Effect of iron content on the structure and disorder of iron-bearing sodium silicate glasses: A high-resolution 29Si and 17O solid-state NMR study

Abstract

Despite its geochemical importance and implications for the properties of natural magmatic melts, understanding the detailed structure of iron-bearing silicate glasses remains among the outstanding problems in geochemistry. This is mainly because solid-state NMR techniques, one of the most versatile experimental methods to probe the structure of oxide glasses, cannot be fully utilized for exploring the structural details of iron-bearing glasses as the unpaired electrons in Fe induce strong local magnetic fields that mask the original spectroscopic features (i.e., paramagnetic effect). Here, we report high-resolution 29Si and 17O solid-state NMR spectra of iron-bearing sodium silicate glasses (Na2O–Fe2O3–SiO2, Fe3+/ΣFe=0.89±0.04, thus containing both ferric and ferrous iron) with varying XFe2O3 [=Fe2O3/(Na2O+Fe2O3)], containing up to 22.9wt% Fe2O3. This compositional series involves Fe–Na substitution at constant SiO2 contents of 66.7mol% in the glasses. For both nuclides, the NMR spectra exhibit a decrease in the signal intensities and an increase in the peak widths with increasing iron concentration partly because of the paramagnetic effect. Despite the intrinsic difficulties that result from the pronounced paramagnetic effect, the 29Si and 17O NMR results yield structural details regarding the effect of iron content on Q speciation, spatial distribution of iron, and the extent of polymerization in the iron-bearing silicate glasses. The 29Si NMR spectra show an apparent increase in highly polymerized Q species with increasing XFe2O3, suggesting an increase in the degree of melt polymerization. The 17O 3QMAS NMR spectra exhibit well-resolved non-bridging oxygen (NBO, Na–O–Si) and bridging oxygen (BO, Si–O–Si) peaks with varying iron concentration. By replacing Na2O with Fe2O3 (and thus with increasing iron content), the fraction of Na–O–Si decreases. Quantitative consideration of this effect confirms that the degree of polymerization is likely to increase with iron content and that Fe3+ is predominantly a network-former. The 17O NMR spectra suggest a moderate degree of preferential partitioning of iron between NBO and BO clusters. The present results bear strong promise for studying iron-bearing silicate glasses using solid-state NMR techniques, constraining the effect of iron content on the degree of polymerization. The observed changes in atomic structures of iron-bearing sodium silicate glasses will be helpful for unraveling atomic origins of the properties of natural silicate melts.