Fig. 8.

(A) Free ion product development vs. normalized time (black solid line) during a typical titration experiment. The black dashed line indicates the solubility product K_sp, compared with the reported solubility product of ACC (K_{sp ACC}) (33) and vaterite (K_{sp vat}) (56). (B) POM indicating micrometer-sized entities at 1.03 t_{σ max} (yellow arrows), which show birefringence when the surrounding solution (dark gray) retracts (Inset). (C) SEM at 1.03 t_{σ max} showing a typical spherical–framboidal vaterite morphology. (D) In situ ATR-FTIR spectra from t_{σ max} to 1.40 t_{σ max} showing typical vibrations of vaterite at 875 cm⁻¹ (CO₃²⁻ ν₂ out of plane bend), 1,087/1,072 cm⁻¹ (CO₃²⁻ ν₁ symmetric stretch), and 1,467 cm⁻¹ (CO₃²⁻, ν₃ asymmetric bond stretch) (57) gradually increasing in time. The Top Inset shows an enlargement of the ν₁ peak at 1.40 t_{σ max}, while the spectrum on the far Right displays the magnified ν₂ spectral region.