Effect of pH on the Formation of Gibbsite-Layer Films at the Muscovite (001)-Water Interface

The heterogeneous nucleation and growth of aluminum (oxy)hydroxide films in aqueous environments can be controlled by the speciation of dissolved Al species and the charge and structure of underlying mineral surfaces. The structure of gibbsite films nucleated at the interface between the muscovite (001) surface and 1 mM AlCl3 solutions was investigated as a function of pH using in situ X-ray reflectivity. The growth of well-ordered gibbsite films was observed at pH between 3 and 4, even when the solutions were undersaturated with respect to gibbsite. The ordering of these gibbsite films likely resulted from the structural similarity (i.e., epitaxy) between the basal planes of gibbsite and muscovite. The gibbsite films formed on the muscovite surface dissolved more than two orders of magnitude faster than bulk gibbsite in pH 4 HCl solution, indicating that they can be described as a metastable phase. In contrast, no film growth was observed at pH between 9 and 12 where the solutions were supersaturated with respect to gibbsite. These results indicate that adsorption and accumulation of aqueous Al(III) species (i.e., Al3+ and AlOH2+ at acidic pH) is a critical step for the formation of secondary minerals on the negatively charged muscovite surface.