Synthesis of All Crystalline Phases of Anhydrous Calcium Carbonate

Usually, the metastable vaterite phase transforms to the thermodynamically most stable calcite phase. We report here the phase transformation of vaterite to aragonite, which is seldom observed in the synthetic processes of calcium carbonate. The vaterite, upon equilibration with distilled water at room temperature, gradually transforms to aragonite. This transformation process is further accelerated when the vaterite sample is refluxed in distilled water. Under a different set of conditions, when equilibrated in the mother liquor, the same vaterite sample transforms to calcite without going through aragonite as an intermediate phase. The formation of all three crystalline polymorphs of calcium carbonate is achieved without the use of any additive. These polymorph transformations have been monitored using powder X-ray diffraction and FTIR spectroscopy. The morphological changes of the calcium carbonate samples isolated during the course of phase transformation processes are captured using scanning electron microscopy. The transformation either to aragonite or to calcite has been explained based on the dissolution−recrystallization mechanism.