The transformation of amorphous colloidal calcium carbonate into single microcrystals was observed in supersaturated solutions in situ. The observations are done by simultaneous time-resolved small-angle and wide-angle X-ray scattering experiments (TR-SAXS/WAXS) at a third generation synchrotron source. TR-SAXS/WAXS demonstrates that the particles generated by reaction of Ca2+ and CO32- ions are amorphous. The transformation of these amorphous CaCO3 particles proceeds via dissolution and subsequent heterogeneous nucleation of the crystalline modification on the walls of the quartz capillary containing the reacting mixture. The crystalline modifications thus generated could be identified. No solid-solid transition is observed. TR-SAXS/WAXS is therefore well-suited to follow the mineralization from aqueous solution in great detail.