Effects of Mg and Hydrogel Solid Content on the Crystallization of Calcium Carbonate in Biomimetic Counter-diffusion Systems

Abstract

Carbonate biominerals are nanocomposites with an intimate association of organic and mineral components. Here we investigate the crystallization of CaCO₃ in gelatin hydrogels (2.5 and 10 wt % solid content) in the presence of Mg (0.01 M) in the growth medium. The precipitate consisted mainly of calcite in all experiments. A wide variety of morphologies and incorporated Mg contents (up to 26 mol % in sphere-like aggregates grown in 10 wt % gelatin) was observed. Etching experiments uncovered an intimate relationship between the inorganic component and a polymeric network in the calcite crystal aggregates. The characteristics of this network varied for hydrogels with different solid contents. When Mg was not present in the growth medium, we obtained 200 nm to 1 μm thick incorporations that were bordered on both sides by a delicate gelatin network. As Mg was added, the incorporations became thinner (∼50–60 nm), and the gelatin network became compact. Electron backscatter diffraction evidenced that the calcite usually consists of aggregates of mutually misoriented crystals with an internal mosaic spread. Crystals with high lattice co-orientation, which occur rather rarely, are terminated by regular rhombohedral (104)-type faces. The irregular-shaped and mosaic-structured aggregates occasionally have a rim of such rhombohedral crystallites. In the experiment with 10 wt % solid gelatin content and Mg in the growth medium, the calcite consisted of crystallites with fan-like small-angle misorientations (split growth), leading to spherulitic microstructures. We attribute these frequent and characteristic small-angle boundaries to dislocations that relax misfit strain, which is associated with selective Mg incorporation at acute growth steps. We ascribe our observations to the acidic functional groups of the gelatin promoting the desolvation of the hydrated Mg²⁺ ions, leading to an increased incorporation of Mg into calcite and a reduced inhibition of calcite nucleation and growth