Experimental Investigation of Oxide Leaching Methods for Li Isotopes

To examine the applicability of different leaching methods used to extract secondary oxides from silicate solids for lithium isotope (delta Li-7) measurement, this study has conducted leaching experiments on five different types of silicate solids, including a fresh basalt, two weathered basalts, a Yellow River sediment (loess-dominated) and a shale. Four factors were assessed in the experiments: the concentration of the leaching reagent hydroxylamine hydrochloride (HH), the leaching temperature (20 degrees C vs 95 degrees C), the leaching time and the reagent/solid ratio. Based on elemental concentrations and Li isotopes, 0.04 mol l(-1) hydroxylamine hydrochloride (HH) in 25% v/v acetic acid at room temperature for 1 h with 40 ml g(-1) reagent/solid ratio is recommended. At high temperatures, low delta Li-7 and high magnesium/iron ratios indicate that minerals other than secondary oxides are dissolved. With increased leaching time, there is no evidence for Li isotopic fractionation at room temperature. However, longer leaching time or increased reagent/solid ratios may increase the risk of leaching from non-oxide phases. Meanwhile, results suggest that low concentrations of HH are not sufficient to target the secondary oxides evenly, while high concentrations of HH can leach out more non-oxides. We also examined the optimal oxide leaching method within a full sequential leaching procedure (i.e., exchangeable, carbonate, oxide, clay and residual phases). Elemental concentrations show that no elements exist exclusively in oxides, so it is essential to analyse multi-elemental concentrations to verify that the leaching has accessed this phase in a given sample. Comparing secondary oxides with their corresponding solutions, we estimate the isotopic fractionation (Delta Li-7(oxide-solution)) is -16.8 parts per thousand to -27.7 parts per thousand.

Automated summary

This study conducted leaching experiments on different types of silicate solids to examine the applicability of various leaching methods for extracting secondary oxides for lithium isotope measurement. The concentration of hydroxylamine hydrochloride, leaching temperature, leaching time, and reagent/solid ratio were assessed as factors. The recommended conditions for effective leaching were determined based on elemental concentrations and Li isotopes. The study also compared secondary oxides with corresponding solutions to estimate isotopic fractionation.