Iron Meteorites As their name suggests, iron meteorites are made of iron metal, with up to 15% nickel by weight. They are very dense and highly magnetic. Iron meteorites, therefore, resemble and feel like pieces of steel, which is an approximate description of their composition. The making of iron meteorites Indeed, iron meteorites have been formed in a process very similar to that in which stainless steel is made in industrial furnaces. During steel making, iron ore, (iron oxide) is mixed with a reducing agent, such as carbon (coal), then heated. In the smelting process, iron oxide is reduced to iron metal, which sinks to the bottom of the furnace and is tapped off for steel making. The by-product of the metal is a light, more stony slag, which floats to the top of the furnace. The parent asteroids of iron meteorites can be regarded as similar to industrial furnaces. The original material of the parent bodies is iron, magnesium silicate (or iron oxide mixed with silicon and magnesium); carbon is also present, as is sulphur, to act as reducing agents. The asteroids were heated; the heat source was partly gravitational heating, a result of the size of the asteroids; partly impact heating through collisions; and partly, indeed mostly, from heat generated by the decay of radioactive isotopes. Asteroid cores In this natural smelting process, the iron silicates were reduced to iron metal, which sank to the centre of the asteroid forming a metal core. The asteroids cooled and solidified. Collisions between asteroids broke them up, eventually revealing their cores. The cooling rate for asteroids that melted was very slow - in some cases around 1°C per million years, a much slower rate than the quenching of metal produced by smelting for steel-making. One result of the slow cooling is the formation of the Widmanstätten pattern. This lattice-like pattern is produced following diffusion of nickel atoms through the iron metal. As the metal cools, the iron and nickel atoms separate, forming regions of different nickel concentrations. The core of the Earth The smelting process is also taking place within the Earth: our planet has an iron core, and the crust on which we stand is the by-product of planetary smelting. We cannot reach the core of the Earth directly; study of iron meteorites helps us to understand planetary formation and differentiation processes and sets boundaries as to when and how the Earth's core formed. Iron and stony-iron meteorites act as markers for iron-silicate segregation and core formation.

The 945g chunk of iron shown in this picture is an iron meteorite approximately 13 cm long of unknown origin. Perhaps a local collector threw it into a box with other rocks, or a geology teacher purchased it at a rock shop. Many such meteorites exist in personal collections.