Hydroxyl Orientations and Interlayer Bonding in Amesite

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Abstract
The hydroxyl orientations in the 2H2 polytype of amesite, Mg~AI(SiAI)O~(OH)4, have been de- termined by minimizing the electrostatic potential energy as a function of OH orientation. The angles, p, between the hydroxyls and (001) vary between 81,5 and 88.8 ~ All surface hydroxyls form hydrogen bonds with oxygens of the adjacent layer. The OHs tend to tilt away from the higher charged A1 v1 ions, and the spread in p values is due to variations in the positions of the receptor oxygens. The inner hydroxyls no- ticeably weaken the interlayer bonding. The substitution of AI in both T and M sites creates a dipolar layer charge and the resulting attraction between layers forms an important part of the interlayer bonding. The 2H2 polytype of amesite has substantial interlayer bonding even with all surface hydroxyls replaced by fluorine. This is not true for a composition of Mg3SizQ(OH)4 where a similar F for OH substitution destroys all interlayer bonding as in kaolinite.

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