Electronic structure calculations, performed at the density functional theory level, were employed to study the surface termination of the {1014} calcite surface in contact with a gaseous phase containing water and carbon dioxide. A surface phase diagram was generated to investigate the change in surface termination as a function of temperature, pressure, and gas-phase composition. This diagram revealed that a nonstoichiometric termination could occur in atmospheric conditions at high relative humidity, hence suggesting that nonstoichiometric surfaces can play a major role in the chemistry of calcite surfaces.