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

机译：高pH在二氧化硅溶液中混合碱土碳酸盐的结晶

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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.
机译：二氧化硅影响碱土金属碳酸盐矿化的能力是在没有任何有机物质的情况下形成仿生结构的杰出例子。在合适的条件下，二氧化硅稳定的碳酸盐纳米晶体可以自发地自组装成具有复杂形态的分层材料，通常被称为二氧化硅生物形态。然而，这些晶体聚集体的生长在很大程度上限于碱土系列中较高的同系物，即SrCO 3和BaCO 3，而非常重要的碳酸钙的相应结构很难实现。为了系统地解决这个问题，我们使用了钡和锶的氯化钙溶液，其中所含的Ca2 +摩尔分数不断增加，因此在高pH的二氧化硅存在下使金属碳酸盐结晶。分析了所得材料的组成，结构和晶体学。获得的数据表明，生长过程已经受到少量钙的强烈影响。确实，在添加的Ca2 +的某些阈值以上，通常观察不到SrCO3和BaCO3的形态。相反，由于二氧化硅中毒导致碳酸盐晶体的分形支化，生成了球状和半球形结构。这些生长行为的变化归因于硬钙离子与溶液中硅酸盐物质的相对较强的相互作用，使它们的形态向更高的低聚物转移，甚至引起部分凝结。通过增加离子强度的其他实验证实了这一观点。我们的结果进一步表明，观察到的半球形颗粒表现出明显的多态性，在较低的Ca2 +含量下形成了正交晶固溶体（文石型（Sr，Ca）CO3和（Ba，Ca）CO3），而以Sr2 + / Ba2 +取代的方解石占优势在较高的Ca2 +分数下。此外，在Ba 2+ / Ca 2+混合物的情况下，存在一个中间范围，在该范围内，实际上相同的形态被证实是掺杂Ba 2+的球ate石。这些发现扩展了在这些简单系统中可获得的结构和组成的多样性，并且可以解释以前在标准条件下制备CaCO3生物形态的尝试中遇到的困难。

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