Classification and Petrography of Some Yamato Chondritic Meteorites

Sixteen Yamato meteorites including the Yamato-69,-74,and -75,all ordinary chondrites, are newly classified. The method for classification adopted here is based on customary microscopic observations, paying attention to the criteria for petrologic types proposed by VAN SCHMUS and WOOD (Geochim. Cosmochim. Acta, 31,741,1967). Compared with this, the convenient method for classification of chondrites proposed by YANAI et al. (Mem. Natl Inst. Polar Res., Spec. Issue, 8,110,1978) is examined, and its availability is ascertained. However, the difference between petrologic types of the Yamato-74492 obtained by these two methods is clearly noticed. It may suggest that this meteorite is a monomict breccia. Since the meteorites studied here are equilibrated ordinary chondrites, several characteristic features of the thermal metamorphism in parent bodies, such as devitrification of glass, mineralogical features of pyroxenes, and type of chondrule, are discussed in relation to petrologic types. For this purpose the samples studied here also include the meteorites described by KIMURA et al. (Mem. Natl Inst. Polar Res., Spec. Issue, 8,156,1978). With the Yamato-74445 extension of veining due to shock effect is described