THE SURFACE-CHEMISTRY OF MANGANIFEROUS SILICATE MINERALS AS INFERRED FROM EXPERIMENTS ON TEPHROITE (MN2SIO4)

The dissolution rate of tephroite in oxygen-free solutions decreases with increased pH over the interval 2 less-than-or-equal-to pH less-than-or-equal-to 8 and with decreased temperature over the range 25-45-degrees-C. The pH-dependence is similar to other orthosilicate minerals at 25-degrees-C and presumably relates to variations in the concentration of adsorbed hydrogen ions. The rate order with respect to solution pH increases with temperature and the experimental activation parameters vary strikingly with solution pH. These variations are interpreted to result from contributions of enthalpy to the experimental activation energy (E(exp)) from proton adsorption and from coulombic interactions among charged sites on the mineral surface. The dependence of dissolution rate on the logarithm of solution pH is only approximately linear with pH; the dependence is sensitive to changes in temperature via the conditional equilibrium constant that describes the concentrations of charge sites on the mineral surface. X-ray photoelectron spectroscopic (XPS) measurements of the reacted mineral surfaces indicate that Mn remains in the divalent valence state at all conditions. The near-surface region of the mineral,after acid dissolution has a Mn/Si ratio which is lower than the unreacted material while the opposite result is observed after reaction at basic-pH conditions.