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Thorium (Th) is a naturally occurring radioactive element found in the environment, and recent advancements have been made in identifying and characterizing Th-bearing nanoparticles (NPs). However, the main focus is still on synthesized Th-bearing NPs and knowledge about natural Th-bearing NPs remains limited. Here, high-resolution transmission electron microscopy (HRTEM) observations of thorite from the Huayangchuan uranium ore deposit in Shaanxi Province, Central China, have revealed the nanoscale mineral characteristics of thorite. In this study, thorite NPs ranging from 5-10 nm in size were identified within the uranium ore. A combination of transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) elemental mapping and corresponding HRTEM images alongside Selected Area Electron Diffraction (SAED) and Fast Fourier Transform (FFT) patterns revealed a complex nanoscale structure in the thorite NPs, consisting of both amorphous and crystalline nanodomains with abundant defects. These nanostructures are associated with a metamictization mechanism in micrometer-sized thorite. Our findings indicate that the metamictization process can generate numerous thorite nanoparticles. Given the high penetrability and mobility of these NPs, the metamictization of thorite poses new challenges for the long-term stability of radioactive substances and the storage containers for radioactive waste. Furthermore, considering the likelihood of environmental release and the chemical toxicity, radioactivity, and nanotoxicity of natural Th-bearing NPs, increased attention should be given to the presence of natural thorite NPs in the ore deposit., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Accessory minerals in granitic rocks are unlikely significant radionuclide contributions to groundwater due to their remarkable durability. However, accessory minerals incorporating U and Th may suffer structural damages due to the radioactivity and become highly susceptible to alteration. This study investigates geochemistry coupled with textural analysis of the U-Th bearing accessory minerals using a field emission scanning electron microscope and an electron probe micro-analyzer. Altered zircons with numerous open structures related to the radioactive decay show higher contents of U and Th and low analytical totals. Some thorites show high contents of U and non-formula elements due to the hydrothermal alteration in the metamicted thorite. The cerianite including U occurs as micro-veinlet in fracture with trace of Fe and Mn oxides, which indicates secondary phase formation from the decomposed accessory minerals in an oxidizing environment. Some accessory minerals with the high content of U and Th have been found in Mesozoic granite terrain in South Korea, where high concentration levels of radionuclide in groundwater were also reported. The leaching of U may be more likely when the accessory minerals are highly metamicted or altered as found in our samples. The altered zircon and thorite of the study area could be major carriers of radioelement in Mesozoic granitic aquifers where the occurrence of soluble U-minerals has not been reported., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)