STRUCTURAL ENVIRONMENT OF Nb5+ IN DRY AND FLUID-RICH (H2O, F) SILICATE GLASSES: A COMBINED XANES AND EXAFS STUDY

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

“…We hypothesize that increasing dissolution of water causes increasing distortion of local structure surrounding Ta sites in the hydrous glass/melt and in the silicate bearing aqueous fluid, as P-T conditions are increased. This is consistent with the local distortion about Nb sites observed from XAS measurements of Nb-bearing peraluminous glasses of varying water concentrations [3]. To that end, a TaO 6 Si 5 Al cluster model was devised.…”

“…The errors in part reflect the statistical uncertainties from fitting of the χ data using IFEFFIT. The six-fold oxygen coordination of Ta is consistent with structural data from various Ta-oxide materials [21][22][23][24] and with coordination of Nb in glasses of similar composition [3]. Next, we make a qualitative examination of the χ data and the Fourier transforms (FT) of the k 3 -weighted χ data of the XAS spectra measured from the Ta glass + H 2 O sample to 960°C and~0.6 GPa, shown in Fig.…”

“…This was well illustrated in a structural study of a variety of silicate glasses showing that Zr 4+ is in most cases 6-fold coordinated but exhibits 8-fold coordination by oxygen atoms in the most polymerized glass [2]. In a separate study, the local structural environment of Nb 5+ was found to be sensitive to melt depolymerization and water content, among other factors, in peraluminous and peralkaline glasses [3]. Tantalum silicate glasses show promise for blue laser and luminescent material applications [4], high-transmission optical fibers [5], and for nonlinear-effect photonic structures [6].…”

“…Using XANES and EXAFS spectroscopy, Pillonen et al [3] determined that the local structure of Nb 5+ is distorted due to water dissolution in peraluminous (ASI = 1.2) glasses; peralkaline glasses show no such distortion (ASI = 0.6) glasses (ASI: alumina saturation index = Al/ (Na + K) ratio). This is attributed by the authors to a greater degree of depolymerization of the network structure caused by water dissolution in the peraluminous glass than in the peralkaline glass.…”

Solubility of manganotantalite and manganocolumbite in pegmatitic melts

“…This would explain a different geochemical behavior (Wolff 1984). However, we are not aware of any spectroscopic data of this element that supports the presence of Nb 3+ (Piilonen et al 2006).…”