This study explores the complex relationship between particle size, mineral composition, surface characteristics, elemental distribution, and the mechanisms of rare earth element (REE) adsorption in a unique ionic clay from South America. Differing from the common ionic clays that primarily exhibit physisorption, this sample demonstrates a mix of REE adsorption methods, including chemisorption. The research includes dividing the clay into three particle size groups: S1 (<0.25 mm), S2 (0.25–0.5 mm), and S3 (0.5–2 mm), and thoroughly analyzing each for elemental and mineral composition, surface area, morphology, elemental distribution, and REE absorption processes. The findings show that most REEs that can be desorbed are physisorbed, mainly due to kaolinite, which is generally associated with physisorption. Heavy rare earth elements (HREEs) are noticeably preferred in adsorption compared with light rare earth elements (LREEs), a pattern attributed to the weathering processes during the formation of the clay, promoting movement and concentration of HREEs. The ionic clay contains a significant amount of mineralized REEs, suggesting more intensive extraction methods like acid baking and water leaching for complete REE extraction. In terms of desorbable REEs, physiosorption is predominant, accounting for over 80%. Chemisorbed REEs are found in association with various minerals, including kaolinite, quartz, and goethite. This research reveals the detailed interaction between particle sizes, mineral makeup, surface features, and REE adsorption behaviors in a non-standard ionic clay sample, offering valuable insights into the factors influencing REE adsorption in ionic clays and highlighting the need to consider different adsorption methods in future extraction processes.