Systematics and Evaluation of Meteorite Classification

Classification of meteorites is largely based on their mineralogical and petrographic characteristics and their whole-rock chemical and O-isotopic compositions. According to the currently used classification scheme, meteorites are divided into chondrites, primitive achondrites, and achondrites. There are 15 chondrite groups, including 8 carbonaceous (CI, CM, CO, CV, CK, CR, CH, CB), 3 ordinary (H, L, LL), 2 enstatite (EH, EL), and R and K chondrites. Several chondrites cannot be assigned to the existing groups and may represent the first members of new groups. Some groups are subdivided into subgroups, which may have resulted from asteroidal processing of a single group of meteorites. Each chondrite group is considered to have sampled a separate parent body. Some chondrite groups and ungrouped chondrites show chemical and mineralogical similarities and are grouped together into clans. The significance of this higher order of classification remains poorly understood. The primitive achondrites include ureilites, acapulcoites, lodranites, winonaites, and silicate inclusions in IAB and IIICD irons and probably represent recrystallization or residues from a low-degree partial melting of chondritic materials. The genetic relationship between primitive achondrites and the existing groups of chondritic meteorites remains controversial. Achondrites resulted from a high degree of melting of chondrites and include asteroidal (angrites, aubrites, howardites-diogenites-eucrites, mesosiderites, 3 groups of pallasites, 15 groups of irons plus many ungrouped irons) and planetary (martian, lunar) meteorites.