Molecular Mechanisms of Magnesium Sulfate Crystallization: Bond Length Inversion and the Role of Hydration in Mineral Formation

Abstract Understanding the molecular mechanisms governing the crystallization of hydrated minerals is critical for advancing both fundamental crystallization theory and applications in geochemistry, materials science, and planetary research. Here, we present a multi-technique investigation that reveals unprecedented insights into the crystallization pathways of magnesium sulfate hydrates. We identify a previously unrecognized bond length inversion phenomenon that fundamentally challenges conventional crystallization models by integrating time-resolved Raman spectroscopy, X-ray pair distribution function (PDF) analysis, and density functional theory calculations. Contrary to common assumptions, we demonstrate that S-O bonds in magnesium sulfate are shorter in the liquid phase (1.43-1.44 Å) compared to crystalline phases (1.47 Å), indicating that crystal packing forces extend bond lengths beyond their solution-phase preferences. The systematic splitting of the SO4 ν1 symmetric stretch vibration into distinct components (ν1h and ν1w) correlates directly with measured S-O bond length distributions, providing spectroscopic fingerprints of the local coordination environments that evolve during crystallization. Our data reveal that crystallization proceeds through the progressive organization of hydrated clusters containing Mg(H2O)6 octahedra surrounded by SO4 tetrahedra, with water activity controlling the sequential formation of increasingly dehydrated phases. Significantly, we observe structural reorganization during crystal maturation, evidenced by an inversion point where bond length relationships between dissolved and crystalline species reverse after approximately 350 minutes. These findings provide experimental evidence supporting non-classical nucleation theories, including two-step nucleation and pre-nucleation cluster pathways, while offering molecular-level explanations for Ostwald’s rule of stages. Beyond advancing crystallization science, our results have important implications for interpreting the environmental history of magnesium sulfate deposits on Earth and Mars, and for developing strategies to control crystallization pathways in materials synthesis and processing in ancient and modern lacustrine environments.