Impact of coating structure on the rates of coupled dissolution-precipitation reactions

Coupled dissolution-precipitation reactions are central to many processes in Earth’s crust, significantly impacting natural and engineered systems. During these reactions, secondary mineral coatings often form on primary mineral surfaces, creating armoring layers that passivate the surface and dramatically reduce reaction rates. While the structure of secondary coatings is thought to influence their effect on coupled reactions, few studies have focused on characterizing their porous nature. Here, we investigated the reaction between calcite and sulfuric acid, which causes gypsum to precipitate as an armoring layer on the dissolving calcite surface. Our experiments revealed a two-stage growth pattern: initial rapid formation (<0.5 h) of a ∼20 μm thick layer of gypsum, followed by constant growth at a rate of approximately 0.71 μm h⁻¹for 768 h, ultimately producing a layer over 500 μm thick. Consistent with previous studies, the secondary mineral layer causes the reaction rate to be several orders of magnitude lower than expected given the low pH conditions. High-resolution FIB-SEM imaging revealed a complex heterogeneous and anisotropic structure in the secondary mineral coating. The layer closest to the calcite interface exhibited extremely low porosity (1 %) with poorly connected nanoscale pores, while the outer region showed much higher porosity (31 %) with a well-connected pore network. Notably, numerous parallel cracks extended from the calcite interface into the high-porosity zone, potentially serving as the primary conduits for solute transport through the gypsum layer. A novel diffusion–reaction model containing 2 porous layers with contrasting diffusivities successfully reproduced the reaction rates we observed in our experiments. Our results demonstrate that the micrometer and nanometer scale structure of the coatings surrounding reacting minerals is a crucial factor controlling the rates of coupled dissolution-precipitation reactions in geochemical systems.