The onset of spherulitic growth in crystallization of calcium carbonate

Batch and semi-batch crystallization experiments were performed in aqueous solutions for the three anhydrous polymorphs of calcium carbonate vaterite, aragonite and calcite to investigate the effect of crystallization parameters on the onset of spherulitic growth and particle morphology. At a low initial supersaturation ratio of S(0) =3.1 (90 degrees C) aragonite crystallizes as monocrystalline needles while at a supersaturation ratio of S=9.9 the aragonite growth is on the edge of switching to a polycrystalline mode. Moreover, a lower temperature (30 degrees C) seems to favour spherulites with a higher degree of branching than that was observed at 90 C. This clearly demonstrates that besides the supersaturation ratio the temperature itself has a significant effect on the growth mode of the calcium carbonate particles. Calcite grows as monocrystalline cubical particles at 30 degrees C, at the expense of metastable vaterite (S=1.9). Polycrystalline crystals of calcite start to form at a temperature of 10 degrees C. However, unlike for aragonite this cannot be solely ascribed to the temperature as the supersaturation ratio is 16.3, and thus magnitudes are higher at 30 degrees C. For the vaterite polymorph it is shown that the particle morphology exhibits smaller crystalline subunits at a higher initial supersaturation ratio of S(0)=3 as compared to S(0)=1.4. It is furthermore shown that the subunit size of particles can be changed arbitrarily (also from large to small) by manipulating the level of supersaturation. (C) 2010 Elsevier B.V. All rights reserved.