Many organisms orchestrate the controlled precipitation of minerals. This physiological process takes place at ambient conditions, using soluble ions as building blocks. A widespread strategy for such crystallization processes is using a multistep route, where the initial phase is metastable and gradually transforms into the mature mineral phase. Even though the maturation of these intermediate phases has been intensively studied, it remains unclear how the initial, far from equilibrium phase can form within the cellular context. A model system for controlled biomineralization is the production of coccoliths by marine microalgae. Coccoliths are calcium carbonate crystalline arrays that form within the intracellular environment, at very low calcium concentrations. Here, we used coccolith-derived and synthetic polymers to study, in vitro, the chemical interactions between calcium ions and organic macromolecules that precede coccolith formation. We used in situ analyses, including state-of-the-art cryo-electron tomography and liquid-cell atomic force microscopy, to study the interactions in bulk solution and on organic surfaces simultaneously. The results unveil a chemical process in which a functional surface induces the precipitation of a polymer–Ca dense phase, or a coacervate, at chemical conditions where precipitation in solution is kinetically inhibited. This strategy demonstrates how organisms can form dense Ca-rich phases from the submillimolar concentration of calcium within organelles. This Ca-rich phase can then transform into a mineral precursor in a subsequent step, without posing challenges to cellular homeostasis.