Deciphering strontium sulfate precipitation via Ostwald's rule of stages: From prenucleation clusters to solution-mediated phase tranformation

Multiple-step nucleation pathways have been observed during mineral formation in both inorganic and biomineral systems. These pathways can involve precursor aqueous species, amorphous intermediates, or metastable phases. Despite the widespread occurrence of these processes, elucidating the precise nucleation steps and the transformation mechanisms between each step remains a challenging task. Using a suite of potentiometric, microscopic, and spectroscopic tools, we studied the nucleation pathway of SrSO₄ as a function of the physico-chemical solution parameters. Our observations reveal that below a threshold supersaturation, nucleation is driven by bound species, akin to the prenucleation cluster model, which directly leads to the formation of the stable phase celestine, SrSO₄. At higher supersaturations, this situation is altered, with nucleation dominated by the consumption of free ions. Importantly, this change in nucleation mechanism is coupled to the formation of a hemihydrate metastable phase, SrSO₄ · 1/2H₂O, which eventually transforms into celestine, adhering to Ostwald's rule of stages. This transformation is a solution-mediated process, also occurring in the presence of a fluid film and is controlled by the physico-chemical parameters of the surrounding environment. It proceeds through the dissolution of the metastable phase and the de novo crystallization of the final phase. Overall, our results reveal that ion association taking place during the prenucleation stage dictates whether the nucleation pathway goes through an intermediate phase or not. This also underlines that although Ostwald's rule of stages is a common process, it is not a prerequisite for mineral formation-even in systems where it can occur.

The Role and Implications of Bassanite as a Stable Precursor Phase to Gypsum Precipitation

Calcium sulfate minerals such as gypsum play important roles in natural and industrial processes, but their precipitation mechanisms remain largely unexplored. We used time-resolved sample quenching and high-resolution microscopy to demonstrate that gypsum forms via a three-stage process: (i) homogeneous precipitation of nanocrystalline hemihydrate bassanite below its predicted solubility, (ii) self-assembly of bassanite into elongated aggregates co-oriented along their c axis, and (iii) transformation into dihydrate gypsum. These findings indicate that a stable nanocrystalline precursor phase can form below its bulk solubility and that in the CaSO4 system, the self-assembly of nanoparticles plays a crucial role. Understanding why bassanite forms prior to gypsum can lead to more efficient anti-scaling strategies for water desalination and may help to explain the persistence of CaSO4 phases in regions of low water activity on Mars.