The Kaolinite‐Mullite Reaction Series: IV, The Coordination of Aluminum

The AlKα emission wave length is determined by the state of coordination of Al atoms and a clear distinction can be made between Al atoms in fourfold and in sixfold coordination to oxygen and also between Al atoms in fourfold coordination to oxygen and to nitrogen. Measurements of Δ(2θ) (the difference in diffraction angle for AlKα radiations from a sample and from metallic aluminum) are used to distinguish Al in fourfold and in sixfold coordination in normal and in heat‐treated kaolin minerals. The Al atoms in meta‐kaolin are shown to have fourfold coordination to oxygen in agreement with the structure proposed by Brindley and Nakahira. After heat‐treatment at 1024°C., the coordination of Al reverts toward sixfold, and this is more marked for disordered kaolinite and halloysite than it is for well‐crystallized kaolinite and dickite. Infrared absorption measurements by Stubičan and Roy provide further confirmation of the coordination changes of the Al ions in these reactions.