The effect of stoichiometry on the new formation and subsequent growth of CaCO₃ was investigated over a large range of solutionstoichiometries (10^–4 < r_aq < 10⁴, where r_aq = {Ca²⁺}:{CO ₃^2–}) at various, initially constant degrees of supersaturation (30 < Ω_cal < 200, where Ω_cal = {Ca²⁺}{CO₃^2–}/K_sp), pHof 10.5 ± 0.27, and ambient temperature and pressure. At r _aq = 1 and Ω_cal < 150, dynamic light scattering (DLS) showed that ion adsorption onto nuclei (1–10 nm) was the dominant mechanism. At higher supersaturation levels, no continuum of particle sizes is observed with time, suggesting aggregation of prenucleation clustersinto larger particles as the dominant growth mechanism. At r _aq ≠ 1 (Ω_cal = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. Cross-polarized light in optical light microscopy was used to measure the time needed for new particle formation and growth to at least 20 μm. This precipitation time depends strongly and asymmetrically on r_aq. Complementary molecular dynamics(MD) simulations confirm that r_aq affects CaCO₃ nanoparticle formation substantially. At r _aq = 1 and Ω_cal ≫ 1000, the largest nanoparticle in the system had a 21–68% larger gyration radius after 20 ns of simulationtime than in nonstoichiometric systems. Our results imply that, besides Ω _cal, stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results mayhelp us to improve the understanding, prediction, and formation of CaCO ₃ in geological, industrial, and geo-engineering settings.