The formation of leached layers on albite surfaces during dissolution under hydrothermal conditions

Abstract

Hydrothermally altered (225°C) albite was compositionally depth-profiled using X-ray photoelectron spectroscopy (XPS)coupled with calibrated Ar ion sputtering. Solution data were collected during dissolution runs for the same crystals which were spectroscopically analyzed. We found that leached zones depleted in Na, Al, and O develop during the initial, incongruent phase of dissolution. Angle resolved XPS (ARXPS) demonstrated that Na and Al are significantly depleted from the upper few monolayers. Depths of leaching, which range from 10 to 900 Å, decrease with increasing pH in theacid region and increase with pH in the basic region. Based on calculated dissolution rates the depth of leaching can be roughly correlated with the release rate of Si. From the observation that the equivalents of H + consumed always exceed the equivalents of Na + and Al 3+ released, hydrolysis cannot be considered to be a simple ion exchange process. The XPS spectra also revealed the presence of Cl − over the entire leaching depth for samples run at pH < pH zpc (zero point of charge) and Ba 2+ at pH > pH zpc, suggesting electrostatic adsorption of aqueous species at charged sites within the leached layer. The presence of Cl − and Ba 2+ also show that preferential leaching creates a porous and open structure which allows for the large-scale influx of solvent molecules. Preliminary evaluations of diffusion transport rates through leached layers suggest that dissolution is not rate limited by diffusion. Instead, the kinetics of dissolution seem to be related to the intrinsic rate of structural hydrolysis. Using the XPS and solution data in conjunction with theoretical and experimental studies in the literature, we propose a dissolution mechanism based on initial ion exchange followed by the hydrolysis of Al and Si, which is modeled as the breakdown of activated complexes formed at bridging oxygen (O br) sites. Elemental mass balances based on comparisons between the XPS and solution data suggest that dissolution occurs non-uniformly and is probably preferentially constrained to dislocations and macroscopic defects within the structure.