Metal carbonates, which are ubiquitous in the near-surface mineral record, are a major product of biomineralizing organisms and serve as important targets for capturing anthropogenic CO₂ emissions. However, pathways of carbonate mineralization typically diverge from classical predictions due to the involvement of disordered precursors, such as the dense liquid phase (DLP), yet little is known about DLP formation or solidification processes. Using in situ methods we report that a highly hydrated bicarbonate DLP forms via liquid–liquid phase separation and transforms into hollow hydrated amorphous CaCO₃ particles. Acidic proteins and polymers extend DLP lifetimes while leaving the pathway and chemistry unchanged. Molecular simulations suggest that the DLP forms via direct condensation of solvated Ca²⁺⋅(HCO₃⁻)₂ complexes that react due to proximity effects in the confined DLP droplets. Our findings provide insight into CaCO₃ nucleation that is mediated by liquid–liquid phase separation, advancing the ability to direct carbonate mineralization and elucidating an often-proposed complex pathway of biomineralization.

金属碳酸盐在近地表矿物记录中无处不在，是生物矿化生物的主要产品，是捕获人为 CO₂ 排放的重要目标。然而，由于涉及无序的前驱体，例如致密液相 （DLP），碳酸盐矿化的途径通常与传统预测不同，但人们对 DLP 的形成或凝固过程知之甚少。使用原位方法，我们报道了高度水合的碳酸氢盐 DLP 通过液-液相分离形成并转化为中空的水合无定形 CaCO₃ 颗粒。酸性蛋白质和聚合物可延长 DLP 的使用寿命，同时保持通路和化学成分不变。分子模拟表明，DLP 是通过溶剂化 Ca²⁺⋅（HCO₃⁻）₂ 复合物的直接缩合形成的，这些复合物由于受限 DLP 液滴中的邻近效应而发生反应。我们的研究结果为液-液相分离介导的 CaCO₃ 成核提供了见解，提高了直接碳酸盐矿化的能力，并阐明了经常提出的复杂生物矿化途径。