Dissolution of albite glass and crystal

Abstract

When normalized by initial surface area, crystalline and amorphous albite release Si and Al at the same rate within error (±40%) as measured at pH 2, 5.6, and 8.4 at 25°C. Differences in density and tetrahedral ring structure between the glass and crystal structures, however, lead to more extensive Na and Al depletion from the glass surface, especially in acid. X-ray photoelectron spectroscopy (XPS) indicates that the chemistry of the altered layers on glass and crystal must be significantly different at a depth of ∼17Å–87Å. Nevertheless, angle-resolved XPS (ARXPS) indicates that the outermost 17Å of the glass and crystal surface are compositionally similar. In neutral and weakly basic conditions, XPS indicates less extensive depletion of Na and Al from reacted glass and crystal surfaces than in acidic conditions. Al enrichment was not observed at any pH on either the crystal or glass surface. At steady state, Al release was stoichiometric for all phases and all pH values, but Na release was always faster than release of Si, especially for the glass. These results are consistent with a model where only the outer surface controls dissolution and the deeper layers of the altered surface do not significantly affect dissolution rate. The similarity in dissolution rate between glass and mineral, if consistent for other phases, may also indicate that some future studies of mineral dissolution could be completed more efficiently by investigation of glass because such studies could reveal the chemical effects in dissolution independent of the microstructure and defects that populate natural mineral samples.