The formation of Al (oxy)hydroxide on the basal surface of muscovite mica was investigated to understand how the structure of the substrate controls the nucleation and growth of secondary phases. Atomic force microscopy images showed that solid phases nucleated on the surface initially as two-dimensional islands that were <= 10 angstrom in height and <= 200 angstrom in diameter after 16-50 h of reaction in a 100 mu M AlCl3 solution at pH 4.2 at room temperature. High-resolution X-ray reflectivity data indicated that these islands were gibbsite layers whose basic unit is composed of a plane of Al ions octahedrally coordinated to oxygen or hydroxyl groups. The formation of gibbsite layers is likely favored because of the structural similarity between its basal plane and the underlying mica surface. After 700-2000 h of reaction, a thicker and continuous film had formed on top of the initial gibbsite layers. X-ray diffraction data showed that this film was composed of diaspore that grew predominantly with its [040] and [140] crystallographic directions oriented along the muscovite [001] direction. These results show the structural characteristics of the muscovite (001) and Al (oxy)hydroxide film interface where presumed epitaxy had facilitated nucleation of metastable gibbsite layers which acted as a structural anchor for the subsequent growth of thermodynamically stable diaspore grown from a mildly acidic and Al-rich solution.