Investigation of the structural environment of Ta in a silicate glass and water system under high P–T conditions

•In situ XAS was measured from a high-temperature Ta-bearing silicate glass + H2O system.•XANES is used to probe the chemical environment surrounding Ta in a glass + H2O system.•Dissolved H2O distorts the local structure of Ta in the melt and silicate-rich fluid.•The quasi-bound upper 5d level occupation is reduced with more Ta–Q-species distortion.

In situ Ta L3-edge XAS measurements have been made from a Ta (~ 1400 ppm)-bearing peraluminous silicate glass + H2O system to 960 °C and ~ 0.6 GPa. A white-line doublet separated by ~ 4 eV occurs in the Ta L3-edge XANES and results from octahedral crystal field splitting of the Ta 5d levels due to the local structure surrounding Ta coordinated by Qn-species (n = number of bridging oxygen atoms shared between SiO4 and AlO4 units) in the silicate glass/melt + H2O system. The XANES spectra measured from the hydrous silicate glass/melt and from the silicate-rich aqueous fluid have been analyzed using multi-peak fitting techniques. The white-line doublet intensity varies with increasing P–T conditions of the silicate glass/melt + water system, indicating a shift in the electronic density of states in the vicinity of quasi bound Ta 5d states probed by the 2p3/2 core photoelectron. Ab initio modeling of the XANES indicates that water dissolution causes distortion of local structure surrounding the 6-fold coordinated Ta–Qn clusters in the hydrous silicate glass/melt and in the silicate-rich aqueous fluid. Calculation of the angular-momentum projected density of states (l-DOS) shows that the upper doublet level quasi-bound d-DOS is steadily reduced with increasing distortion of the local structure surrounding Ta–Qn clusters.