Classification and composition

Chondrite classes

The chondrite classes that were introduced in Chapter 1 are the enstatite chondrites, at the reduced end of the spectrum and with large metal–silicate fractionation between the so-called EH and EL chondrites. There are many published reviews of these meteorites (Keil, 1968; 1989; Sears et al., 1982b; Zhang et al., 1995). The ordinary chondrites appear in the middle of the Urey–Craig plot with metal–silicate fractionation separating the H, L, and LL chondrites. Urey and Craig (1953), Keil and Fredriksson (1964), and Fredriksson et al. (1968) discussed these classes. The R (after Rumuruti) chondrites are superficially similar to the ordinary chondrites although they contain considerably more matrix and have unusual oxygen isotope properties (Weisberg et al., 1991; Kallemeyn et al., 1996). The carbonaceous chondrites contain little metallic iron, although some contain sulfides and all contain considerable amounts of iron in the form of clay minerals and magnetite. The carbonaceous chondrites can be subdivided into CO chondrites (McSween, 1977a; Scott and Jones, 1990; Sears et al., 1991), CV chondrites (Van Schmus and Hayes, 1974; McSween, 1977c), CR chondrites (Bischoff et al., 1993b; Weisberg et al., 1993), CK chondrites (Kallemeyn et al., 1991), CM chondrites (McSween, 1979a), and CI chondrites (McSween, 1979b). The carbonaceous chondrites contain little metal and have suffered little metal–silicate fractionation, however, redox state varies. The CO and some CV chondrites are somewhat more reduced than the other C chondrites. Thus the CV chondrites are subdivided into reduced and oxidized subgroups.