic rocks as well as many sedimentary rocks. Magnetite 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.

Large deposits of magnetite are found in Kiruna, Sweden, 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.

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.

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