What is an iron meteorite?
An iron meteorite is made mainly of iron-nickel metal. Most are thought to be pieces of the metallic cores of asteroids that melted and separated into layers early in Solar System history.
Iron meteorites are dense, heavy and often surprisingly beautiful. They consist mostly of two iron-nickel alloys: kamacite, which is low in nickel, and taenite, which is richer in it. Many also contain nodules of troilite (iron sulfide), graphite and phosphide minerals such as schreibersite. Nickel content is usually several per cent and can reach far higher in rare examples.
Where did all that metal come from? In the first few million years of the Solar System, some young asteroids were heated, mainly by the decay of short-lived radioactive aluminium-26. They melted, and the heavy metal sank to the centre to form a core, just as Earth's iron core formed. When later collisions smashed these bodies apart, fragments of the cores were scattered through space. Some iron meteorites formed in other ways, for example in pools of metal melted by impacts, but the core story explains most of them.
Scientists classify irons in two ways. By structure, they are hexahedrites, octahedrites or ataxites, depending on the crystal pattern visible after etching. By chemistry, they fall into groups such as IAB, IIAB, IIIAB and IVA, each linked to a different parent body; researchers estimate that iron meteorites sample dozens of separate asteroids.
Irons make up only about 5 per cent of observed falls, but they are prominent in museums because they survive weathering well and look unlike ordinary rocks. Famous examples include Hoba in Namibia, the largest known meteorite at around 60 tonnes; Sikhote-Alin, which fell in eastern Russia in 1947 and showered thousands of fragments; and Gibeon from Namibia, prized for its fine crystal pattern.