The Allende Meteorite and the Primordial Solar Nebula

The constituent units of the Allende meteorite are briefly summarized and their possible origins are presented. Among coarse-grained CAI's, type-B2 CAI's are the most primitive, and type-A CAI's (compact type) are considered to have been formed from the type-B2 via type-B1 by gas reactions in the primordial solar nebula. Fine-grained CAI's or amoeboid olivine inclusions may be primarily aggregates consisting mainly of fassaite with small amounts of spinel and melilite or olivine with variable amounts of opaque minerals and aluminous pyroxene, respectively. These inclusions are thought to have reacted with the low-temperature nebular gas, introducing alkalis and FeO components into them. Chondrules and chondritic clasts may have been formed under conditions similar to those of chondrules in ordinary chondrites, and they were driven to the place where the parental body of Allende was formed.