The effect of oxygen fugacity and Na/(Na+K) ratio on iron speciation in pantelleritic glasses

Abstract

Reduction experiments were conducted to study the structural role (redox state and coordination) of iron in peralkaline rhyolitic glasses as a function of oxygen fugacity and alkali atomic ratio ([Na/(Na+K)]=0.0, 0.69 and 1.0). The glasses investigated, analogues of natural pantellerites of the Kenya Rift Valley, have been synthesized at ambient pressure, 1250°C and equilibrated under controlled redox conditions. Oxygen fugacities, ranging from NNO+7 to NNO- 6.8 (relative to Ni/NiO assemblage buffer), were adjusted by different gases and/or gas mixtures (air, O2, CO2, Ar/H2/H2O, Ar/H2) in a horizontal gas-mixing furnace.Iron oxidation state and coordination geometry have been inferred by Fe K-edge X-ray Absorption Near-Edge Structure (XANES) spectroscopy, which suggested the presence of Fe3+ in four-fold coordination and Fe2+ having an average coordination number intermediate between four- and five-fold in all the glasses.A colorimetric method has been employed to determine the Fe2+/Fetot ratios for all the samples. We verified in the samples a continuous increase of divalent iron for more reducing conditions (e.g. Fe2+/Fetot ranging from 0.09 in air to 0.93 in Ar/H2 atmosphere) but a non-linear relation between [Na/(Na+K)] molar ratio and iron redox was observed.The weak dependence of log (Fe3+/Fe2+) on log (fO2/bar) indicates a strong deviation from ideal mixing of iron species in pantelleritic compositions which suggests a different behaviour of alkali dominated peralkaline melts compared to other aluminosilicate melts in terms of fO2 and alkalis effects.