Calcium carbonate precipitation at differentvalues of the nominal ionic activity product (IAP) is studied innanoliter and picoliter droplets at (20±2°C). Experimentsare carried out through direct mixing of equimolar reactantsolutions using two different setups:first, droplet-basedmicrofluidics using Teflon capillaries (nanoliter experiments)and second, the microinjection technique under oil (picoliterdroplets). Instantaneous precipitation of a metastable CaCO$_3$phase is initially observed. This phase is stabilized in time byreducing the initial volume of the experiments from the nano-to picoliters range and when the CaCl$_2$/Na$_2$CO$_3$ratio approaches 1. Further analysis by X-ray diffraction, transmission electronmicroscopy, and selected area electron diffraction confirms thefirst nucleated phase is CaCO$_3$·6H$_2$O (ikaite) and in few dropletsikaite plus CaCO$_3$·H$_2$O (monohydrocalcite). No evidence of amorphous calcium carbonate (ACC) is found even in conditionswhere the IAP exceeds the solubility product of this phase. The in vitrofinding of ikaite formation and stabilization due tovolume confinement is an unexpected result since it is thefirst time that this hydrous phase is stabilized at room temperature (itis normally found at near 0°C) in the absence of additives. This result can be of interest for those biomineralization processesoccurring in the confined volumes of intracellular vesicles and for biomimetic materials science in general
