An innovative precipitation process, achieved through the thermal decomposition of urea in polyols containing hydrated calcium salts, was used to prepare calcite and aragonite at elevated temperatures (120180 C). The effect of various experimental conditions, such as the media of different polyols (ethylene glycol, EG; diethylene glycol, DEG; and tetraethylene glycol, TEG), the temperature, the reaction time, and the addition of magnesium salts, on the structure, size, and morphology of the obtained solids is described. It was found that the formation of calcium carbonate polymorphs, i.e., calcite or aragonite, their morphology and their size was predominantly influenced by the type of polyols used, and by the diverse growing mechanisms occurring in the different polyol systems. Structurally and morphologically different calcite precipitates were formed in the DEG and TEG solvents via classical crystallization processes, while colloidal and nanostructured aragonite particles were assembled in the EG through a recently reaffirmed mechanism based on the role of nanoscale aggregation processes in the formation of carbonate solids. This study highlights the importance of the presence and concentration of magnesium ions in the inhibition of the crystal growth of nanosized aragonite, and in the stabilization of amorphous calcium carbonate (ACC) precipitates in polyol solutions. (C) 2010 Elsevier Inc. All rights reserved.