Gypsum Dissolution Rate from Atomic Step Kinetics

Abstract

The macroscopic dissolution rate of minerals is generally deduced from solution chemistry measurements. A microscopic dissolution rate can also be deduced from the dynamics of molecular events (etch pits growth, atomic step migration. . .). Both hardly ever agree, even qualitatively, and the elaboration of a general theory linking the kinetics at the two scales is still in progress. We present here microscopic dissolution rates of gypsum, measured by atomic force microscopy (AFM), in quantitative agreement with macroscopic rates. This agreement has been obtained in taking care to neutralize the bias induced by the force applied by the AFM tip on the surface, and to identify clearly the driving molecular mechanism. This result shows that the determination, among the topographic changes during the dissolution of a mineral, of the dominant one, and the measurement of its dynamics, may permit to deduce from AFM experiments a reliable macroscopic dissolution rate.

Tuning Biotic and Abiotic Calcite Growth by Stress

“…This inhibiting effect of stress is reminiscent of the dissolution of crystals in supersaturated solutions, due to the strain induced by some adsorbed inhibitor . This effect is opposite to the case of dissolution, where the elastic energy amplifies the dis-equilibrium …”

“…During dissolution, from a given threshold on, the application of a high force by the scanning AFM tip has been seen to induce a damaging of a gypsum or calcite surface, creating kink sites in the migrating steps. , But the functional dependence of the step speed on the applied force was observed to be completely different from a pressure solution law, following instead a much faster Arrhenius increase. With this in mind, we have ruled out such a corrosive wear here for two reasons.…”

“…Thereby a kymogram was obtained, with the step migration leaving a track in the (X, t) plane (t the time) and the velocity being the slope of the track. 25,26 With this method the scanning AFM tip has a double role: it measures the step velocity and, to achieve this goal, it applies locally a given force to the crystal.…”

“…The step movement rates were measured by aligning the scan direction so that two facing steps, one acute and one obtuse (Figure ), were parallel to the Y scan direction and then disabling this scan direction. Thereby a kymogram was obtained, with the step migration leaving a track in the ( X , t ) plane ( t , time) and the velocity being the slope of the track. , With this method the scanning AFM tip has a double role: it measures the step velocity, and to achieve this goal, it applies locally a given force to the crystal.…”

Identification of the parameters affecting the in situ dissolution process of natural gypsum

“…Most of the knowledge on the dissolution kinetics of gypsum comes from laboratory studies performed at the mineral gypsum-solution interface such as rotating disk setups or flow cell devices, which quantify the dissociation rate of the gypsum mineral under well-controlled flow conditions [8][9][10][11][12][13]. More recent experiments employ atomic force measurements [14,15] or holographic interferometry [16,17] to characterize kinetic processes at a nanometric scale. These previous studies often use gypsum samples under ideal conditions (powder, polished or cleaved gypsum crystals with a high purity criterion) and the solvent used is often pure water so as to have only the effect of the solubility of the mineral.…”