Outstanding Problems in the Kaolinite‐Mullite Reaction Sequence Investigated by 29 Si and 27 Al Solid‐state Nuclear Magnetic Resonance: I, Metakaolinite

Structural models previously proposed for metakaolinite are examined in light of the most recent published experimental data and new information obtained by solid‐state high resolution 29 Si and 27 AI NMR. A new model for metakaolinite is proposed, consisting of anhydrous regions of distorted AI‐0 tetrahedra containing randomly distributed isolated residual hydroxyls associated with A1‐0 configurations of regular octahedral and tetrahedpal symmetry. Such a structure, which can readily be formed from kaolinite by the removal of hydroxyls in certain sequences, accounts for the lack of a well‐defined X‐ray pattern and the persistence of ∼10% residual hydroxyls in metakaolinite and is consistent with the most recent density data, bond lengths, and the new 29 Si and 27 AI NMR data.