The article by Denis Gebauer, Antje Völkel, and Helmut Gölfen explores the formation of stable prenucleation calcium carbonate clusters, which are crucial for the precipitation of calcium carbonate. Despite its significance, the early stages of calcium carbonate nucleation, particularly the prenucleation stage, have been poorly understood. The authors demonstrate that dissolved calcium carbonate can form stable prenucleation ion clusters even in undersaturated solutions, challenging the classical view that metastable clusters are rare and stochastic. Using a multiple-binding model and equilibrium thermodynamics, the researchers show that these clusters are thermodynamically stable and play a dominant role in the prenucleation stage. The formation of these clusters is influenced by pH, with higher pH values promoting cluster formation and increased calcium binding. The clusters, which can be detected through analytical ultracentrifugation, are found to be the precursor species for different calcium carbonate phases, including calcite and vaterite. The study suggests that the prenucleation-stage clusters provide an early structural preformation, which is then transferred to the postnucleation stage, leading to the crystallization of specific polymorphs. This mechanism may also apply to the crystallization of other minerals, such as calcium phosphate and calcium oxalate. The findings highlight the importance of early structural preformation in the nucleation process and provide a new perspective on the classical view of nucleation.The article by Denis Gebauer, Antje Völkel, and Helmut Gölfen explores the formation of stable prenucleation calcium carbonate clusters, which are crucial for the precipitation of calcium carbonate. Despite its significance, the early stages of calcium carbonate nucleation, particularly the prenucleation stage, have been poorly understood. The authors demonstrate that dissolved calcium carbonate can form stable prenucleation ion clusters even in undersaturated solutions, challenging the classical view that metastable clusters are rare and stochastic. Using a multiple-binding model and equilibrium thermodynamics, the researchers show that these clusters are thermodynamically stable and play a dominant role in the prenucleation stage. The formation of these clusters is influenced by pH, with higher pH values promoting cluster formation and increased calcium binding. The clusters, which can be detected through analytical ultracentrifugation, are found to be the precursor species for different calcium carbonate phases, including calcite and vaterite. The study suggests that the prenucleation-stage clusters provide an early structural preformation, which is then transferred to the postnucleation stage, leading to the crystallization of specific polymorphs. This mechanism may also apply to the crystallization of other minerals, such as calcium phosphate and calcium oxalate. The findings highlight the importance of early structural preformation in the nucleation process and provide a new perspective on the classical view of nucleation.