Gypsum Dissolution Rate from Atomic Step Kinetics

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 pit growth, atomic step migration, etc.). 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 deducing from AFM experiments a reliable macroscopic dissolution rate.