magnétite

Magnétite, commonly spelled magnetite in English, is a naturally occurring iron oxide mineral with the chemical formula Fe3O4. It is a member of the spinel group and is one of the most magnetic minerals on Earth. Magnetite is ferrimagnetic at room temperature and has a Curie temperature of about 858 K (585°C). Its crystal structure is the inverse spinel: Fe3+ ions occupy tetrahedral sites and a mix of Fe2+ and Fe3+ ions occupy octahedral sites. The magnetic moments on the sublattices do not fully cancel, giving rise to spontaneous magnetization.

Physical properties include a black to brownish-black color, metallic luster, and a grey-black streak. It is opaque, with a Mohs hardness of 5.5–6.5 and a specific gravity around 5.2. Magnetite commonly forms octahedral or irregular grains and can occur as naturally magnetized crystals or aggregates.

Occurrence and formation span a range of geological environments. Magnetite is common in igneous and metamorphic rocks, particularly ultramafic and basaltic rocks, and also appears as detrital grains in sedimentary rocks and beach sands. It forms by high-temperature magmatic crystallization, as well as by low-temperature oxidation and hydrothermal processes. Magnetite is also a notable component of many meteorites.

Uses and significance include its value as an iron ore, frequently concentrated for steel production. It has long been used in magnetics for compasses and for magnetic separation processes. In geology and archaeology, magnetite-bearing rocks preserve records of Earth's past magnetic fields, underpinning fields such as paleomagnetism and archaeomagnetism.