The oxidation state and coordination environment of antimony in silicate glasses

Abstract

Antimony K-edge X-ray absorption near edge structure (XANES) spectra were recorded for Sb in glasses of five synthetic CaO-MgO-Al2O3-SiO2 (CMAS) compositions and a simplified mid-ocean ridge basalt (MORB). The glasses were quenched from melts equilibrated at 1300 °C, atmospheric pressure, and oxygen fugacities (fO2) from logfO2 = –9 to 0 (ΔQFM from −1.7 to 7.3, where ΔQFM is the fO2 in log units relative to the quartz-fayalite-magnetite buffer), and 1400 °C, 1.0 GPa and logfO2 = +4.7 (ΔQFM+11.0). Comparison with model compounds (Sb, Sb2O3, β-Sb2O4, MnSb2O4 and Zn7Sb2O12) indicates that the oxidation state of Sb changes from Sb3+ to Sb5+ over the range of fO2 investigated. Using the spectra of the most reduced and oxidised glasses as end-members the oxidation state of Sb, Sb5+/ΣSb (where ΣSb = Sb3+ + Sb5+), was determined from the spectra of other samples by linear combination fitting. At terrestrial fO2s nearly all Sb occurs as Sb3+ in these silicate melts. The stability of Sb5+ was found to be positively correlated with the CaO content and optical basicity of the melts. Extended X-ray absorption fine structure (EXAFS) spectra were recorded for glasses prepared at pressures from 1.0 to 3.0 GPa, for which Sb5+/ΣSb = 0 and 1, and for a synthetic Sb-bearing sample of anorthite. The Sb3+O and Sb5+O bond lengths in the glasses are similar (1.938(4) and 1.964(5) Å, respectively) and suggest trigonal pyramidal coordination of Sb3+and octahedral Sb5+. The Sb3+O bond length did not change with pressure. Anorthite (CaAl2Si2O8) was found to readily incorporate Sb as Sb5+ on a tetrahedral site.