  We present a model for the formation of silicon carbide aggregates within the
expanding and cooling supernova remnant. Many SiC-X grains have been found to
be aggregates of smaller crystals which are isotopically homogenous. The
initial condensation of SiC in the ejecta occurs within a interior dense shell
of material which is created by a reverse shock which rebounds from the
core-envelope interface. A subsequent reverse shock accelerates the grains
forward, but the gas drag from the ejecta on the rapidly moving particles
limits their travel distance. By observing the effects of gas drag on the
travel distance of grains, we propose that supernova grain aggregates form from
material that condensed in a highly localized region, which satisfies the
observational evidence of isotopic homogeneity in SiC-X grains.
