REACTIONS OF AQUEOUS ANIONS AND CATIONS AT THE LABRADORITE-WATER INTERFACE - COUPLED EFFECTS OF SURFACE PROCESSES AND DIFFUSION

Incongruent dissolution of labradorite in acidic solutions due to sele ctive leaching of Na, Ca, and Al results in a leached surface layer en riched in Si. Preferential leaching extends several hundred angstroms below the mineral surface. SIMS profiles indicate that the depth from which Na, Ca, and Al are removed in advanced of Si depends not only on concentration but also on speciation of cations and anions in solutio n. Aqueous cations in acidic solution reduce leaching of Na, Ca, and A l relative to Si in the feldspar. On the other hand, certain anions in acidic solution enhance leaching of Na, Ca, and Al relative to Si. Di ssolved cations can be adsorbed or complexed at labradorite surfaces a nd formation of surface metal-ligand complexes facilitates detachment of cations from the surface. Increasing or decreasing dissolved cation concentrations in the vicinity of the mineral surface also affects ch emical potential gradients between unleached labradorite and the solid -solution interface, thereby either decreasing or increasing thickness of the leached layer. Labradorite surface chemistry and depth or volu me of leaching reactions are discussed with reference to dissolved cat ions and anions in solution. Copyright (C) 1997 Elsevier Science Ltd.