Heavy rare earth elements (HREEs), an indispensable resource for modern industry, are extracted mainly from clays in ion adsorption deposits (IADs) in South China. The HREEs in IADs are derived from accessory minerals in parental granites. These precursor HREE phases have low solubility in aqueous environments, and unraveling the mechanism of their decomposition during weathering is critical to understanding how IADs form. Here, we report the micro- to nanoscale structural characteristics of HREE precursor minerals in parental granites from the large Zudong and Zhaibei IADs. High-resolution transmission electron microscopy shows that these minerals are characterized by abundant structural defects that range from lattice dislocations to submicro- to nanoscale crystallite aggregates with a variable proportion of amorphous material. Ubiquitous structural defects make the precursor HREE minerals unstable during weathering, resulting in their rapid decomposition, thereby facilitating the development of clay-hosted mineralization.

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