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Despite similarities in geochemical behavior, Cs and Rb concentrate in the upper continental crust (UCC) preferentially to K. Illite, a K-depleted dioctahedral mica occurring in large proportions in all types of sedimentary rocks, has high selectivity to Cs and Rb with irreversible adsorption in crystallite wedges and frayed edge sites. Analyzing 4000+ samples from 22 basins and sub-basins throughout the world using statistical computation, we found that Cs concentrates almost exclusively in illitic minerals (illite, illite-smectite) while Rb is equally distributed between K-feldspar and illite. Illite has Cs/K ratios up to 3× (median 1.97×) higher than average UCC, whereas its Rb/K variability corresponds to 1.0× to 1.08× the UCC’s. Cs/Rb ratios in illite fall within the range covering average ratios for UCC, loess, and river suspension. Cs/K and Cs/Rb ratios in illite are formation specific and are higher with thinner illite crystallites, where the proportion of frayed edges to basal plane is high. With average Cs content of 17.7 ppm, illite is suggested to be a global sink for Cs on Earth’s surface. Using mass balance, we present that parts-per-million concentrations of Cs adsorbed by illite crystallite edges constitute up to 20% of Cs present in UCC. Based on cation hydration enthalpy, we introduce the “water-incompatibility” concept to explain the Cs > Rb > K order of cation selectivity in illite, which is also the order of the cations’ relative depletion in global seawater with respect to the UCC.

Cesium, a water-incompatible, siloxane-complexed cation in Earth’s upper crust