Extent of disorder in iron-bearing albite and anorthite melts: Insights from multi-nuclear (29Si, 27Al, and 17O) solid-state NMR study of iron-bearing NaAlSi3O8 and CaAl2Si2O8 glasses

Abstract

Knowledge of the detailed structure of iron-bearing aluminosilicate glasses is essential to unraveling the effects of iron on the macroscopic properties of natural silicate glasses and melts. However, the local structures around the framework cations (such as Al and Si) and the extent of network polymerization in iron-bearing aluminosilicate glasses are not well understood. This is mainly because the high-resolution solid-state NMR - one of the effective probes of the structures of aluminosilicate glasses - has limited utility in delving into the local configurations of oxide glasses with paramagnetic elements (i.e., iron). Here, we use multi-nuclear (29Si, 27Al, and 17O) high-resolution solid-state NMR to investigate the effect of iron content on the local structure around framework cations and oxygen species in Na(Al1-XFeX)Si3O8 glasses with varying X [=Fe/(Al + Fe)] (Fe3+/ΣFe = ~0.8), NaAlSi3O8 glasses with varying Fe2O3 (NaAlSi3O8 + Fe2O3, Fe3+/ΣFe = ~0.5), and CaAl2Si2O8 glasses with varying Fe2O3 (CaAl2Si2O8 + Fe2O3, Fe3+/ΣFe = ~0.3).The NMR results for Na(Al1-XFeX)Si3O8 glasses reveal preferential interaction between Fe3+ and Si-rich framework and thus an extensive mixing between [4]Si and [4]Fe3+. The presence of a small amount of [5]Al in Na(Al0.5Fe0.5)Si3O8 glass suggests that the extent of disorder around Al increases by Al-Fe3+ substitution. The NMR results for NaAlSi3O8 + Fe2O3 glasses suggest that Fe2+ interacts more strongly with Al-rich framework and reveal the presence of a non-negligible fraction of Na-NBO, confirming Fe2+-induced structural disorder. The absence of [5]Al in NaAlSi3O8 + Fe2O3 glasses indicates that Fe3+ is more likely to form a highly-coordinated framework (i.e., [5]Fe3+) when excess Fe3+ is added to charge-balanced glasses. The NMR spectra for CaAlSi2O8 + Fe2O3 glasses showed an iron-induced increase in the [5]Al fraction and confirmed that the overall degree of disorder increases with increasing Fe2O3 content. The noble NMR results for iron-bearing aluminosilicate glasses revealed the nature of diverse aspect of iron-induced structural disorder, mainly characterized by the intermixing between Si and Fe3+, preferential interaction between Al and Fe2+, and formation of highly coordinated [5]Fe3+. The detailed iron-induced structural evolution is largely dependent on the composition of melts as well as the valence state of iron. The observed increase in the extent of structural disorder in charge-balanced Na- and Ca-aluminosilicate glasses with increasing Fe2+ and Fe3+ can account for the iron-induced decrease in viscosity of the corresponding iron-bearing aluminosilicate melts.