Interferometric study of the dolomite dissolution: a new conceptual model for mineral dissolution

The dissolution rate and mechanism of three different cleavage faces of a dolomite crystal from Navarra (near Pamplona), Spain, were studied in detail by vertical scanning interferometry techniques. A total of 37 different regions (each about 124 × 156 (mu)m in size) on the three sample surfaces were monitored as a function of time during dissolution at 25°C and pH 3. Dissolution produced shallow etch pits with widths reaching 20 (mu)m during 8 h of dissolution. Depth development as a function of time was remarkably similar for all etch pits on a given dolomite surface. On the basis of etch pit distribution and volume as a function of time, the calculated dissolution rate increases from near zero to 4 × 10-11 mol cm-2 s-1 over 5 h. The time variation is different for each of the three cleavage surfaces studied. In addition, the absolute dissolution rates of different parts of the dolomite crystal surface can be computed by using a reference surface. The different surfaces yield an "average" rate of 1.08 × 10-11 mol cm-2 s-1 with a standard deviation of 0.3 × 10-11 mol cm-2 s-1 based on about 60 analyses. The mean absolute rate of the dolomite surface is about 10 times slower than the rate calculated from etch pit dissolution alone. On the other hand, earlier batch rate data that used BET surface areas yield rates that are at least 30 to 60 times faster than our directly measured mean dissolution rate for the same pH and temperature. A conceptual model for mineral dissolution has been inferred from the surface topography obtained by the interferometry investigations. 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.