Regolith-hosted rare earth elements (REEs) deposits received great attention due to the increasing incorporation of REEs in modern technologies. In lateritic Sc deposits and ion-adsorption deposits (IADs), Sc behaves quite differently from REEs: REEs adsorb as outer-sphere complexes on clay surface in IADs, while Sc could enter the lattice of clay minerals in lateritic Sc deposits. The unique behavior of Sc has not been well understood yet. Here, by using first-principles molecular dynamics techniques, we show that the complexation mechanisms of Y3+ and Sc3+ on clay edge surfaces are distinctly different. Y3+ preferentially adsorbs on Al(OH)2SiO site with its coordination water protonated. Sc3+ is found to behave similarly to other first-row transition metals (e.g., Ni2+) due to its smaller ionic radius and prefers adsorbing on the vacancy site, from where Sc3+ can be readily incorporated in the clay lattice. The H2O ligands of Sc3+ get deprotonated upon complexation, providing new binding sites for further enrichment of Sc3+. These processes prevent Sc3+ from being leached during weathering and lead to the formation of Sc-rich clay minerals found in lateritic deposits. Based on these results, it is revealed that the small ionic radius and high affinity to enter the vacancy on edge surfaces make Sc compatible with clay minerals and are the origin of its unique geochemical behavior.

中文翻译：

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了解 Sc 在与粘土矿物相互作用中的独特地球化学行为

由于现代技术中越来越多地采用稀土元素，风化层中的稀土元素 (REE) 矿床受到了极大的关注。在红土钪矿床和离子吸附矿床（IAD）中，钪的行为与稀土元素有很大不同：稀土元素在IAD中作为外球络合物吸附在粘土表面，而钪可以进入红土钪矿床中粘土矿物的晶格。Sc 的独特行为尚未得到很好的理解。在这里，通过使用第一原理分子动力学技术，我们证明了 Y3+ 和 Sc3+ 在粘土边缘表面上的络合机制明显不同。Y3+优先吸附在Al(OH)2SiO位点上，其配位水被质子化。人们发现 Sc3+ 的行为与其他第一行过渡金属（例如 Ni2+）类似，因为它的离子半径较小，并且更喜欢吸附在空位位点上，从空位位点 Sc3+ 可以很容易地结合到粘土晶格中。Sc3+ 的 H2O 配体在络合后去质子化，为进一步富集 Sc3+ 提供新的结合位点。这些过程可防止 Sc3+ 在风化过程中被浸出，并导致红土矿床中富含 Sc 的粘土矿物的形成。基于这些结果表明，小离子半径和进入边缘表面空位的高亲和力使得Sc与粘土矿物相容，并且是其独特地球化学行为的根源。