Magnetite is a magnetic mineral form of both iron(II) oxide and iron(III) oxide or (iron(II,III) oxide), 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 a valuable source of iron ore. It dissolves slowly in hydrochloric acid.
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.
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.
Crystals of magnetite have been found in some bacteria (e.g., Aquaspirillum 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.