Initiation of Vaterite−Aragonite CaCO₃ Particles from Silicate−Casein Sols

31 P NMR difference spectra of sodium caseinate sols with and without silicate ions provide direct evidence of interactions between silicate ions and casein serine phosphate groups. The addition of Ca 2+ to sodium caseinate solution without silicate ions and, subsequently, the diffusion of atmospheric CO 2 to the resulting mixture do not lead to CaCO 3 mineralization, whereas comparable experiments in the presence of silicate ions induce the precipitation of hemispherical three-component microstructures composed of silica, casein, and calcium carbonate. Apparently, the silicate−protein interaction plays a role as promoter for calcium carbonate mineralization in aqueous sols. X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) analyses reveal that vaterite is the crystalline phase of the composites. The observed materials are flat on one side and curved outward on the other side. In time, the flat surface cracks to display a star-like shape. Occasionally, in the center of the crack, layer-by-layer sphere-like particles grow, probably due to a secondary nucleation. These spheres are composed of a large number of two-dimensional aragonitic sheets, which are densely packed and form a multilayered structure.