The dissolution of calcite (CaCO3) in aqueous solution at pH below ca, 5-6 is known to proceed via the direct reaction of protons at the solid surface. However, exposure of the mineral to sulfuric acid is shown to lead to the rapid formation of an overlayer of calcium sulfate (gypsum) which completely passivates the surface against further reaction and dissolution. The overlayer is nucleated instantaneously on CaCO, surfaces, Scanning electron and atomic force microscopy show that strongly adherent overgrowths are encouraged by a lattice matching between the cleavage (211) plane of calcite and the (010) plane of CaSO4 . 2H(2)O.