Composition of calcium carbonate polymorphs precipitated using ultrasound

The precipitation of calcium carbonate (CaCO3) from saturated solutions prepared using calcium chloride and sodium hydrogen carbonate under the influence of ultrasound (20 kHz, intensity ranging from 1.5-18.5 W cm(-2)) has been studied. For short reaction times or treatment at low intensity, precipitates containing up to 90% vaterite were recovered; longer (> 30 min) times or higher intensities yielded up to 100% calcite. Changes in the crystal sizes were noted. The results are consistent with the literature model of CaCO3 crystallization by the initial formation of vaterite followed by a dissolution recrystallization process. Although a complete explanation cannot be offered, the results help to explain some of the apparently conflicting information previously published on this topic.