Magnetite is a mineral and one of the main minerals of iron. With the chemical formula Fe3O4, it is one of the iron oxides. Magnetite is ferrimagnetic attracts a magnet and can be magnetized to become a permanent magnet. It is the most magnetic of all the natural minerals on Earth. Naturally magnetized pieces of magnetite, called mudstones, will attract small pieces of iron, which is how ancient people first discovered the property of magnetism. Today it is mined as iron ore.
Small grains of magnetite occur in almost all igneous and metamorphic rocks. Magnetite is black or brownish-black with a metallic luster, has a Mohs hardness of 5-6, and leaves a black stripe.
The IUPAC chemical name is iron oxide and the common chemical name is ferrous-ferric oxide.
In addition to igneous rocks, magnetite also occurs in sedimentary rocks, including banded iron formations, and in lake and marine sediments such as detrital grains and magnetophosiles. Magnetite nanoparticles are also believed to form in soils, where maghemite is likely to rapidly oxidize.
Magnetite is sometimes found in large quantities in beach sand. Such black sands (mineral sands or iron sands) are found in various places, such as Lung Kwu Tan of Hong Kong, California of the United States, and the west coast of the North Island of New Zealand. The magnetite is carried to the beach through rivers of erosion and is concentrated through the action of waves and currents. Huge deposits have been found in banded iron formations. These sedimentary rocks have been used to infer changes in the oxygen content of Earth's atmosphere.
Due to its high iron content, magnetite has long been an important iron ore. It is reduced in blast furnaces to cast iron or sponge iron for conversion to steel.
Audio recording with magnetic acetate tape was developed in the 1930s. The German tape recorder used magnetite powder as the recording medium. After World War II, 3M Company continued to work on German design. In 1946, 3M researchers discovered that they could improve magnetite-based tape, which used cubic crystal powders, by replacing magnetite with needle-shaped particles of gamma ferric oxide (γ-Fe2O3).
Magnetite is the catalyst for the industrial synthesis of ammonia.
