Hydroxyl Orientations and Interlayer Bonding in Amesite

The hydroxyl orientations in the 2H 2 polytype of amesite, Mg 2 Al(SiAl)O 5 (OH) 4 , have been determined by minimizing the electrostatic potential energy as a function of OH orientation. The angles, ρ , 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 VI ions, and the spread in ρ values is due to variations in the positions of the receptor oxygens. The inner hydroxyls noticeably weaken the interlayer bonding. The substitution of Al 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 2H 2 polytype of amesite has substantial interlayer bonding even with all surface hydroxyls replaced by fluorine. This is not true for a composition of Mg 3 Si 2 O 5 (OH) 4 where a similar F for OH substitution destroys all interlayer bonding as in kaolinite.