Bicarbonate Liquid Condensate Controls Manganese Partitioning in Calcite

Crystals formed in aqueous solutions often encode chemical memories of their growth environment. However, the molecular pathway of element partitioning that links solution chemistry to solid composition remains elusive when growth proceeds through nonclassical intermediates. In this study, we investigated calcite growth mechanisms in the presence of aqueous manganese (Mn2+) under chemostat conditions using a custom titration method. At near-neutral pH, where bicarbonate ions dominate, we show that Mn2+ triggers the formation of bicarbonate-rich, ion-paired liquid condensates that accumulate at the calcite-water interface and control Mn incorporation into the solid phase. Time-resolved in situ X-ray scattering and Raman spectroscopy, combined with cryogenic transmission electron microscopy and molecular dynamics simulations, reveal a pathway in which these condensates deprotonate and dehydrate into Mn-bearing amorphous carbonate that crystallizes into polycrystalline calcite-rhodochrosite interfacial solid solutions with progressively increasing Mn content. These results identify an interfacial, bicarbonate-rich liquid condensate as a mechanistic link between crystallization pathway and element partitioning, suggesting that nanoscale interfacial fluid organization can dominate selective ion uptake during crystallization in multicomponent aqueous environments that are relevant to geoscience, environmental science, chemistry, or materials science.