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

Abstract

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 29Si and 27AI 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 29Si and 27AI NMR data. Copyright © 1985, Wiley Blackwell. All rights reserved