TEM study of kaolinite thermal decomposition by controlled-rate thermal analysis

The dehydroxylation of kaolinite leading to metakaolinite was studied by transmission electron microscopy. TEM techniques allow the study of morphology, structure (by selected-area electron diffraction) and lattice imaging (by high resolution) in a non-destructive way. The samples were prepared using a controlled-rate thermal analysis method which provides a well-defined material (near thermodynamic equilibrium). The dehydroxylation begins near the defects of the structure (stacking defects, dislocation and edges) and progresses from them slowly, producing metakaolinite without any intermediate state. The formed metakaolinite is still organized: the two-dimensional hexagonal structure of the ab plane remains, but with a shorter extension (approximate to 50 nm). A lamellar structure of a few layers with an interlayer period of about 1.2 nm appears in place of the kaolinite layers (0.71 nm). This structure is explained by the tetrahedral SiO4 sheet which remains but is highly deformed by a distortion wave due to the new aluminium and oxygen (remaining from the OH sheet) distribution after the loss of water.