Crystallization of Mixed Alkaline-Earth Carbonates in Silica Solutions at High pH

The ability of silica to influence the mineralization of alkaline-earth carbonates is an outstanding example for the formation of biomimetic structures in the absence of any organic matter. Under suitable conditions, silica-stabilized carbonate nanocrystals can spontaneously self-assemble into hierarchical materials with complex morphologies, commonly referred to as silica biomorphs. However, growth of these crystal aggregates has largely been restricted to the higher homologues in the alkaline-earth series, i.e., SrCO3 and BaCO3, while corresponding architectures of the much more relevant calcium carbonate are quite difficult to realize. To systematically address this problem, we have crystallized metal carbonates in the presence of silica at high pH, using barium and strontium chloride solutions that contained increasing molar fractions of Ca2+. The resulting materials were analyzed with respect to their composition, structure, and crystallography. The obtained data demonstrate that the growth process is already strongly affected by small amounts of calcium. Indeed, morphologies typically observed for SrCO3 and BaCO3 remained absent above certain thresholds of added Ca2+. Instead, globular and hemispherical structures were generated, owing to fractal branching of carbonate crystals as a consequence of poisoning by silica. These alterations in the growth behavior are ascribed to relatively strong interactions of hard calcium ions with silicate species in solution, shifting their speciation toward higher oligomers and even inducing partial coagulation. This notion is confirmed by additional experiments at increased ionic strength. Our results further demonstrate that the observed hemispherical particles exhibit distinct polymorphism, with orthorhombic solid solutions (aragonite-type (Sr,Ca)CO3 and (Ba,Ca)CO3) being formed at lower Ca2+ contents, whereas Sr2+/Ba2+-substituted calcite prevails at higher Ca2+ fractions. In the case of Ba2+/Ca2+ mixtures, there is moreover an intermediate range where virtually identical morphologies were confirmed to be Ba2+-doped vaterite. These findings extend the variety of structures and compositions accessible in these simple systems, and may explain difficulties previously encountered in attempts to prepare CaCO3 biomorphs at standard conditions.