In addition, Figure demonstrates a linear increase in particle size as pH decreases. It shows that submicron particles are only obtained in conditions where pH > 11.5. The particles are ∼400 nm in size, when pH ≈ 12.5, but grew larger with decreasing pH, approaching 4.5 µm when pH = 9. This increase in particle size with decreasing pH is consistent with ob- servations in the corresponding SEM micrographs (Figure ). It indicates that additive Ca 2+ ions lose their growth inhibitory efficiency in low pH conditions. From Figure , the pH also affected the morphology and monodispersity of the particles. Fairly monodisperse truncated calcite cubes were synthesized at initial pH = 12.5 (Figure ), but they grew larger when the pH was decreased to 12 (Figure ). Increasingly large and polydisperse rhombohedral calcite particles are obtained as the pH was further decreased. These results support the conclusion that a high pH condition is necessary for the synthesis of monodisperse submicron particles, even when additive Ca 2+ ions are available for growth inhibition.