The time-evolution of calcium phosphate precipitation by vapor diffusion has been studied by in situ confocal Raman microspectroscopy. A hanging drop configuration within a device known as “crystallization mushroom” was employed in order to improve the Raman signal coming from growing crystals. This innovative methodology allowed to identify and follow the evolution of the precipitates formed at different areas of the drops containing mixed solutions of Ca(CH₃COO)₂ and (NH₄)₂HPO₄ due to the diffusion of CO₂ and NH₃ gases released from NH₄HCO₃ solutions at different concentrations (30 mM, 100 mM and 2 M). Time-dependent in situ Raman spectra indicated that amorphous calcium phosphate (ACP) was the first precipitate appearing just after mixing the Ca- and PO₄-containing solutions. A few minutes later, it transformed to dicalcium phosphate dihydrate (DCPD). The lifetime of DCPD strongly depends on the concentration of the NH₄HCO₃ solutions and thus on the pH increase rate. The pathway for the phase transformation from ACP to DCPD and then to octacalcium phosphate (OCP) followed a dissolution–reprecipitation mechanism. Additionally, OCP acted as temporal template for the heterogeneous nucleation and crystallization of biomimetic carbonate–apatite nanocrystals (cAp). The characterization by TEM, XRPD and Raman spectroscopy of the freeze-dried powders obtained after seven days confirmed that OCP and cAp were the remaining phases when using 30 mM and 100 mM NH₄HCO₃ solutions. By contrast, working with the highest NH₄HCO₃ concentration the system evolved to the precipitation of elongated calcite crystals.