KINETICS OF DOLOMITE DISSOLUTION STUDIED BY INTERFEROMETRY AND A NEW CONCEPTUAL MODEL FOR MINERAL DISSOLUTION

The dissolution rate and mechanism of three different rhombohedral cleavage faces of a dolomite crystal from Navarra (near Pamplona), Spain were studied in detail using vertical scanning interferometry (VSI) techniques. A total of 37 different regions (each about 124 x 156 microns in size) on the three sample surfaces were monitored as a function of time during dissolution at 25ºC and pH 3. Dissolution leads to shallow etch pits with widths reaching 20 microns during 8 hours of dissolution. Depth development as a function of time was remarkably similar for all etch pits on a given dolomite surface.

Based on etch pit distribution and volume as a function of time, the calculated dissolution rate increases from near zero to 4 x 10^-11 moles/cm²/sec over 5 hours. The time variation is fairly similar for each of the three cleavage surfaces studied. In addition, the absolute dissolution rates of different parts of the dolomite crystal surface were computed by using a reference surface. All the different surfaces yielded an "average" rate of 1.08 x 10^-11 moles/cm²/sec with a standard deviation of 0.3 x 10^-11 moles/cm²/sec based on about 60 analyses. The mean absolute rate of the dolomite surface is about ten times slower than the rate calculated from etch pit dissolution alone. On the other hand, earlier batch rate data using BET surface areas yielded rates that are at least 30-60 times faster than our measured mean dissolution rate based on direct observation of the surface for the same pH and temperature.

A conceptual model for mineral dissolution has been inferred from the surface topography variations obtained by the VSI investigations including the absolute reference surface. In this model, mineral dissolution is not dominated by etch pit formation itself but rather by extensive dissolution stepwaves that originate at the outskirts of the etch pits. These stepwaves control the overall dissolution as well as the dependence on temperature and saturation state.