The precipitation and dissolution processes of CaCO₃ are essential to the marine carbon cycle and are relevant to climate research. Nevertheless, one of the nagging questions is why the precipitation of CaCO₃ rarely occurs without the help of biological activity even in the surface seawater, where the calcite saturation state (Ω, expressed as the ratio of the calcium and carbonate ion concentration product to the stoichiometric solubility constant of calcite) is up to 5, but commonly takes place in hard-water lakes, springs and the low-salinity zone (practical salinity <5) of estuaries. Studies have found that Mg²⁺ plays a significant role in inhibiting all aspects of CaCO₃ precipitation, namely crystal formation, solubility, and precipitation rate. However, whether the presence of Mg²⁺ in seawater is sufficient to inhibit CaCO₃ precipitation has not been clearly stated, and the specific inhibitory role of Mg²⁺ in CaCO₃ precipitation still requires clarification. In this study, we used four different seed materials to induce CaCO₃ precipitation in artificial seawater of four Mg:Ca ratios and examined the crystal morphologies of the resulting precipitates using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). We found that Mg²⁺ inhibits the precipitation at the first stage of nucleation, which drastically increases the difficulty of calcite nucleation. At a seawater Mg:Ca ratio of 5.2, aragonite preferentially nucleates instead of calcite. Among the tested materials, only calcite and aragonite seeds exhibit marked catalysis of CaCO₃ precipitation, indicating that not all the particles in natural seawater can catalyze CaCO₃ precipitation. Our results demonstrate while the precipitation of CaCO₃ is inhibited by Mg²⁺ at all stages, the inhibition at the nucleation stage is the main reason massive spontaneous CaCO₃ precipitation does not occur under the surface seawater conditions, supporting the notion that the incorporation of foreign ions such as Mg²⁺ increases the surface energy and the solubility of the overgrowth or cluster, and thus inhibits CaCO₃ precipitation.

CaCO ₃的沉淀和溶解过程对海洋碳循环至关重要，与气候研究相关。然而，一个棘手的问题是为什么 CaCO ₃的沉淀很少在没有生物活动帮助的情况下发生，即使在表层海水中，方解石饱和状态（Ω，表示为钙和碳酸根离子浓度乘积与方解石的化学计量溶解度常数）高达 5，但通常发生在硬水湖泊、泉水和河口低盐度区（实际盐度 <5）。研究发现，Mg ²⁺在抑制CaCO _{3 的}各个方面都有显着的作用沉淀，即晶体形成、溶解度和沉淀速率。但是，海水中Mg ²⁺的存在是否足以抑制CaCO ₃沉淀尚未明确，Mg ²⁺在CaCO ₃沉淀中的具体抑制作用仍有待阐明。在本研究中，我们使用四种不同的种子材料在四种 Mg:Ca 比例的人工海水中诱导 CaCO ₃沉淀，并使用扫描电子显微镜 (SEM)、能量色散 X 射线光谱 (EDS) 检查所得沉淀物的晶体形态) 和 X 射线衍射 (XRD)。我们发现 Mg ²⁺抑制了第一阶段成核的析出，大大增加了方解石成核的难度。在海水 Mg:Ca 比为 5.2 时，文石优先成核而不是方解石。在被测材料中，只有方解石和文石种子对CaCO ₃沉淀有显着的催化作用，表明并非天然海水中的所有颗粒都能催化CaCO ₃沉淀。我们的结果表明，虽然 CaCO ₃的沉淀在所有阶段都被 Mg ²⁺抑制，但成核阶段的抑制是大量自发 CaCO _{3 的}主要原因在表层海水条件下不会发生沉淀，支持了外来离子（如 Mg ^{2+ ）}的掺入增加了过度生长或簇的表面能和溶解度，从而抑制了 CaCO ₃沉淀的观点。