An amorphous phase is instantaneously formed upon CaCO 3 precipitation. At room temperature, it transforms spontaneously into calcite through an intermediate vaterite phase. This process is solution mediated: the intermediate phase dissolves and recrystallizes into calcite, which is the most thermodynamically stable phase. Aragonite phase is formed, instead, in higher temperatures. Besides temperature, this dissolution-recrystallization process is also affected by the presence of additive ions or impurities. Presented in Figure The spherical particles obtained are composed of smaller growth units, which is peculiar to polycrystalline vaterite. In high C S conditions, however, the samples are scalenohedral calcite particles, particularly when C P and β are relatively low (Figures and 3j-l). They ions relative to the volume of precipitated particles.