DO SILICATE DISSOLUTION MECHANISMS DEPEND ON PH?

Among those parameters influencing mineral and glass dissolution rates, solution pH has been perhaps the most extensively studied. Evaluation of major rock forming silicate dissolution rates reveals that the degree to which dissolution mechanisms may or may not change with pH depends on the structure and bond types present in each silicate. Silicates whose frameworks are comprised of Si-O and tetrahedral Al-O bonds, such as albite and basaltic glass, appear to have pH independent dissolution mechanisms. The logarithm of both albite and basaltic glass far from equilibrium, constant temperature dissolution rates plot as a single linear function of log (a(H+3)/a(Al3+)), where a refers to the activity of the indicated aqueous species, at all pH. In contrast, silicates whose structures contain Si-O and octahedral Al-O bonds, such as kaolinite and muscovite, have pH dependent dissolution mechanisms. Logarithms of kaolinite and muscovite far from equilibrium, constant temperature dissolution rates plot as single functions of log (a(H+3)/a(Al3+)), at acidic conditions, but at basic conditions also depend on aqueous silica activity. Similarly, forsterite far from equilibrium, constant temperature dissolution rates are independent of aqueous Mg and SiO2 activity at acidic conditions, but depend on aqueous SiO2 activity at basic conditions.

The pH dependence of a multi-oxide silicate dissolution mechanism can be deduced from the corresponding pH dependence of the relative breaking rates of its constituent metal oxygen bonds. These relative breaking rates can be inferred from the dissolution rates of single metal (hydr)oxides. Corundum, gibbsite, and brucite dissolution rates are significantly faster than that of quartz at low pH, but appear to be slower than that of quartz at high pH. In contrast, the rates of breaking tetrahedral Al-O bonds appear to be faster than that of Si-O bonds at all pH. Consequently those silicates containing Si-O and either octahedral Al-O or Mg-O bonds exhibit pH dependent dissolution mechanisms. It seems likely that these same concepts can be used to deduce the degree to which multi-oxide silicate dissolution mechanisms vary with other factors, such as temperature, aqueous organic and inorganic species activities, and silicate composition.