meteorite

The recovered fragment of a meteoroid that has survived passage through the Earth's atmosphere. Individual meteorites are normally named after the place where they fell. Studies of the paths of a small number of meteorites observed as fireballs, and subsequently recovered, show they were in orbits originating in the asteroid belt. The chemical and mineralogical composition of meteorites is studied with considerable interest since they appear to be samples from remote parts of the solar system and thus provide clues to its origin and evolution. There are three main classes of meteorite: irons (siderites), stony-irons (siderolites or lithosiderites) and stones (aerolites). Stony meteorites are further divided into two important categories: the achondrites and the chondrites. Chondrites are characterized by the presence of chondrules, small spherical inclusions, which may be of metal or of silicate or sulphide materials. Chondrules are not present in achondrites. The chemical composition of the chondrites is very similar to that of the Sun, except that they contain no free hydrogen and helium and there is more lithium and boron. The interpretation is that the chondrites represent primitive solar system material that has not been altered significantly by heating, though there is evidence for some metamorphism and for alteration by water. Carbonaceous chondrites have the highest proportion of volatiles and the composition closest to the Sun's. "Ordinary" chondrites have the lowest proportion of volatiles, while enstatite chondrites are intermediate between the two. Among the achondrites, numerous subtypes are identified according to detailed chemical and mineralogical composition. In Antarctica, where large numbers of meteorites have been preserved and concentrated in certain areas of the ice sheet, examples have been found with compositions very close to those of lunar samples returned by the Apollo missions. Stony-iron meteorites contain free metal and stony material in roughly equal proportions. Pallasites consist of olivine grains enclosed in metal; mesosiderites are agglomerates of metal and silicates. Iron meteorites consist almost entirely of iron and nickel. Over forty different minerals have been identified in them, though the basic constituents are two forms of iron-nickel alloy, kamacite and taenite. Iron meteorites are categorized according to the proportion of nickel, which determines the crystalline structure. Hexahedrites contain up to 6 per cent nickel, octahedrites between 6 and 14 per cent nickel, and ataxites up to 66 per cent.