Potassium isotopic equilibrium fractionation induced by clay adsorption and incorporation: A density functional theory study

Abstract

Chemical weathering of continental silicate rocks is a key process in controlling the concentration and isotope composition of many elements in seawater, including potassium. K isotopes show great potential for tracing the silicate weathering. However, their application remains constrained by a lack of understanding of K isotopic fractionation mechanisms during chemical weathering, wherein secondary clay minerals adsorb and incorporate K derived from silicate minerals. Using first-principles molecular dynamics simulations and density functional theory (DFT) calculation, the coordination states of K in aqueous fluid and at the kaolinite (001) and (010) surfaces, as well as the K isotopic equilibrium fractionation caused by the adsorption of kaolinite surface and the incorporation of illite and glauconite, have been investigated. The results show that five (^VK) and six (^VIK) coordinated K dominate in aqueous fluids (∼88 %) and on kaolinite (001) (∼84 %), while ^IVK and ^VK prevail on (010) surfaces (∼80 %). The 1000lnβ values of aqueous fluids and kaolinite (001) and (010) surface configurations do not correlate well with structural factors (e.g., K–O bond lengths, the distortions of K–O bond length and O–K–O angle of coordination polyhedral). Rather, the 1000lnβ values exhibit a strong linear correlation with the force constants acting on the K⁺ ion in minerals and fluids. This process results in K isotopic equilibrium fractionation between the kaolinite (001) and (010) surfaces and aqueous fluids of 0.63 ± 0.37 ‰ and 0.58 ± 0.42 ‰, respectively, at 25 °C. The tetrahedral Al absence alters the K isotope equilibrium fractionation between clay minerals and aqueous fluids (0.25 ± 0.30 ‰ vs. −0.37 ± 0.24 ‰ for illite; −0.06 ± 0.22 ‰ vs. −0.35 ± 0.23 ‰; for glauconite; at 25 °C). These theoretical calculations may be used to model K isotope evolution in surface weathering environments to gain a more detailed understanding of K isotope fractionation under varying conditions of temperature, pH, ionic strength, and salinity.