Abstract. Calcite has been shown to incorporate trivalent actinides into its crystal lattice, highlighting its potential to contribute to radionuclide retention processes in the environment. Earlier studies on calcite's elemental uptake were conducted under controlled laboratory conditions, which do not fully capture the complexities of natural environments. To gain a deeper understanding of calcite's uptake capacity under natural conditions, a sample from the Wenzel ore mine, Germany, was analyzed, originating from a calcite vein that formed under conditions relevant to deep geological-waste repositories. The chemical analogy between rare earth elements (REEs) and trivalent actinides helps to evaluate the retention potential and incorporation mechanisms of trivalent radionuclides. A micro-X-ray fluorescence (µXRF) element map revealed that the investigated calcite consists of an euhedral crystal core exhibiting sector zoning, characterized by trace element heterogeneity across coevally grown crystal faces. High-resolution laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) further refined and quantified the elemental distribution, revealing preferential incorporation of specific elements, particularly REEs, within one sector of the calcite. Remarkably, this sector showed REE concentrations over 200 times higher than those in the depleted sector, indicating a significant potential for the retention of trivalent actinides. Furthermore, the data indicate that charge equilibration of incorporated trivalent cations occurs via two processes: coupled substitution with monovalent cations and vacancies in the crystal structure. Overall, these results demonstrate that sector-zoned calcite formed under repository-relevant conditions can maintain high retention potential for trivalent actinides, even in environments depleted in monovalent cations.
Baumeister et al. (Mon,) studied this question.