INTRODUCTIONCalcium carbonate, a material of considerable practical importance [1][2][3], exists in two anhydrous, metastable polymorphs (vaterite and aragonite) and one stable (under ordinary conditions) polymorph (calcite). Each of these polymorphs can be prepared in the form of particles of different morphologies. As a rule, vaterite particles are spheroidal and calcite particles are rhomboidal [4, 5].The vaterite-to-calcite phase transition [6,7] is believed to occur either through spontaneous recrystallization of individual particles or through dissolution of initially precipitating vaterite in an aqueous medium (the solubility of vaterite in water is higher than that of calcite [8]). It seems likely that this transition may also follow a third mechanism, so-called "relay" recrystallization, a process in which metastable particles transform into stable ones through collisions in a stirred suspension. The heat released in the contact region as a result of a collision increases the amplitude of thermal vibrations of atoms (ions, molecules) in the surface layer of the particles, giving rise to structural changes and, eventually, to the transformation of the metastable phase into the stable one. After the collision, the particles may remain in contact (aggregate) or persist as individual particles. In this way, a fast phase transition occurs in all the particles of the metastable phase. The mechanism of such relay processes is schematically illustrated in Fig. 1.The relay mechanism was found earlier to underlie the amorphous-to-crystalline phase transformation of ultrafine hydroxyapatite particles dispersed in an aqueous medium [9] and the crystallization of supercooled melt drops in an inert liquid [10]. Here, we describe a relay vaterite-to-calcite phase transition in an aqueous suspension during vigorous ultrasonic stirring. EXPERIMENTALTo prepare an aqueous vaterite suspension, an aqueous sodium carbonate solution was introduced from above into a fountain of fine droplets of an aqueous calcium nitrate solution, produced by sonication, The starting chemicals were reagent-grade Ca(NO 3 ) 2 and Na 2 CO 3 . We used freshly prepared 0.1 M solutions. The Ca(NO 3 ) 2 : Na 2 CO 3 volume ratio was 1 : 2. Syntheses were carried out in the apparatus shown schematically in Fig. 2. The reaction vessel ( 4 ) was 8 cm in diameter and had a 1.8-cm-diameter acoustic window ( 2 ) fitted with polymer film ( 3 ). The vessel was filled with a Ca(NO 3 ) 2 solution maintained at 20 ± 1 ° C.Abstract -The vaterite-to-calcite phase transition in spheroidal vaterite particles produced by reacting aqueous Ca(NO 3 ) 2 and Na 2 CO 3 solutions at 20 ± 1 ° 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 t...

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Primary nanoparticles of vaterite (a metastable form of calcium carbonate) precipitated in aqueous solutions under 2.64-MHz sonication are found to experience layered agglomeration. The vaterite particles are characterized by electron microscopy and x-ray diffraction. The particle size distribution of the precipitate is determined. A mathematical model for layered agglomeration is proposed, and the conclusion is made that, under the experimental conditions used, heterogeneous nucleation of primary vaterite particles takes place.