NEAR-SURFACE COMPOSITION OF ACID-LEACHED LABRADORITE INVESTIGATED BY SIMS

The composition of the near-surface of acid-leached labradorite was in vestigated by depth-profiling with secondary ion mass spectrometry for H, D, O, Na, Al, Si, Cl, and Ca. Specimens were leached at room tempe rature at pH 1 for 500 h. and pH 2 and 3 for 1200 h, in solutions acid ified with HCl. Complete leaching of Na, Ca, and Al occurred to a dept h of 100 nm after 1200 h of reaction at pH 3 and produced sigmoidal de pletion/concentration profiles. Depletion depths of these cations are shown to increase further with H+ -activity and time. The presented el ement profiles, therefore, represent non-steady-state conditions. Calc ium is removed from between 10 and 30 nm deeper within a single profil e than the corresponding Al depletion depth. After removal of Na+ and Ca+, a sequence of reaction steps results in preferential leaching of Al over Si and formation of silanol groups around the tetrahedral vaca ncy. This is immediately followed by spontaneous condensation of adjac ent silanol groups, which eliminates a fraction of oxygen from the lea ched layer. The O/Si ratio of the residual structure is thus reduced, and a high degree of cross-linkage between Si-tetrahedra is maintained . Throughout the leached layer, the O/Si atomic ratio is reduced from 3.3 (fresh labradorite) to values between 2.5 and 2.0. The leached lay er is enriched in H in all specimens, with average concentration plate aus of about 7.10(21) atoms/cm(3), or 11 at%, if the leached layer is assumed to have a density equal to amorphous SiO2. At the time of anal ysis the leached layer is anhydrous and composed solely of H, Si, and O. The H concentrations are in accord with an O/Si atomic ratio of 2.2 in terms of charge balance. The analytical composition corresponds to SiO1.8(OH)(0.4). Good agreement between leaching depths determined by SIMS profiling and as calculated from element release rates during ki netic dissolution experiments suggests that the sites for ion-exchange and depolymerization reactions are uniformly distributed over the ent ire mineral surface, and that any changes in reactive surface area hav e been small during these experiments. Copyright (C) 1997 Elsevier Sci ence Ltd.