Hydroxyl orientations in gibbsite and bayerite

Abstract

The hydrogen positions in bayerite and gibbsite [both AI(OH)a] have been determined by minimizing the electrostatic energy as a function of hydroxyl orientation for a fixed O-H distance. The hydrogen positions in gibbsite are known from an a~curate X-ray refinement and one of the H-H distances is short (2.122 ,~). The point charge model (fully ionized atoms) separates these hydrogens resulting in deviations from the observed OH orientations of approximately 18 ° . Several models with reduced charges on the ions were refined with no improvement in the hydroxyl orientations. The addition of a modifying function to simulate sp 3 hybridization of the oxygens resulted in hydrogen positional parameters which are within two standard deviations (X-ray) of the X-ray refinement for 16 of the 18 positional parameters. Hydrogen positions have been determined for all six geometrically possible hydroxyl orientations and the correct one has the lowest electrostatic energy. There are three geometrically possible hydroxyl orientations for bayerite and all three have been refined. The model with the lowest energy has, between the layers, one hydroxyl nearly normal to the hydroxide layer, (001), participating in a single hydrogen bond, a second hydroxyl inclined at 58.3 ° to (001) participating in a single hydrogen bond and a third hydroxyl in the surface of the hydroxide layer participating in a bifurcated bond.

Comparison of Orientations of OH-Bonds in Layer Silicates by Diffraction Methods and Electrostatic Calculations

“…The studies by Giese and co-workers Giese and Datta, 1973;Datta and Giese, 1971;Giese, 1976), and the results of the present paper establish thus two classes of minerals for which the electrostatic calculations predict hydroxyl orientations if detailed information on the geometry of the structure is available. In some minerals, such as protolithionite, the comparison of the calculated and experimental positions of the protons assists in understanding the distribution of cations throughout nonequivalent positions.…”

“…Therefore, before applying this method, it is necessary to make sure of the reliability of the results obtained. Giese and coworkers showed that results obtained by this method are in agreement with the diffraction data for compounds of the XOOH type (X = A1, Fe, Mn, and Y) and for some hydroxides Giese, 1976). Because of the absence of the experimental proton positions for all refined layer silicates they were only able to check their calculations in the case of muscovite-2M1 and in the less important case of phlogopite (Giese, 1979).…”

The Surface Coulomb Energy and Proton Coulomb Potentials of Pyrophyllite {010}, {110}, {100}, and {130} EDGES

“…A complete discussion of the method is found in these references. Giese (1976Giese ( , 1979Giese ( , and 1984 demonstrated the importance of hydroxyl-bond orientations in the stability of minerals. This paper uses optimized OH bond-vector orientations to allow for this effect on surface energy.…”

Hydroxyl Orientations in 2:1 Phyllosilicates

“…Determining the hydroxyl orientation by minimizing the calculated electrostatic potential energy has proven to be an accurate and straightforward procedure (Giese, 1971(Giese, , 1976. The electrostatic potential energy is calculated as a function of the OH orientation, the minimum energy giving the equilibrium orientation.…”