Influence of Gelatin Hydrogel Porosity on the Crystallization of CaCO3

We investigated the influence of the porosity of the growth medium on the crystallization of calcium carbonate in hydrogels with different gelatin solid contents (2.5, 5, and 10 wt %). In all experiments, the precipitate consisted of calcite with occasional occurrences of some vaterite and aragonite. The calcite grew as compact radial intergrowths of crystals that show rhombohedral external faces. The crystal surfaces consist of identical 1–10 μm sized rhombohedral sub-blocks. Electron backscatter diffraction (EBSD) uncovered the radial intergrowth structure of the aggregates. EBSD also documented the internal microscale mosaicity and mesocrystal-like constitution of the gel-grown calcite. Raman spectroscopy and thermogravimetric analysis confirmed the presence of gelatin within the crystals. It reached up to ∼2 mass % in the calcite-gelatin composites that formed in hydrogels with 10 wt % gelatin content. Calcite morphology and mosaicity varied with the gelatin content of the hydrogel, such that an increase in gelatin content initiated the growth of smaller crystal aggregates having progressively rougher surfaces, increasing amounts of incorporated gel, and increasing degrees of misorientation in the internal mosaic structure. Apart from biospecific morphology, the gel growth experiment successfully mimics many characteristics of calcite biomineralization such as formation of a hierarchical hybrid composite, crystal mosaicity, and mesocrystal-like constitution.