REACTION MECHANISM FOR LIGAND-PROMOTED DISSOLUTION OF FORSTERITE AT LOW PH

            A series of 104 olivine dissolution experiments in the presence of oxalate over the pH range 1<pH<7 shows that the addition of 0.02 m oxalate increases the dissolution rate by about 1.5 log units.  Factorial experiments with variable oxalate concentrations and pH show that the rates depend upon both factors.  Amounts of total oxalate varied from 3.0x10^-4 to 3.5x10^-1 m.  The combined rate law produced by our experiments for proton-promoted plus ligand-promoted dissolution is

r=10^-6.47a_H+^0.51a_H20^m1 + 10^-5.00a_H+^-0.47a_Ox^0.33.

Based on these experimental results and the observation that the surfaces of forsterite dissolving at low pH are silica enriched, we propose that upon initial contact with an acidic solution, precursor reactions cause Mg²⁺ to be quickly released, enriching the surface in Si.  This Si-enriched surface is relatively stable and the silica is released by a relatively slow ligand-exchange reaction which releases H₄SiO₄ into solution.  However, the bond between the Mg and the bridging oxygen is strong and will not be broken unless the O has been protonated, weakening its attachment to the Mg.  Because protonation is necessary for the ligand-exchange to occur, this reaction has a strong pH dependence.

These results suggest that because both a proton and a ligand (either an anion or a water molecule) must be present in the activated complex, the distinction between proton-promoted and ligand-promoted reactions may be misleading. Because the charge on the water dipole is much smaller than the charge on the oxalate ion, water molecules mount a much weaker nucleophilic attack on the Mg center than oxalate ions do, so the rate constant for dissolution in a simple water solution (first term in the rate law) is much less than the rate constant for the oxalate assisted rate (second term in the rate law).