Crystallization of chondrules in ordinary chondrites

Abstract The process of crystallization and the origin of chondrules are discussed, in terms of the phase relations of the minerals in chondrules in six ordinary chondrites of the Yamato-74 meteorites, especially the Yamato-74191 (L3). Chondrules are classified into six types. The bulk compositions of chondrules projected onto the MgO-FeO-SiO 2 system show that the compositions of chondrules vary widely. Investigations by means of the MgO-Al 2 O 3 -SiO 2 system indicate that porphyritic chondrules can be regarded as products of supercooling crystallization. The growth rates of crystals in porphyritic chondrules were fairly small. The difference between types of chondrules is interpreted in terms of the compositions of chondrules and the nucleation temperatures of the supercooled droplets. All these observations and estimations must be taken into account for discussing the origin of chondrules. The impact and dust fusion theories do not appear to be plausible. Molten droplets due to these mechanisms will be glassy spherules, or crystallize at equilibrium. Only a liquid condensation theory can well explain the characteristic features and the process of the crystallization of chondrules.