Mineralization of calcium carbonate in aqueous solutions starting from the very early stages was studied by time-resolved small-angle neutron scattering (SANS). Homogeneous crystallization of CaCO3 involves an initial formation of thin plate-shapednuclei which subsequently reassemble to 3-dimensional particles, first of fractal and finally of compact structure. The presence of the egg-white protein ovalbumin leads to a different progression of mineralization and appears in several stages; the first step represents amorphous CaCO3, while the other phases are of crystalline structure. The formation and dissolution of the amorphous phase is accompanied by Ca2+-mediated unfolding and complexation of about 50 protein monomers showing the characteristic scattering of linear chains with a large statistical segment length. The protein complexes act as nucleation centers for the amorphous phase because of their enrichment by Ca2+-ions. The amorphous calcium carbonate particles as well as the subsequently formed phases of vaterite and aragonite were identified by contrast variation. The sequential formation of CaCO3 from amorphous to the crystalline polymorphs of vaterite and aragonite, e.g. the formation from less dense to more dense polymorphs follows the Ostwald-Volmer rule and are widespread in nature.
Progression of the CaCO3 mineralization (P4) with time. during mineralization in four 0.1M CaCl2 aqueous H2O solutions (pH10). Three processes become visible as indicated by the dashed lines.