Mechanism of the Vaterite-to-calcite phase transition under sonication

The vaterite-to-calcite phase transition in spheroidal vaterite particles produced by reacting aqueous Ca(NO3)(2) and Na2CO3 solutions at 20 +/- 1 degrees C under 2.6-MHz sonication was studied by scanning electron microscopy and x-ray diffraction. The results are interpreted as evidence that the transformation of vaterite, a metastable phase under ordinary conditions, to calcite follows a "relay" recrystallization mechanism: vigorous ultrasonic stirring leads to energetic collisions between vaterite and calcite particles, resulting in a transition of the metastable phase to the stable one, followed by the formation of calcite aggregates consisting of 2 to 14 particles.