In-situ, real-time replacement of calcite under geometrical confinement.

Mineral replacement by dissolution-precipitation reactions that occur under confinement critically influences subsurface deformation by modifying rock porosity, permeability, and cohesion, and by inducing fracturing. Yet real-time, experimental observations of these phenomena at the nano- to microscale remain insufficient. In this work, we follow the in-situ replacement of confined calcite crystal using a surface force apparatus (SFA) technique. Calcite undergoes dissolution under low pH conditions, followed by replacement into three various Ca-minerals: calcium oxalate, calcium sulfate (gypsum), or calcium phosphate (brushite), depending on the initial composition of the solution. We monitor these reactions in real time, map the spatial distribution of precipitates as a function of confinement gap size, and evaluate how epitaxy between the secondary phases and the parent calcite governs preferred nucleation and growth sites. In addition, we measure forces that act on the confining pore wall during the replacement and estimate the associated crystallization pressures. This work contributes to the understanding of the mineral growth under confinement and its consequences for porous rock integrity, with immediate relevance to subsurface fluid and gas storage operations, where rapid mineralization is common.