Abstract
The coordination environment of Fe(II) has been examined in seven anhydrous ferrosilicate glasses at 298 K and 1 bar using
57Fe Mössbauer, 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): Li
2FeSi
3O
8 (LI2: 1.33), Rb
2FeSi
3O
8 (RB2: 1.33), Na
l.08Fe
l.l7Si
3.l3O
8 (NAl: 1.09), Na
l.46Ca
0.24Fe
l.08Si
2.97O
8 (NC6: 1.38), Na
l.09Ca
0.51Fe
0.72Si
3.10O
8 (NC2: 1.15), Na
0.99Ca
0.92Fe
0.24 Si
3.17O
8 (NCl: 1.04), and Na
0.29Mg
0.53Ca
0.52Fe
0.56Al
0.91Si
2.44O
8 (BAS: 1.05). Mössbauer, XANES, and EXAFS information suggests that iron is dominantly ferrous in all glasses (<10 atom% Fe(III)) with an average first-neighbor Fe(II) coordination varying from ∼ 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
−l, 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 ∼8500 cm
−1, ∼6700 cm
−1, and ∼4500 cm
−1. The behavior of these MCD bands as a function of temperature (1.6 K to 300 K) and magnetic field strength (1 T to 7 T) indicates that they most likely arise from three distinct Fe(II) 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 M
2FeSi
3O
8 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.
