Interaction between dissolved silica and calcium carbonate: 1. Spontaneous precipitation of calcium carbonate in the presence of dissolved silica

• Published in: Geochimica et cosmochimica acta 2010-05, Vol.74 (9), p.2655-2664
• Main Authors: Lakshtanov, L.Z., Stipp, S.L.S.
• Format: Article
• Language: English
• Subjects: Calcite; Calcium carbonate; Dissolution; Nucleation; Precipitation; Saturation; Silicon dioxide; Spontaneous
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2010.02.009

The kinetics of spontaneous precipitation of CaCO 3 from aqueous solution in the presence of dissolved silica was investigated by recording pH as a function of time. The presence of dissolved silica, at concentrations below saturation with respect to the amorphous phase, decreases induction time for CaCO 3 nucleation, but does not affect CaCO 3 polymorphism. For a “pure” system without silica, the surface free energy, σ, determined from classical nucleation theory is 42 mJ m −2. This agrees well with values reported in the literature for vaterite and indicates some degree of heterogeneous nucleation, which can occur because of the relatively low degree of supersaturation used for the experiments. In the presence of 1 and 2 mM silica, σ is 37 and 34 mJ m −2, indicating an increasing degree of heterogeneous nucleation as the amount of polymeric silica increases. The ratio of Ca 2+ to CO 3 2− activity was a governing parameter for determining which CaCO 3 polymorph precipitated. At high Ca 2+ to CO 3 2− activity ratios, almost all initial solid was vaterite, whereas at low ratios, a mixture of vaterite and calcite was observed. In solutions with low Ca 2+ to CO 3 2− activity ratios, the presence of silica at concentrations above saturation with respect to amorphous silica led to formation of only calcite and strongly influenced the crystalline structure and morphology of the precipitates. At high Ca 2+ to CO 3 2− ratios, system behaviour did not differ from that without silica.