Iron and phosphorus in calcium silicate quenched melts

Abstract

Mössbauer and Raman spectroscopy has been used to describe the relationships between redox equilibria of iron and phosphorus content and anionic structure of calcium silicate quenched melts. Phosphorus was added as Ca 3(PO 4) 2 [P-series of glasses where Ca ( Si + P) is constant] and as P 2O 5 [PW-series of glasses where Ca ( P + Si) decreases with increasing P ( P + Si) ]. Raman spectroscopic data show that in either case, phosphorus is dissolved in Fe-free melts as an orthophosphate (PO 3− 4) complex with a narrow ∼20–25-cm −1 full width at half height) P-0 stretch band near 950 cm −1. Its frequency as well as those of the Raman bands assigned to SiO stretching modes, are not affected by variations in P ( P + Si) in the range studied (0.00–0.16). There is a positive correlation between ln( Fe 3+ Fe 3+ ) and P ( P + Si) in the P-series of Fe-bearing quenched melts, whereas in the PW-series, the correlation is negative at the same temperature and oxygen fugacity. This different behavior results from two competing effects on Fe 3+ Fe 2+ . Phosphate complexing with Fe causes an increase in the redox ratio as observed in the P-series of glasses. In the PW-series, the degree of polymerization increases with P ( P + Si) , and this increase results in a decrease in the redox ratio. For quenched Fe-bearing melts equilibrated in air (under these conditions Fe 3+ Σ Fe = 0.58–0.75 depending on temperature and composition), the hyperfine parameters (from Mössbauer spectroscopy) of both ferric and ferrous iron are insensitive to composition (for 298-K spectra, isomer shift, IS Fe 3+ = 0.314 ± 0.007 mm s −1, quadrupole splitting, QS Fe 3+ = 1.165 ± 0.021 mm, IS Fe 2+ = 1.012 ± 0.012 mm s −1 and QS Fe 2+ = 1.952 ± 0.011 mm s −1), consistent with tetrahedrally coordinated Fe 3+ and probably octahedrally coordinated Fe 2+. When Fe 3+ Σ Fe is reduced below 0.4, the IS Fe 3+ increases to above 0.5 mm s −1, consistent with a transformation of Fe 3+ from tetrahedral to octahedral coordination.