Multi-spectroscopic study of Fe(II) in silicate glasses: Implications for the coordination environment of Fe(II) in silicate melts

The coordination environment of Fe(II) has been examined in seven anhydrous ferrosilicate glasses at 298 K and 1 bar using Fe-57 Mbssbauer, Fe K-edge X-ray near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS), UV-Vis-NIR, and magnetic circular dichroism (MCD) spectroscopies. Glasses of the following compositions were synthesized from oxide melts (abbreviation and nonbridging oxygen:tetrahedral cation ratio (NBO/T) in parentheses): Li2FeSi3O8 (LI2: 1.33), Rb2FeSi3O8 (RB2: 1.33), Na1.08Fe1.17Si3.13O8 (NAl: 1.09), Na1.46Ca0.24Fe1.08Si2.97O8 (NC6: 1.38), Na1.09Ca0.51Fe0.72Si3.10O8 (NC2: 1.15), Na0.99Ca0.92Fe0.24 Si3.17O8 (NCI: 1.04), and Na0.29Mg0.53Ca0.52Fe0.56Al0.91Si2.44O8 (BAS: 1.05). Mbssbauer, XANES, and EXAFS information suggests that iron is dominantly ferrous in all glasses (< 10 atom% Fe(III)) with an average first-neighbor Fe(H) coordination varying from similar to 4 to 5.2 (+/- 0.2) oxygens. The UV-Vis-NIR spectrum of each sample exhibits intense absorption centered near 8100-9200 cm(-1) and weak absorption near 5000 cm(-1), which cannot be assigned unambiguously. The MCD spectrum of NC6 glass, which is the first such measurement on a silicate glass, shows three transitions at similar to 8500 cm(-1), similar to 6700 cm(-1), and similar to 4500 cm(-1). The behavior of these MCD bands as a function of temperature (1.6 K to 300 K) and magnetic field strength (I T to 7 T) indicates that they most likely arise from three distinct Fe(H) sites with different ground states, two of which are 5-coordinated and one of which is 4-coordinated by oxygens. The combined results suggest that Fe(II) predominantly occupies 5- and 4-coordinated sites in each glass, with the ratios differing for the different compositions. Small amounts of 6-coordinated Fe(II) are possible as well, but primarily in the more basic glass compositions such as BAS. The substitution of Li(I) for Rb(I) in the M2FeSi3O8 base glass composition causes a weakening of the average Fe(II)-O bond, as indicated by the longer Fe(II)-O distance in the latter. The basalt composition glass was found to have the largest Fe(II) sites relative to those in the other glasses in this study. A bond valence model that helps predict the coordination number of Fe(II) in silicate glasses is proposed. The structural information extrapolated to Fe(II)-bearing melts is parameterized using bond valence theory, which helps to rationalize the melt-crystal partitioning behavior of ferrous iron in natural and synthetic melt-crystal systems. Copyright (c) 2005 Elsevier Ltd.