Study of hydrogen isotope equilibrium and kinetic fractionation in the ilvaite-water system

Abstract

Hydrogen isotope fractionation between ilvaite and water has been studied experimentally at the temperature of 750-250°C. Small quartz capsules impermeable to hydrogen are used in the experiments. All samples are sealed and products are collected under vacuum, so that the contamination of atmospheric water can be eliminated and the precisions of experiments can be significantly improved. At the temperature of 750-550°C, the hydrogen isotope equilibrium fractionation factor between ilvaite and water can be described by a linear equation: 10 3 In α ilvaite − H 2 O e = − 29.95 ( 10 6 / T 2 ) − 60.62 ; while, at 550-350°C, 10 3 In α ilvaite − H 2 O e = − 105.0 ± 0.7 , which is independent of temperature. At 250°C, the precision of equilibrium fractionation factors calculated by interpolation is too low because of low exchange rate. The hydrogen isotope equilibrium fractionation factors between hydrogen-bonded minerals and water are affected by the hydrogen bond structures and chemical compositions. They have a linear correlation with the hydrogen bond distances and chemical compositions [ d −( X Fe/10)], as well as with the hydroxyl stretching frequencies and chemical compositions [ f;- ( X Fe·100)]. The activation energy for hydrogen diffusion in ilvaite is calculated to be 118.5 kJ/mol by the cylinder model, and 115.5 kJ/mol by the plate model. The closure temperature of hydrogen isotope exchange between ilvaite and water is much lower than the formation temperature of ilvaite. The exchange mechanism is dominated by diffusion.