The compositional classification of chondrites

New data are presented from a neutron activation analysis of four enstatite chondrites including the taxonomically important St. Sauveur, and the classification of enstatite chondrites is discussed. The enstatite chondrites can be divided into two compositionally distinct sets; in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1.5 to 2.0 x higher than in the other. A well-resolved compositional hiatus separates these two sets. The differences in composition are as great as those between the groups of ordinary chondrites, and therefore it appears best to treat these sets as separate groups. By analogy with the symbols used for ordinary chondrites it is proposed to designate the high-Fe, high siderophile group EH and the low-Fe, low-siderophile group EL. Known members of the EH group belong to petrologic types 4 and 5, whereas all EL members are petrologic type 6. Cosmic-ray ages for EH chondrites are 0.5 to 7 Ma, while those for EL chondrites are 4 to 18 Ma. Relative to Bjurbole, I-XE formation intervals are - 1.3 +- 0.6 Ma for EH chondrites and 2.9 +- 0.5 Ma for EL chondrites. The weight of the chemical and physical evidence indicates that the EH and EL groups formed separate bodies at similar distances from the Sun. (author)