The kinetics of calcite dissolution and precipitation in geologically relevant situations of karst areas: 2. Closed system

Abstract

A general theory of dissolution and precipitation rates of calcite in the pure CaCO 3CO 2H 2O system under closed-system conditions is presented. The three rate-limiting processes, surface-controlled dissolution or precipitation at the CaCO 3 surface, diffusion of the molecular and ionic species, and slow conversion of CO 2 into HCO − 3 are treated simultaneously. Dissolution and precipitation rates are calculated as a function of the Ca 2+ concentration in the solution at various temperatures T and partial pressures of CO 2, P CO 2, in the solution. The rates can in all cases be approximated by a linear dependence R = α([Ca 2+] eq − [Ca 2+]), where α = α( T, P CO 2, δ) and 2δ is the thickness of the water film enclosed by two calcite surfaces. Values of α are summarized in two tables which provide the geologist with data from which dissolution rates of pure limestone can be derived easily. The calculated rates for turbulent flow are one order of magnitude higher than those for laminar flow. This provides the first theoretical explanation of the so-called hydraulic jump. We have carried out dissolution experiments to determine the time dependence of the Ca 2+ concentration in stagnant and turbulently stirred H 2O films at given P CO 2, δ and T. From the time constants of the exponential behaviour of [Ca 2+]( t) the values of α have been determined. They are in good agreement with the theory.