Tin and niobium in dry and fluid-rich (H2O, F) silicate glasses

Abstract

The speciation of niobium and tin in 29 silicate glasses was investigated by XAFS at the Nb and Sn K-edges using beamline 4-3 and 11-2 at SSRL (Stanford, USA). The metal content in the glasses varies between 100 ppm and 5 wt.% (to examine the effects of Nb2O5 and SnO2 saturation in the melt composition prior quenching). The glasses show variation in composition from peralkaline to peraluminous, with either zero or ∼2 wt.% fluorine. In addition, hydrous glasses were quenched from water-saturated melts at 850 °C and 2 kbar under relatively reducing conditions. Three “haplogranitic” compositions (a model granite in the system Na2O-K2O-AlO3-SiO2) were investigated: peralkaline (labeled “ASI 0.6”), metaluminous (“ASI 1.0”) and peraluminous (“ASI 1.2”). One series of haplogranitic glasses (“SQ”) were synthesized at intermediate oxygen fugacity conditions (∼10−9 atm.). In addition, 15 model oxide and silicate compounds of tin and niobium were investigated.The tin-bearing glasses show Sn K-edge XAFS spectra that resemble that displayed by eakerite, a Ca-Sn silicate in which Sn(IV)O6 octahedra share corners with SiO4 tetrahedra (⟨Sn-O-Si⟩: 150°). However, the most reduced glasses show significant contributions arising from weakly bound Sn(II)On complexes. The niobium-bearing glasses show Nb K-edge XAFS spectra that most closely resemble that displayed by vuonnemite, a Na-Ti-Nb silicophosphate. Niobium forms NbO6 clusters, also bonded to Si/Al second neighbors. The coordination geometry of niobium in the glasses is strongly influenced by both Nb content and alkalinity; H2O content does not strongly influence the speciation of niobium. Fluorine-bearing Nb-glasses show unusual XANES spectra, which might be related to a decrease in the coordination number of niobium from 6 to 4 (or 5). However, based on Nb K-edge XANES calculations (FEFF 8), fluorine does not appear to complex niobium.