Intergrown mica and montmorillonite in the Allende carbonaceous chondrite

Calcium-, aluminium-rich inclusions (CAIs) in carbonaceous chondrites such as Allende are of considerable interest because of their primitive character. There are, however, differences of opinions as to whether they formed directly by condensation from the early solar nebula¹ or by the evaporation of primitive dust². The mineralogy of the CAIs potentially bears on this question and therefore much effort has been devoted to detailed petrographic studies. We present here high-resolution transmission electron microscopy (HRTEM) observations of a complex mixture of mica and montmorillonite that occurs within a fine-grained CAI in the Allende C3(V) meteorite. In spite of previous intensive petrographic investigations of this and related meteorites, the existence of such CAIs bearing mica and montmorillonite has not been reported previously, and there have been no previous positive identifications of these minerals in carbonaceous chondrites.