Effect of chemical affinity on feldspar hydrolysis rates in two natural weathering systems

Recent studies of alkali-feldspar hydrolysis kinetics have shown that, in the near-neutral pH range, weathering rates in natural systems are up to three orders of magnitude slower than laboratory rates. It has been hypothesized that decelerated rates may result from lower thermodynamic affinities for the hydrolysis reaction in natural systems than in laboratory systems. However, the chemical affinities for the feldspar hydrolysis reaction in two well-constrained natural systems are significantly higher than the threshold value at which affinity would exert detectable influences on the reaction rates. Thus, the hypothesis is rejected, and closer proximity to thermodynamic equilibrium in natural weathering systems does not account for the observed discrepancy between natural and laboratory rates of feldspar hydrolysis. Differences in feldspar weathering rates between natural and laboratory systems are most likely due to a combination of experimental preparation artifacts, loss of reactive surface to the formation of etch pits in naturally weathered feldspars, and inhomogeneous access of reactive fluids to those surfaces.