THE DISSOLUTION KINETICS OF MIXED-CATION ORTHOSILICATE MINERALS

Proton-promoted dissolution rates of orthosilicate minerals of the oli vine and willemite structural classes were examined as a function of c omposition, pH, and temperature. The dissolution rates far from equili brium vary by many orders of magnitude but exhibit a similar dependenc e on pH and temperature. The dissolution rates correlate well with the rates of solvent exchange around the corresponding hydrated, divalent cation. This correlation suggests that the silicate tetrahedra are re leased intact from the reacting surfaces after protonation and hydrati on of bonds between divalent metals and structural oxygens. Surprising ly, the correlation remains sound even for minerals with dissimilar di valent cations in the structure. Rates for intermediate compounds in a binary series vary approximately exponentially between the dissolutio n rates for the endmembers, which greatly simplifies prediction of the reactivities. The similar variation in Arrhenius activation energies with pH for all minerals suggests that the experimental activation ene rgies include large contributions of enthalpy from equilibrium acid-ba se reactions. The acid-base properties are not particularly sensitive to homovalent substitutions df cations in the mineral structure, even though such substitutions dramatically affect the mineral reactivity a nd mineral lattice energies.