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-Al 2 O 3 -SiO 2 (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 ( f O 2 ) from log f O 2 –9 to 0 (ΔQFM from −1.7 to 7.3, where ΔQFM is the f O 2 in log units relative to the quartz-fayalite-magnetite buffer), and 1400 °C, 1.0 GPa and log f O 2  = +4.7 (ΔQFM+12). Comparison with model compounds (Sb, Sb 2 O 3 , β-Sb 2 O 4 , MnSb 2 O 4 and Zn 7 Sb 2 O 12 ) indicates that the oxidation state of Sb changes from Sb 3+ to Sb 5+ over the range of f O 2 investigated. Using the spectra of the most reduced and oxidised glasses as end-members the oxidation state of Sb, Sb 5+ /ΣSb (where ΣSb = Sb 3+  + Sb 5+ ), was determined from the spectra of other samples by linear combination fitting. At terrestrial f O 2 s nearly all Sb occurs as Sb 3+ in these silicate melts. The stability of Sb 5+ 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 Sb 5+ /ΣSb = 0 and 1, and for a synthetic Sb-bearing sample of anorthite. The Sb 3+ O and Sb 5+ O bond lengths in the glasses are similar (1.938 (4) and 1.964 (5) A, respectively) and suggest trigonal pyramidal coordination of Sb 3+ and octahedral Sb 5+. The Sb 3+ O bond length did not change with pressure. Anorthite (CaAl 2 Si 2 O 8 ) was found to readily incorporate Sb as Sb 5+ on a tetrahedral site.