magnetite

Magnetite is a ferric iron oxide mineral with the chemical formula Fe3O4. It is one of the most magnetic natural minerals and serves as a major ore of iron. The name derives from the Greek magnetis lithos, meaning “the Magnesian stone,” in reference to lodestone, the naturally magnetized form used since ancient times for navigation.

Crystal structure and properties: Magnetite crystallizes in the cubic system with an inverse spinel structure. In this arrangement, Fe2+ ions occupy some octahedral sites and Fe3+ ions occupy both octahedral and tetrahedral sites, producing ferrimagnetism. It has a high Curie temperature of about 858 K (585 °C), making its magnetic properties stable under typical conditions. Magnetite is typically black with a metallic luster, hardness of 5.5–6.5 on the Mohs scale, and a specific gravity around 5.15.

Occurrence and formation: Magnetite occurs in a wide range of settings, including igneous and metamorphic rocks, as well as detrital grains in sedimentary rocks and heavy-mineral sands. It forms through magmatic differentiation, high-temperature hydrothermal processes, and, in some environments, secondary oxidation and weathering.

Uses and significance: As a principal iron ore, magnetite is economically important for steel production, often processed by magnetic separation. It is also used as a pigment and as a distinctive geological indicator. In modern technology, magnetite nanoparticles are employed in biomedical imaging and other applications, while lodestone historically demonstrated the Earth’s magnetism and guided early navigational compasses.