NEW EXPERIMENTAL DEVELOPMENTS FOR IN SITU XAFS STUDIES OF CHEMICAL REACTIONS UNDER HYDROTHERMAL CONDITIONS

Abstract

New experimental developments for in situ X-ray absorption fine structure spectroscopy (XAFS) studies of hydrothermal systems are described. Improvements in materials of construction and the design of spectroscopic cells have allowed application of XAFS as a powerful method to derive the molecular structures of reacting species. Two different spectroscopic cells are described that are suitable for in situ XAFS studies of aqueous solutions under hydrothermal conditions. One cell consists of a titanium alloy body with a corrosion-resistant platinum-iridium insert and diamond windows for X-ray transmission. Using this cell, XAFS spectra were acquired on aqueous tungstate solutions up to 400°C. The results demonstrate how XAFS can be used to study the speciation of isopolytungstates and their equilibria at high temperatures. Results from model calculations using the FEFF ab initio code are used to explain the observed spectral changes with changes in pH and temperature. The second XAFS cell consists of a simple fused-silica capillary having a 180 μm inner diameter. High-quality spectra of a 0.2 molal nickel bromide solution were collected in these small capillaries demonstrating the feasibility of the approach to the study of a wide range of geochemical systems. Further, the XAFS spectra show that the nickel bromide solution irreversibly reacts with the fused-silica capillary at 425°C. Based upon these observations, a potential use of the fused-silica capillary is suggested for in situ studies of silicate formation, or, in general, for studies of chemical interactions of hydrothermal brine solutions with quartz.