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

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.