The onset of spherulitic growth in crystallization of calcium carbonate

At a low initial supersaturation ratio of S0=3.1 (90 °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 °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 °C, at the expense of metastable vaterite (S=1.9). Polycrystalline crystals of calcite start to form at a temperature of 10 °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 °C. For the vaterite polymorph it is shown that the particle morphology exhibits smaller crystalline “subunits” at a higher initial supersaturation ratio of S0=3 as compared to S0=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.