THE COMPOSITIONAL CLASSIFICATION OF CHONDRITES .6. THE CR CARBONACEOUS CHONDRITE GROUP

New analytical data combined with recent studies by other researchers allow the definition of a Renazzo (CR) group of carbonaceous chondrite s. We analyzed nine CR chondrites (Acfer 187, Acfer 209, El Djouf 001, Elephant Moraine 87747, Elephant Moraine 87770, Elephant Moraine 8784 7, MacAlpine Hills 87320, PCA91082, and Yamato 793495) constituting at least five independent fall events by instrumental neutron activation analysis for twenty-seven elements. Along with previously analyzed Re nazzo, six or more closely related fall events are represented. Key CR properties include refractory lithophile abundances approximately 1.0 x CI levels, Zn/Mn ratios approximately 0.3 x CI levels, metal conten ts of 100-160 mg/g, (unusually high for a carbonaceous chondrite-group ), relatively large chondrules (mean size approximately 700 mum), and the presence of magnetite framboids. Al Rais is a close relative but t oo different in chemical and isotopic composition to be considered a n ormal member of the CR group; we suggest that it be treated as an anom alous member (CR-an), but that its properties not be included in CR ra nges and means. MAC87320, PCA91082, EET87770, and Acfer 187 were studi ed petrographically along with Renazzo and Al Rais. Renazzo has a line ation possibly caused by fluid-lubricated, impact-induced shearing. Th e CR chondrites experienced some reduction during weak thermal metamor phism; the heating must have taken place prior to hydrothermal alterat ion. We suggest that formation of magnetite (and framboidal magnetite in particular) in CI and CR chondrites is due to hydrothermal alterati on of metal-rich (or opaque-rich) precursors, and that the low abundan ce of magnetite in CM chondrites relative to CI indicates that the CM precursors were metal poor. Some carbonates in CI and CR chondrites ma y have formed by H2O reaction with cohenite or poorly crystallized gra phite.