Abstract The properties of amorphous calcium carbonate provide the foundation for understanding mineralization of calcified skeletons but inconsistencies between studies suggest the physical picture is incomplete. This study quantifies the metastable solubility and structure of ACC produced under a series of chemical conditions. Using complementary in situ methods that quantify ACC structure (in situ PDF analysis) in parallel with chemical composition (TGA, ICP-OES), we find two short-range structures are produced that also show distinct morphological differences. Conditions with high carbonate ion concentrations stabilize a Ca-rich ACC with short-range order that is independent of Mg content. In contrast, low carbonate solutions favor Mg-rich ACC with mixed Ca- and Mg-short range order. Measurements of solubility product determined from supersaturated and undersaturated conditions find the distinctive structures are related to solution carbonate activity while solubility is primarily determined by the magnesium content of the solid. Bulk solubility is a composite that concurs with established values. The findings demonstrate the type of ACC that forms is tuned by Mg2+ and CO3-2- concentration and suggest ACC structure, not bulk composition, is a reliable indicator of intermediate phase. This chemical framework also provides a basis for interpreting apparent differences between observations of ACC in synthetic and natural systems that reconciles previous structural studies.

中文翻译：

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无定形碳酸钙 (ACC) 的亚稳态溶解度和局部结构

摘要 无定形碳酸钙的性质为理解钙化骨骼的矿化提供了基础，但研究之间的不一致表明物理图片不完整。该研究量化了在一系列化学条件下产生的 ACC 的亚稳态溶解度和结构。使用互补的原位方法量化 ACC 结构（原位 PDF 分析）与化学成分（TGA、ICP-OES）并行，我们发现产生了两种短程结构，它们也显示出明显的形态差异。具有高碳酸根离子浓度的条件以与镁含量无关的短程有序稳定富含钙的ACC。相比之下，低碳酸盐溶液有利于具有混合 Ca 和 Mg 短程有序的富含 Mg 的 ACC。从过饱和和欠饱和条件确定的溶度积的测量发现独特的结构与溶液碳酸盐活性有关，而溶解度主要由固体的镁含量决定。体积溶解度是与既定值一致的复合物。研究结果表明，形成的 ACC 类型由 Mg2+ 和 CO3-2- 浓度调节，并表明 ACC 结构，而不是整体成分，是中间相的可靠指标。这种化学框架还为解释合成和自然系统中 ACC 观察结果之间的明显差异提供了基础，这些差异与先前的结构研究相协调。