Calcium carbonate has been precipitated from aqueous solutions containing magnesium under conditions of high supersaturation. The precipitates were analysed over time using X-ray diffraction (XRD) and Infrared Spectroscopy (IR), and it was demonstrated that in all cases amorphous calcium carbonate (ACC) was the first phase formed. The magnesium content of the ACC was defined by the Mg:Ca ratio in the solution, and increased systematically with increasing Mg:Ca. In turn, ACC occluding higher magnesium concentrations was significantly more stable than low magnesium ACC The crystalline phase produced on transformation of ACC depended on the Mg:Ca ratio, and ACC precipitated from solutions containing higher Mg concentrations crystallised to yield high-Mg calcites (occluding up to 47 mol% Mg), together with CaCO3.H2O and/or CaCO3.6H(2)O. The magnesium content of the ACC phase also directly affected the final morphology of the precipitates, such that continuous sheets of CaCO3 were produced at the air/solution interface at solution ratios of 3:1 Mg:Ca and above. Previously, this effect has only been observed for ACC stabilised with organic additives. These results have relevance to the mechanism of formation of biological magnesian calcites, and suggest that the stabilisation of ACC by magnesium may provide organisms with a mechanism for controlling crystal morphologies. (C) 2003 Elsevier Science B.V. All rights reserved.