Lauer A R, Hellmann R, Montes-Hernandez G, Findling N, Ling W L, Epicier T, Fernandez-Martinez A, Van Driessche A E S
Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, ISTerre, 38000 Grenoble, France.
Université Grenoble Alpes, CEA, CNRS, IBS, F-38000 Grenoble, France.
J Chem Phys. 2023 Feb 7;158(5):054501. doi: 10.1063/5.0136870.
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 SrSO 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, SrSO. 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, SrSO · 1/2HO, 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.
在无机和生物矿物系统的矿物形成过程中，均观察到了多步成核途径。这些途径可能涉及前体水相物种、无定形中间体或亚稳相。尽管这些过程广泛存在，但阐明精确的成核步骤以及每一步之间的转变机制仍然是一项具有挑战性的任务。我们使用了一系列电位、显微镜和光谱工具，研究了SrSO₄的成核途径与物理化学溶液参数的关系。我们的观察结果表明，在阈值过饱和度以下，成核由结合物种驱动，类似于预成核簇模型，直接导致稳定相天青石SrSO₄的形成。在较高的过饱和度下，这种情况会发生变化，成核主要由自由离子的消耗主导。重要的是，这种成核机制的变化与半水亚稳相SrSO₄·1/2H₂O的形成相关，该亚稳相最终会转变为天青石，遵循奥斯特瓦尔德阶段规则。这种转变是一个溶液介导的过程，在存在液膜的情况下也会发生，并且受周围环境的物理化学参数控制。它通过亚稳相的溶解和最终相的重新结晶来进行。总体而言，我们的结果表明，预成核阶段发生的离子缔合决定了成核途径是否会经过中间相。这也强调了尽管奥斯特瓦尔德阶段规则是一个常见的过程，但它不是矿物形成的先决条件——即使在可能发生该规则的系统中也是如此。
