Structural analysis of the transition phases in the kaolinite-mullite thermal sequence

Dehydroxylation phases of a kaolinite from Zettlitz, Czechoslovakia, were investigated by radial electron density distribution (RED) and X-ray spectroscopy. Special emphasis was placed on the 2 amorphous or poorly crystallized transition phases, i.e. metakaolinite (600 degrees to 800 degrees C) and the spinel structure (900 degrees C). The RED diffraction method, which is particularly suited to the study of such materials, indicated (1) a particularly expanded tetrahedral configuration for the Al atoms in metakaolinite and (2) a pure Al/sub 2/O/sub 3/ spinel structure. The first result confirms earlier observations; the second eliminates, for the experimental conditions and kaolinite studied, the possibility of the formation of an Al-Si spinel structure with the Al and Si occupying the octahedral and tetrahedral interstices, respectively. Complementary measurements on the Al/b K/ alpha line shifts for the same samples support the present interpretation. Determination of the apparent Si coordination number by the same spectroscopy procedure revealed a regular 4 coordination for all phases except the end-member mullite.