Equilibrium isotopic fractionation in the kaolinite, quartz, water system: Prediction from first-principles density-functional theory

Isotopic fractionation factors for oxygen, hydrogen and silicon have been calculated using first-principles methods for the kaolinite, quartz, water (ice and gas water) system. Good agreement between theory and experiment is obtained for mineral-water oxygen isotope fractionation. This approach gives reliable results on isotopic fractionation factors as a function of temperature, within a relative precision of typically 5%. These calculations provide independent quantitative constraints on the internal fractionation of oxygen in kaolinite, the fractionation of silicon isotopes at equilibrium, or hydrogen fractionation between kaolinite and water. Calculated fractionation factors at 300 K are 12.5‰ for the kaolinite internal-fractionation of oxygen, and 1.6‰ for silicon fractionation between quartz and kaolinite.