**2. Polymorphism of calcium carbonate**

Calcium carbonate has six different polymorphs: one amorphous CaCO3, two hydrated crystalline ones (hexahydrate CaCO36H2O and monohydrate CaCO3H2O), and three anhydrous crystalline polymorphs that are the hexagonal vaterite, the orthorhombic aragonite and the rhombohedra calcite [22–25]. The X-ray diffraction (XRD) diagrams of these polymorphs are shown in **Figure 2**. For CaCO3 polymorphs, the solubility increases in the order of calcite, aragonite, and vaterite [26, 27]. Calcite is the most stable polymorph under ambient atmospheric conditions [26]. In the presence of water, vaterite, which is the least thermodynamically stable polymorph, easily transforms into the more stable calcite or aragonite [28]. Environmental conditions play effective roles in crystal growth and morphological changes, namely, the solution supersaturation, calcium concentration, pH, and temperature. These experimental parameters should operate together and control the polymorphism of CaCO3.
