The formation of biogenic CaCO₃ has fascinated scientists over decades. A specific polymorph can be selected by living organisms and used to assemble complex morphology for biological functions, as seen in corals, coccoliths, bivalves and others. Many studies focus on how organic matter scaffolds or selects polymorphism, while others recreate in vitro these morphologies by using organic additives. Nevertheless, complex morphologies similar to biominerals can be obtained within inorganic matrix. These “biomorphs” trigger the question of how much we should trust complex morphology to be the trace of organic matter or even living organisms [1]. Crystal nucleation is a critical remaining question in CaCO₃ crystallization to understand its crystal growth and design. Several crystallization pathways, classical and non-classical nucleation [2], have been proposed, but no consensus has been reached due to the complexity of the system. Its low solubility and sensitivity to pH can lead to different observations and pathways. Furthermore, precipitation protocols and analytical techniques vary widely across literature. In situ Transmission Electron Microscopy (TEM) provides nano-scale details of crystal formation, though with a limited field of view. Spectroscopic means (FT-IR, Raman, SAXS, WAXS, neutron scattering etc.) could capture the signatures of various precipitation stages, but a spectrum is typically an average of ensemble.  Surprisingly, bright field optical microscopy is not used for key insights, despite being well-suited to bridging the length scale gap between TEM and ensemble spectroscopy approaches. This may be due to the limited spatial resolution and difficulty in assigning polymorphs without a characterization technique. In our study, we combined optical microscopy with in situ Raman microspectroscopy as a mesoscale technique [3]. Using this method, we conducted unstirred precipitation experiments over a wide range of supersaturations, varying initial pH and concentrations of CaCl₂ and Na₂CO₃, and observed the evolution of mixed solutions up to 24 hours. This allowed us to obtain a landscape of crystallization processes and categorize precipitates formed at specific supersaturations. At high supersaturation, a gel formed whilst at low supersaturation, calcite crystals appeared first, followed by amorphous calcium carbonate (ACC). Morphologies reported in literature with additives, such as calcitic spherulites, aragonite and biomorphic vaterite, were observed without additives in our study. We highlight the coexistence, evolution of phases, and formation of a gel, as a metastable precursor to ACC. Its Raman signature and morphology differ from ACC, while the inversion test supports its classification as a gel. Our contribution will emphasize the importance of studying the system at the mesoscale scale as well as that of initial conditions so that the debates can converge in near future [3].

[1] Juan Manuel García-Ruiz, Emilio Melero-García, Stephen T Hyde, Science, 2009, 323 (5912), 362–365.

[2] Huachuan Du , Esther Amstad, Angew. Chem. Int. Ed., 2020, 59, 1798–1816

[3] Natercia Barbosa, Tetiana Khakhula, Johanna Brazard, Takuji B. M. Adachi, Cryst. Growth Des., 2025, 25, 210−224.