Petrology of the St. Mesmin chondrite

Fifteen samples of clasts and matrix material from the St. Mesmin LL-group chon drite were examined petrogaphically. Their olivines and low-calcium pyroxenes were partially analyzed (for Fe, Ca and Mg) with the electron microprobe. The data confirm and extend the conclusion of Fredriksson et al. (1968) that St. Mesmin is unequilibrated. Though some of its xenolitbs have textures and mineral assemblages appropriate to type 6 and 7 (Dodd, 1972) chondritic material, other materials within it show no evidence of recrystallization. The components of St. Mesmin can be classified by petrologic type according to the system of Van Schmus and Wood (1967); the meteorite as a whole cannot. St. Mesmin is also polymict. Two dark xenoliths in the material studied are fragments of intensely shocked H-group (H-4 ?) chondrites. Two others are fragments of olivine microporphyry which may represent shock-generated magma. Within St. Mesmin and other unequilibrated LL-group chondrites (Fredriksson et al., 1968), the iron contents of olivine and orthopyroxene vary directly with petrologic type. Literature data suggest a similar relationship among the LL-group chondrites, but with many exceptions. In view of the potential importance of this observation for interpretation of the metamorphic history of LL-group chondrites, a detailed restudy and reclassification of these meteorites seems to be in order.