Many sedimentary calcian dolomites commonly display a modulated microstructure when viewed with the transmission electron microscope. This structure has been attributed to a disordered incorporation of excess calcium in the dolomite structure. We describe here two more highly ordered metastable superstructures (γ and δ dolomite) that are considered intermediate between disordered calcian dolomite as formed by dissolution-reprecipitation processes and—probably as a result of diagenesis—decomposition to stable stoichiometric dolomite and calcite. Small platelike regions of coherent calcite are observed in dolomite from the Devonian Lost Burro Formation in southeastern California and in heat-treated calcian dolomite. These observations document the evolution of sedimentary carbonates from metastable to stable phases under favorable kinetic conditions.