Effect of oxygen fugacity on the coordination and oxidation state of iron in alkali bearing silicate melts

In this study the effect of oxygen fugacity (fO(2)) on the oxidation state and coordination of Fewas investigated in different alkali trisilicate glasses (Rb2Si3O7 = RFS, K2Si3O7 = KFS; Na2Si3O7 = NFS; Li2Si3O7 = LFS) doped with similar to 5wt.% of Fe2O3 with main focus on K- and Na-bearing compositions. Most of the experiments were conducted at ambient pressure in a gas mixing furnace at 1250 degrees C with controlled redox conditions (log fO(2)/bar: -0.68 to -16.18). The quenched glasses were analyzed using several methods. Analyses by a colorimetric wet chemistry method revealed a continuous increase in Fe2+/Fe-total towards more reducing conditions without reaching 100% Fe2+ even at extremely reducing conditions (range of Fe2+/Fetotal: from 0.08 in air to 0.93 in H-2 atmosphere). X-ray absorption near edge structure (XANES) spectroscopy shows an increase of Fe coordination with decreasing ionic radius of the coexisting alkali, while the average coordination number seems to be independent on the oxidation state of iron aside from the largest studied alkali Rb, which seems to support lower coordinated Fe (tetrahedral) at more oxidizing conditions. The Fe2+/Fe-total ratios inferred by XANES, using an intensity ratio based calibration of W ilke et al. (2004), are systematically higher by 10% compared to the wet chemistry results of this study, which may be due to the different external Fe2+/Fetotal determination method (Mossbauer spectroscopy) used in their calibration. A new calibration curve based on wet chemistry and centroid positions is proposed for alkali silicate glasses. In optical spectroscopy, the position of the main Fe2+-related peak shifts to lower wavenumbers with increasing ionic radius of the incorporated alkali and with increasing abundance of ferrous iron. Absorption coefficients epsilon(Fe(II)) and epsilon(Fe(III)) were calculated for the absorbance band at similar to 9000 and similar to 26,000 cm(-1), respectively. A decrease in eFe(II) was detected with decreasing ionic radius of the incorporated alkalis (epsilon Fe(II)KFS = 31.8 +/- 2.6 L.mol(-1).cm(-1), epsilon Fe(II)NFS = 30.7 +/- 2.3 L.mol(-1).cm(-1) and epsilon Fe(II)LFS = 23.6 +/- 1.7 L.mol(-1).cm(-1)). Finally, the results of this study are compared with recent models, which predict fO(2) based on the knowledge of the Fe2+/Fe-total ratio. All models overestimate Fe2+/Fe-total in alkali silicate melts at very reducing conditions probably due to an unanticipated stabilization of Fe3+ by adjacent Fe2+. (C) 2015 Elsevier B.V. All rights reserved.