Discontinuous Dissolution Mechanism of Olivine Deduced from a Topography Observation Method

Olivine dissolution plays an important role in environmental science and technology, from controlling global element circulation to carbon capture for climate change mitigation. Most studies have been focused on investigating its dissolution rates by monitoring chemical effluent changes under various conditions. However, only by observation of surface reactivity can we unravel the actual mechanism (s) of dissolution. Here, we studied the dissolution of an olivine (010) plane in a flow-through reaction cell with an acidic solution, a surface-controlled regime, and far-from-equilibrium conditions. Direct mineral surface topography measurements using vertical scanning interferometry and atomic force microscopy allowed for quantitative analyses of the spatial and temporal changes in the dissolution rate. The (010) plane dissolved discontinuously in time for different surface sites, resulting in a heterogeneously distributed rate map. Pits with different depths showed opposite dissolution rate distributions from the dislocation center to further out from the etch pit. Based on the step-wave model, we propose a mechanism of dissolution that is governed by the competition between Gibbs free energy of the dissolution process, ΔG, and the critical free energy of the opening of etch pits, i.e., ΔG_crit. The migration of step waves, the distribution of surface defects, the strain field of etch pits, and other dynamic elements, resulting in the instantaneous change of ΔG_crit on the surface, are important factors leading to the discontinuous dissolution of crystal materials. This discontinuous dissolution provides new insight into the guidance of crystalline mineral applications and the prediction of material properties regarding mineral dissolution variation.