The Gibbs free energy change, ΔGr, dependence of the anorthite dissolution rate in a supercritical CO2–water system was measured as part of a geochemical assessment of CO2 geological sequestration. Being bounded at ΔGrcrit, the critical ΔGr required for an opening of the etch pit at a screw dislocation, it has been accepted that mineral dissolution follows etch pit formation assisted by dislocations below ΔGrcrit, whereas the horizontal step retreats without the etch pit above ΔGrcrit. The experiment described herein, however, revealed that another mode of dissolution occurs more distant from equilibrium by spontaneous formation of the etch pit over the entire surface, as observed on calcite. The dissolution rate is higher by more than one order of magnitude than that in the dislocation-assisted mode. Therefore, including the rate gap at ΔGrcrit, a nonlinear curve with three steps instead of a sigmoidal curve is proposed for the ΔGr dependence of the anorthite dissolution rate. Extremely slow rates were observed depending on observed points for the same ΔGr condition. Although the reason for such a rate difference remains unknown, it is likely related to the defect density on the crystal surface. It is possible that initial spreading of the dissolved surface attributable to the etch pit formation assisted by defects provides some trigger for subsequent explosive etch pit formation. These findings suggest that the initial transient process can strongly influence the kinetics of geochemical reactions that occur during CO2 geological sequestration.