Many organisms use amorphous calcium carbonate (ACC) and control its stability by various additives and water; however, the underlying mechanisms are yet unclear. Here, the effect of water and inorganic additives commonly found in biology on the dynamics of the structure of ACC during crystallization and on the energetics of this process is studied. Total X‐ray scattering and pair distribution function analysis show that the short‐ and medium‐range order of all studied ACC samples are similar; however, the use of in situ methodologies allow the observation of small structural modifications that are otherwise easily overlooked. Isothermal calorimetric coupled with microgravimetric measurements show that the presence of Mg²⁺ and of PO₄³⁻ in ACC retards the crystallization whereas increased water content accelerates the transformation. The enthalpy of ACC with respect to calcite appears, however, independent of the additive concentration but decreases with water content. Surprisingly, the enthalpic contribution of water is compensated for by an equal and opposite entropic term leading to a net independence of ACC thermodynamic stability on its hydration level. Together, these results point toward a kinetic stabilization effect of inorganic additives and water, and may contribute to the understanding of the biological control of mineral stability.

中文翻译：

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无定形碳酸钙的结晶受离子杂质和水的动力学控制

许多生物体使用无定形碳酸钙（ACC）并通过各种添加剂和水控制其稳定性；然而，其根本机制尚不清楚。在此，研究了生物学中常见的水和无机添加剂对结晶过程中 ACC 结构动力学以及该过程的能量学的影响。总 X 射线散射和对分布函数分析表明，所有研究的 ACC 样品的短程和中程顺序相似；然而，使用原位方法可以观察到容易被忽视的微小结构变化。^{等温量热法与微重量测量结果表明，ACC 中 Mg 2+}和 PO ₄ ³⁻的存在延缓了结晶，而增加的水含量则加速了转变。然而，ACC 相对于方解石的热函似乎与添加剂浓度无关，但随着水含量的增加而降低。令人惊讶的是，水的焓贡献由相等且相反的熵项补偿，导致 ACC 热力学稳定性对其水合水平的净独立性。总之，这些结果表明无机添加剂和水具有动力学稳定作用，并可能有助于理解矿物稳定性的生物控制。