Mg2+ and the mixture of Mg2+/DL-aspartic or L-tyrosine were used as effective crystal nucleation and growth modifiers to control the crystallization of CaCO3. The polymorph and morphology of crystals were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The results demonstrated that various morphologies such as rod-shaped particles, spinose sphere particles, cauliflower-like crystals, and the spindly particles produced in above aqueous solutions. When the molar ratio of Mg2+ to Ca2+ was less than 8, the products were all the mixtures of calcite and aragonite. The ability of Mg2+ to induce the formation of aragonite was enhanced as the molar ratio of Mg2+ to Ca2+ increased. When the molar ratio reached 8, the crystals obtained were all aragonite. In the mixed systems of Mg2+/amino acids, the presence of amino acids was helpful for the formation of aragonite. The possible formation mechanisms of CaCO3 in different systems were discussed in the paper. (c) 2006 Elsevier B.V. All rights reserved.