any meteorite consisting largely of rock-forming (silicate) minerals. Stony meteorites, which are the most abundant kind of meteorite, are divided into two groups: chondrites and achondrites. Chondrites are physically and chemically the most primitive meteorites in the solar system. They appear to be primarily aggregates of material that formed in the solar nebula prior to or during planet formation. They also contain some material that predates formation of the solar system. Most chondrites have been either thermally or aqueously altered when they were part of their parent asteroids, but none have experienced significant melting. Achondrites, on the other hand, were all produced by melting of their parent bodies. Although most achondrites are fragments of asteroids, a few come from the Moon or Mars.