Time-dependent CVs provided information on the repeatable phase transition between ACC and crystalline phases, as observed in Fig. The c-axis length (18.20-18.66 Å) of the sampled R3/c SG with 6 Ca 2+ sheets, as indicated in Table , is longer than that of stable calcite crystals (17.6 Å). These structures can be identified as metastable calcite structures, with a slightly shorter a-axis and longer c-axis (∼0.14 Å and 1 Å, respectively) than stable calcite crystals, as shown in Table . The resulting density is about 2% lower than that of calcite crystals but higher than that of vaterite crystals (∼2.54 g/cm 3 ). Notably, a calcite structure perfectly matching experimental data did not emerge, likely due to the use of XRD CVs tailored for vaterite crystals.