Liquid-liquid phase separation into reactant-rich precursors during mineral crystallization

Abstract

Liquid-liquid phase separation (LLPS) is increasingly been recognized as a critical intermediate in non-classical crystallization pathways, representing a paradigmatic shift from early descriptions invoking single-step nucleation. While extensively documented in organic systems, LLPS in mineral systems presents unique experimental challenges due to accelerated crystallization kinetics and the difficulty of distinguishing true liquid droplets from amorphous solid precursors. This review synthesizes current evidence for LLPS across diverse mineral systems, examining cases by decreasing order of experimental confidence—from the well-known calcium carbonate system to emerging discussions in oxalates, oxides, metallic nanoparticles, sulfur sols, sulfates, calcium phosphates, metal organic frameworks and sodium chloride. A fundamental challenge lies in definitively establishing liquid character, as Transmission Electron Microscopy (TEM) and X-ray scattering methods cannot distinguish between liquid and solid amorphous structures, while liquid-phase TEM observations are prone to interfere with the real crystallization process. Understanding when and why LLPS occurs remains challenging, complicated by inconsistent reporting practices and the predominant use of thermodynamic interpretations where kinetic factors may actually govern the process. Operating far from equilibrium, these systems may require alternative mechanisms beyond classical thermodynamic treatments. Key research frontiers include rigorous demonstration of true liquid character, systematic exploration of structure and dynamics across mineral systems down to the atom and sub-millisecond scales, and integrated experimental-theoretical approaches capturing both thermodynamic and kinetic factors—essential for the rational design of materials and controlled nanoparticle morphologies through LLPS-mediated pathways.