THE DISSOLUTION KINETICS OF CALCITE, DOLOMITE, AND DOLOMITIC ROCKS IN THE CARBON DIOXIDE - WATER SYSTEM

The dissolution kinetics of calcite, dolomite, and dolomitic rocks were studied in laboratory experiments in aqueous carbonate solutions at 0(DEGREES) to 25(DEGREES)C. Emphasis was placed upon understanding how the effects of solvent motion and of carbonate lithology modify dissolution rates.;Observed single crystal calcite dissolution rate was constant from pH = 4 to pH = 5-5.5 and decreased sharply near pH = 5.7-5.8. Rate exhibited some dependence on spinning speed indicating an element of mass transport control in rate, but only far from equilibrium. Constant forward rate was dependent upon H(,2)CO(,3). Hydration of CO(,2)(aq) near the crystal surface may be the slow step in the net dissolution reaction far from equilibrium. Empirical backward rate is approximately first order with H('+). The dissolved crystal surface displayed a uniformly rugged topography of crystallographically controlled etch features. For conditions close to equilibrium, surface reactions control calcite dissolution rate.;Dissolution rates for single crystal dolomite, coarse-grained dolomite marble, and microcrystalline sedimentary rock are similar in form and value. Surface morphology of the dissolved single crystal was characterized by deep narrow holes while the rocks dissolved along grain boundaries. The grain size effects on dissolution rate were insignificant. A spinning rate dependence was observed far from equilibrium, but it became a less important factor as saturation was approached. From initial conditions, the dissolution rate decreased as the solution became more saturated. At solution conditions still far from equilibrium (ion activity product = 10('-19)), rate dropped off sharply to a very low value. Evidence of high activation energies and very high reaction orders indicate surface reactions are controlling dolomite dissolution rate.;The experimental design provided for accurate control of the hydrodynamic conditions. The samples were held on the end of a motorized spinning shaft which extended into the reaction solution. The disk-shaped samples had one face exposed to the solution. The diffusion boundary layer on a rotating disk is a constant thickness everywhere, providing a uniformly accessible surface with characterizable mass transfer properties. Free drift dissolution experiments were monitored for changes in solution pH, Ca('2+), Mg('2+), HCO(,3)('-), and conductivity over time.