As suggested by the name, iron meteorites are comprised entirely of metal. They are dense meteorites have are composed of an iron-nickel alloy in the form of two exotic minerals called kamacite and taenite. These two minerals are only found on the Earth’s surface in iron meteorites as terrestrial metals commonly oxides (bonding with oxygen to form FeO, Fe2O3, or NiO). Iron meteorites also have a unique texture called Widmanstätten pattern caused by the interlocking of different types of Fe-Ni alloys.
These unique meteorites formed early in the solar system as dust particles accreted into larger masses, such as asteroids. As the mass of these bodies grew, the iron and nickel within these asteroids would “sink” to the centre of the body through a process we call planetary differentiation. This process occurs on all large planetary bodies and is why the crust and mantle are dominated by silicate minerals and the core contains liquid and solid Fe-Ni alloys. Therefore iron meteorites represent the cores of ancient bodies that have long been destroyed.
Stony Iron Meteorites
Stony iron meteorites are a mix of silicate material (e.g. rock) and an iron-nickel alloy. Unlike chondrites, where the metallic phases can make up to 15% of the meteorite, the iron-nickel alloy is approximately 50% of these meteorites. There are two main groups of these meteorites: pallasites and mesosiderites. While both of these meteorites contain even portions of silicate material, their formation process is different. Pallasites are a product of planetary differentiation and are representative of the boundary between the iron-nickel core and the silicate material of the mantle. Therefore they are the products of bodies that have undergone differentiations but have consequently been broken up and landed on Earth. Mesosiderites, on the other hand, are the formed by the collision of silicate rich-material and metallic rich-material in the Solar Systems.