The precipitation process of calcium carbonate front highly supersaturated solutions was observed in situ by mixing CaCl2 and Na2CO3 aqueous solutions at about 20degreesC under an optical microscope and an infrared microspectroscope. After an amorphous phase forms, spherulitic vaterite and calcite in a rhombohedral shape nucleate simultaneously but separately, and grow by forming the precipitate-free zone around them. This transformation is solvent-mediated, and the measurement of the growth rate suggests that the rate-control ling process is the diffusion of elements in the earlier stage of this process, which then changes to surface kinetics. (C) 2002 Elsevier Science B.V. All rights reserved.