The degree of polymerization and structural disorder in (Mg,Fe)SiO3 glasses and melts: Insights from high-resolution 29Si and 17O solid-state NMR

Abstract

Probing the degree of polymerization and structural disorder of (Mg,Fe)SiO3 glasses and melts remains a challenging problem, despite their strong implications for the properties of natural volcanic glasses and dense melts in Earth’s surface and interior. Here, we explore the iron-induced changes in silicon and the oxygen configurations of (Mg1−XFeX)SiO3 glasses (Fe3+/ƩFe = 0.22 ± 0.07) with varying XFe [=Fe/(Mg + Fe), containing up to 13.5 wt% FeO] using high-resolution 29Si and 17O MAS, and 2D 17O 3QMAS NMR, unveiling the effect of Fe2+ on the extent of disorder in these glasses. The 29Si MAS NMR spectra show the apparent peak broadening in less polymerized species (i.e., Q1) and an increase in the fraction of highly polymerized (i.e., Q3) species with increasing XFe. These changes imply the selective decrease in NMR signal intensity of Q1 and Q2 due to their spatial proximity to Fe2+ (i.e., paramagnetic effect), and slight increase in the degree of polymerization due to the presence of minor fraction of Fe3+. The 2D 17O NMR spectra revealed that the fraction of Si-O-Si apparently increases with XFe and more significant changes in peak width for Mg-O-Si are observed with varying XFe. The results indicate that Mg-O-Si is more strongly affected by Fe2+ than Si-O-Si, suggesting a moderate degree of preferential proximity between Fe2+ and non-bridging oxygen. The results allow us to conceptualize the scheme to account for paramagnetic interaction between the nuclear spins and Fe2+. The scheme provides a systematic protocol to interpret the NMR spectra of diverse iron-bearing silicate glasses. The current NMR results, in conjunction with the results in the previous study for Fe3+-dominant sodium silicate glasses, suggest that paramagnetic effect depends on the valence state of Fe, indicating the presence of a moderate degree of preferential interaction between Fe2+ and NBO and less polymerized Si units as well as that between Fe3+ and highly polymerized Si units and BO. Though qualitative, the current NMR results for (Mg,Fe)SiO3 glasses shed light on an opportunity to explore the detailed structures of iron-bearing natural volcanic glasses and can improve understanding of the transport properties of the basaltic melts.