An iron meteorite is any meteorite consisting mostly of iron, usually combined with a small amount of nickel. When such meteorites pass through the Earth's atmosphere, they can form a thin black crust of iron oxide that quickly weathers and rusts. Although these meteorites make up only about 5 percent of observed meteorite falls, they are relatively easy to distinguish from terrestrial rocks and last longer in the soil than rock meteorites; therefore, they are found more often than stone or iron-stone meteorites.
Iron meteorites consist of two minerals, nickel-poor kamacite and nickel-rich taenite, which often occur together. The mutual crystal clusters of the two minerals form a characteristic arrangement, the so-called Widmanstätten pattern, which indicates the relatively low pressure at which iron meteorites form. Historically, irons were grouped according to their crystal structure, which can be revealed by etching a polished section of the meteorite with dilute acid. There are three groups that classify each other: hexahedrites, octahedrites, and ataxites. Hexahedrites are usually composed entirely of kamacite and lack Widmanstätten patterns. Octahedrites contain both kamacite and taenite and constitute the largest group of iron meteorite finds. Most ataxites, which are the rarest group, are pure taenite; some ataxite specimens contain up to 69 percent nickel. More recently, this structural classification has been replaced by a chemical classification based on the abundances of the elements gallium, germanium, and nickel.
Iron meteorites of various types from worldwide localities, in natural or altered form as slices, can be found in this category.