Iron Meteorite

Early in the solar system, heat from the decay of radioactive elements melted many asteroids; their dense iron sank to the centre and formed metallic cores. After collisions broke these bodies apart, the core metal cooled extremely slowly, at roughly 1 °C per million years according to the Natural History Museum. Fragments later knocked off these asteroids reach Earth as iron meteorites. Iron meteorites are named after the place where they are found, such as Gibeon (Namibia), Campo del Cielo (Argentina) or Sikhote-Alin (Russia).

Iron meteorites are much heavier than ordinary rocks, strongly magnetic, and show a bright white metal surface when a corner is ground; fresh pieces have a fusion crust with deep thumbprint-like pits (regmaglypts), which weathers to rusty brown. When cut, polished and etched with dilute nitric acid, most show the Widmanstätten pattern of interlocking kamacite and taenite plates. This pattern grows only during cooling over millions of years deep inside a parent body, so the AMNH notes that metal showing it on Earth is from a meteorite; band width (from over 3 cm to under 0.2 mm) sets the structural class. Hexahedrites, each a single large kamacite crystal, and very fine-grained ataxites show no visible pattern. Magnetite is the material most often mistaken for meteorites: it is magnetic but less dense than steel and gives a black streak. In jewellery, etched iron meteorite may be plated; GIA identified rhodium-plated pieces (over copper and nickel layers) as genuine octahedrites, although their bright white lustre first suggested imitations.

Iron meteorites rust readily because of their high iron content; even slight humidity can cause rapid corrosion. GIA notes that they are commonly kept in humidity-controlled conditions or with moisture-absorbing silica gel, or coated with oil or hard clear lacquer. In jewellery, rhodium plating is used mainly to keep moisture away from the iron, but rust can still start where the plating fails to adhere.

Before iron smelting was widespread, meteoritic iron was a prized material. Nine small beads from burials at Gerzeh in northern Egypt, dated to about 3200 BCE, are the earliest known iron artefacts and are made of meteoritic iron. A 2016 study of the iron dagger blade found in 1925 on the mummy of Tutankhamun (14th century BCE) measured 10.8% nickel and 0.58% cobalt, a composition that strongly supports a meteoritic origin. The study also notes that a new Egyptian term literally meaning "iron of the sky" came into use in the 13th century BCE. The Widmanstätten pattern is named after Count Alois von Beckh Widmanstätten, who described it in 1808; a British scientist, G. Thomson, had noticed the same pattern a few years earlier, and it is sometimes called the Thomson structure.