The oxidation state and coordination environment of antimony in silicate glasses

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 fO₂in log units relative to the quartz-fayalite-magnetite buffer), and 1400 °C, 1.0 GPa and logfO₂ = +4.7 (ΔQFM+12.0). 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.