CHONDRITE

any stony meteorite characterized by the presence of chondrules (see chondrule). Chondrites represent the most abundant meteorite class, constituting more than 86 percent of meteorite falls and 51 percent of known meteorites. The structure of chondrites in some respects resembles that of such terrestrial rocks as volcanic tuffs; two distinguishing features are the eccentricity of the point from which the fibres in the chondrules diverge and the not uncommon enclosure of one chondrule by another. Chondrites are primary in origin, having crystallized 4,700,000,000 years ago, and are divided into three main groups according to chemical composition: hypersthene chondrites, in which the ratio of magnesium (Mg) to iron (Fe) atoms lies between two and four; bronzite chondrites, with a magnesium to iron ratio between four and nine; and enstatite chondrites, with a magnesium to iron ratio larger than nine. Chondrites are chemically similar to one another and also to the solar atmosphere, the only discrepancy being in the amount of iron. Most chondrites contain the anhydrous silicate minerals olivine, ortho- and clino-pyroxenes, and plagioclase, as well as kamacite, taenite, and troilite. Some contain claylike hydrous silicates and carbonaceous material (see carbonaceous chondrite).