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The direct crystallization of dolomite from an aqueous solution at temperatures between 60-220 C was followed in situ through time-resolved synchrotron-based energy-dispersive X-ray diffraction combined with offline high-resolution imaging, X-ray diffraction, and infrared spectroscopy. Crystalline CaMg(CO 3 ) 2 phases form through a three-stage process. In the first stage, a nanoparticulate magnesium-deficient, amorphous calcium carbonate (Mg-ACC) with a nominal formula of Ca 0.606 Mg 0.394 CO 3 1.37H 2 O forms. After a temperature-dependent induction time, during stage 2 the Mg-ACC partially dehydrates and orders prior to its rapid (<5 min) crystallization to non-stoichiometric proto-dolomite. This occurs via the dissolution of Mg-ACC, followed by the secondary nucleation of proto-dolomite from solution. The proto-dolomite crystallization proceeds via spherulitic growth that follows a growth front nucleation mechanism with a de-nuovo and continuous formation of nanocrystalline proto-dolomite subunits that form spherical aggregates. In stage three of the reaction, the proto-dolomite transforms to highly crystalline and stoichiometric dolomite on a much longer timescale (hours to days), via an Ostwald-ripening mechanism. Such a three-stage crystallization can explain microbially induced proto-dolomites observed in modern hypersaline settings and may also be the route by which the Cryogenian cap dolomite deposits of the Neoproterozoic formed.