Uniform needle-like aragonite particles were obtained by aging solutions of calcium salts in the presence of urea at 90 degrees C. The effects of various experimental conditions, such as the concentration of reactants, temperature, aging time, agitation, mixing procedure, and the addition of divalent cations, surfactants, and polymers, on the precipitation of aragonite were investigated. It was found that the formation of different polymorphs of calcium carbonate was affected mostly by agitation and concentration of reactants, as well as by the mixing procedure. Thus, under otherwise the same experimental conditions, needles of aragonite were formed without stirring or at low-power ultrasound; rhombohedral calcite appeared when aging experiments were carried out under the influence of magnetic stirring, while irregular particles of mixed vaterite and calcite crystal structure were generated at higher CaCl(2) concentrations. Mixing the preheated reactant solutions resulted in unique "flower-type" vaterite particles. Finally, the presence of sulfonate or sulfate ions promoted precipitation of vaterite and calcite. Copyright 1999 Academic Press.