Direct atomic-scale observation reveals rare earth element fractionation at mineral-water interfaces.

High-resolution atomic force microscopy shows that Si-basal planes on kaolinite, illite, and mica selectively enrich heavy rare earth elements, with pH-enhanced adsorption capacity across pH 4.8–6.4.

• Why it matters: Atomic-scale mechanisms for preferential rare earth adsorption and fractionation at mineral-water interfaces are poorly constrained, limiting interpretation of weathering archives and critical-element cycling.
• What they did: In situ atomic force microscopy quantified adsorption of La, Y, and Yb on Si-basal, Al-basal, and edge terminations under environmentally relevant pH (4.8–6.4), supported by synchrotron X-ray absorption spectroscopy and surface complexation modeling.
• The result: Si-basal adsorption is linked to outer-sphere complexation, while Al-basal/edge adsorption is linked to inner-sphere binding; modeling indicates Si-dominated to Al- and edge-controlled adsorption as pH increases.

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