Feldspar dissolution in acidic and organic solutions: Compositional and pH dependence of dissolution rate

The steady-state dissolution rates of plagioclase feldspars into inorganic acid solutions in a flow-through reactor increased with Al content of the mineral from 1.4 . 10(-11) mol Si/m(2)/s for albite to 5.6 . 10(-9) mol Si/m(2)/s for bytownite. A similar trend was observed for minerals dissolved in neutral solutions although the rates were lower. The results of these experiments are used to develop a simple empirical equation to describe the dissolution of tectosilicates (quartz + feldspars): R(H) = k(H)a(H+)(nH) where R(H) is the dissolution rate of tectosilicates in acid solution, a(H+) is the activity of H+ ion, and k(H) and n(H) are dependent on the aluminum fraction in the tectosilicate framework [Al/Al + Si]: log k(H) = -11.24 + 25.98* [Al/Al + Si](2) and n(H) = -0.052 + 4.23* [Al/Al + Si](2). This model, with its strong dependence on Al fraction, suggests that tectosilicate dissolution in acid solution results primarily from attack at Al sites at the mineral surface. In acidic oxalate solutions the steady-state dissolution rates were, in some cases, up to a factor of 10 higher than dissolution rates in inorganic solutions at the same pH and appeared to have a similar dependence on pH and mineral composition, at least away from the extremes in aluminum fraction (quartz and bytownite). On the basis of the results of the experiments with acidic oxalate and previous experiments showing a linear dependence of feldspar dissolution rate on organic ligand concentration, an empirical expression for the ligand-promoted component of tectosilicate dissolution rates as measured by silica release (R(L)) is proposed: R(L) = (kappa(H)(L) [L] - k(H))a(H+)(n) + R(H)(L) (Si) where the first term describes the effect of competitive proton and ligand attack at Al sites at the mineral surface leading to silica release to solution and R(H)(L)(Si) reflects the smaller rate of attack at Si sites (kappa(H)(L) is a factor depending on the ligand, [L] is the ligand concentration, k(H) and a(H+) are as given above, and n describes the pH dependence of ligand- and proton-promoted dissolution and is taken to be equal to n(H) away from the extremes of aluminum fraction). The strong dependence of dissolution rate in acidic organic solutions on aluminum fraction indicates that both protons and ligands attack the mineral surface at the same, presumably Al, sites.