Calcium carbonate forms scales, geological deposits, biominerals, and ocean sediments. It plays a significant role in biomineralization and geosciences, influencing ocean chemistry and Earth's climate. Despite its importance, the precipitation mechanism of calcium carbonate remains poorly understood. This study demonstrates that stable prenucleation ion clusters of calcium carbonate form even in undersaturated solutions. These clusters are characterized by equilibrium thermodynamics and structural preformation, challenging the classical view of nucleation. The research shows that calcium carbonate clusters form in both undersaturated and supersaturated stages, with binding behavior dependent on carbonate concentration. These clusters are thermodynamically stable, not metastable, and can be detected using analytical ultracentrifugation. The clusters exhibit solute-like behavior, with their size and structure influenced by pH. The study also identifies two different amorphous calcium carbonate (ACC) phases, ACC I and ACC II, with distinct solubility products and polymorphs formed under varying pH conditions. The findings suggest that nucleation occurs through cluster aggregation, with prenucleation clusters acting as early precursor species. The classical view of nucleation, based on stochastic solute clustering, is contrasted with the novel view that prenucleation clusters form based on equilibrium thermodynamics. This mechanism allows for structural preformation, influencing the final crystalline structure. The study also indicates that the nucleation of different ACC phases may be under thermodynamic or kinetic control, though the exact mechanism remains unclear. The results have implications for understanding the crystallization of other minerals, as similar nucleation mechanisms may apply. The study highlights the importance of pH-dependent equilibrium thermodynamics in the formation of calcium carbonate clusters and their role in determining the final crystalline structure. The findings contribute to a deeper understanding of biomineralization and geosciences, offering new insights into the early stages of mineral crystallization.Calcium carbonate forms scales, geological deposits, biominerals, and ocean sediments. It plays a significant role in biomineralization and geosciences, influencing ocean chemistry and Earth's climate. Despite its importance, the precipitation mechanism of calcium carbonate remains poorly understood. This study demonstrates that stable prenucleation ion clusters of calcium carbonate form even in undersaturated solutions. These clusters are characterized by equilibrium thermodynamics and structural preformation, challenging the classical view of nucleation. The research shows that calcium carbonate clusters form in both undersaturated and supersaturated stages, with binding behavior dependent on carbonate concentration. These clusters are thermodynamically stable, not metastable, and can be detected using analytical ultracentrifugation. The clusters exhibit solute-like behavior, with their size and structure influenced by pH. The study also identifies two different amorphous calcium carbonate (ACC) phases, ACC I and ACC II, with distinct solubility products and polymorphs formed under varying pH conditions. The findings suggest that nucleation occurs through cluster aggregation, with prenucleation clusters acting as early precursor species. The classical view of nucleation, based on stochastic solute clustering, is contrasted with the novel view that prenucleation clusters form based on equilibrium thermodynamics. This mechanism allows for structural preformation, influencing the final crystalline structure. The study also indicates that the nucleation of different ACC phases may be under thermodynamic or kinetic control, though the exact mechanism remains unclear. The results have implications for understanding the crystallization of other minerals, as similar nucleation mechanisms may apply. The study highlights the importance of pH-dependent equilibrium thermodynamics in the formation of calcium carbonate clusters and their role in determining the final crystalline structure. The findings contribute to a deeper understanding of biomineralization and geosciences, offering new insights into the early stages of mineral crystallization.