Evidence from X-ray absorption for network-forming Fe²⁺ in molten alkali silicates

Abstract

Fe²⁺ is the most abundant iron species in magmas and in many slags. Its structural role in these liquids is poorly understood, largely because of the difficulty of studying melts at high temperatures by direct structural methods. Whether Fe²⁺ behaves as a network modifier, network former or free-ion complex has not been adequately resolved, yet this is fundamental to understanding the properties of silicate liquids. Also uncharacterized are the structural changes accompanying the melt-to-glass transition in Fe-bearing silicate melts. Here we report the results of a high-temperature synchrotron-based X-ray absorption study of Fe in silicate glasses and melts of compositions near Na₂FeSi₃O₈ and K₂FeSi₃O₈. The glasses were also analysed by ⁵⁷Fe Mossbauer spectroscopy. We conclude that Fe²⁺ is a four-coordinated network former in these melt/glass systems and that little structural relaxation occurs at the iron site during the melt-to-glass transition. These results and structural data for Fe₂²⁺SiO₄ melts¹ suggest the possibility of a pressure-induced change from four- to six-coordination for Fe²⁺ in magmas in the Earth's upper mantle.