IRON METEORITE

any meteorite consisting mainly of iron, usually combined with small amounts of nickel. When such meteorites, called irons, fall through the atmosphere, a thin, black crust of iron oxide may form that quickly weathers to rust. Though irons constitute only about 5 percent of meteorite falls, they are relatively easy to distinguish from terrestrial rock and last longer in soil than do stony meteorites; thus irons are found more often than stony or stony-iron meteorites. (The latter, containing stone and iron in about equal amounts, are the rarest group of finds.) Irons are composed of two minerals, nickel-poor kamacite and nickel-rich taenite, which often occur together. Their interlocking crystals combine to form a characteristic arrangement, the Widmansttten pattern, which indicates the relatively low pressure at which irons were formed. Irons are grouped according to their structure, which can be revealed through etching a polished cross section with dilute acid, and there are three groups grading into one another: hexahedrites, octahedrites, and ataxites. Hexahedrites are usually made up entirely of kamacite and lack the Widmansttten pattern. Octahedrites contain both kamacite and taenite and constitute the largest group of iron finds. Most ataxites are pure taenite, and they are the rarest group; some ataxite finds may contain up to 69 percent nickel.