A meteorite composed primarily of nickel–iron; sometimes also known as a siderite. Some iron meteorites are probably material from the cores of asteroid-sized parent bodies, while others may have originated at shallower depths. Irons are the easiest meteorites to identify, because they are heavy, look metallic, are strongly magnetic, and are usually covered with pits, depressions, and grooves from their passage through the atmosphere. On impact they have a blue-black fusion crust, with traces of metal melted during the flight, but after exposure on Earth they rust, turning brown. Iron meteorites comprise only 4–5% of observed falls, but resist weathering better than stony meteorites and so are found more easily. Iron meteorites are divided into three main subgroups on the basis of the structure of their nickel–iron alloy: hexahedrites, octahedrites (six types), and the ataxites. They can be divided into 13 chemical subgroups based on the composition of heavy trace elements, such as Ga and Ir. Those that cannot be included in one of these groups are termed anomalous. The largest known meteorites are irons; examples are the Cape York and Hoba West meteorites. Meteor Crater in Arizona was formed by the impact of an iron meteorite.