  Fluid-induced alteration of rocks and mineral-based materials often starts at
confined mineral interfaces where nm-thick water films can persist even at high
overburden pressures and at low vapor pressures. These films enable transport
of reactants and affect forces acting between mineral surfaces. However, the
feedback between the surface forces and reactivity of confined solids is not
fully understood. We used the surface forces apparatus (SFA) to follow surface
reactivity in confinement and measure nm-range forces between two rough calcite
surfaces in NaCl, CaCl$_2$, or MgCl$_2$ solutions with ionic strength of 0.01,
0.1 or 1 M. We observed long-range repulsion that could not be explained by
changes in calcite surface roughness, surface damage, or by electrostatic or
hydration repulsion, but was correlated with precipitation events which started
at ${\mu}$m-thick separations. We observed a poorly crystalline or amorphous
precipitate that formed in the confined solution. This liquid-like precipitate
did not undergo any spontaneous ripening into larger crystals, which suggested
that confinement prevented its dehydration. Nucleation was significantly
postponed in the presence of Mg$^{2+}$. The long-range repulsion generated by
nucleation between confined mineral surfaces can have a crucial influence on
the evolution of the microstructure and therefore the macroscopic strength of
rocks and materials.
