In an earlier report, we showed that altered layers formed on the surface of dissolving labradorite feldspar grains, and that the thickness of these layers (up to hundreds of angstroms) is strongly dependent on the pH of the reactant solution. In this paper, we show that the thickness of these altered layers also depends strongly on the composition of the plagioclase feldspar. Secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) have been used to characterize these altered layers. During dissolution, Ca and Al are removed from the solid material to form an altered layer residually enriched in Si with very similar profiles for Ca and Al. In acidic solutions (pH 3.5) for 90 days, the altered layers increase in thickness from a few hundred angstroms to many hundred angstroms in the order: albite, oligoclase, labradorite, bytownite. These results emphasize the central role of hydrolysis of the bridging Si-O-Al bonds in the initial weathering process. Analysis by scanning electron microscopy (SEM) does not provide any evidence for the growth of secondary phases. Extensive dissolution features (etch pits) were observed on many of the reacted surfaces.
