Most iron meteorites are thought to be the cores of asteroids that melted early in their history. They consist mainly of iron-nickel metal with small amounts of sulphide and carbide minerals.
Iron meteorite showing dark fusion crust.
It is thought that when asteroids melted, iron, being dense, sank to the centre to form a metallic core. These melted asteroids are known as differentiated since they have separated into concentric shells, with an iron core surrounded by a silicate mantle and perhaps even a silicate crust.
As this concentric, differentiated structure is similar to that of the terrestrial planets (Mercury, Venus, Earth and Mars), iron meteorites can tell us a great deal about how the metallic cores of planets formed.
Around 5 out of every 100 meteorites that fall are iron. However, because iron is a tough material, they are more likely than stony meteorites to survive the fall to Earth. This means that most meteorite craters are likely to have been caused by iron meteorites.
An iron meteorite showing Widmanstatten texture.
Iron meteorites consist mainly of an iron-nickel metal alloy and most have a distinctive crystalline structure with bands containing low and high levels of nickel known as Widmanstatten texture. The low nickel alloy is the mineral kamacite and the high nickel alloy is taenite. These are the same as the terrestrial minerals ferrite and austite.
There can be wide variation in the texture, as well as the mix of other minerals within iron meteorites, such as iron sulphides and iron carbides. This means there are a lot of different groups and sub-types of iron meteorite and they can be related to both stony achondritic and chondrite meteorites.