²⁹Si solid state NMR and Ti K-edge XAFS pre-edge spectroscopy reveal complex behavior of Ti in silicate melts

An understanding of the mechanisms of Ti is incorporation into silicate glasses and melts is critical for the field of petrology. Trace-element thermobarometry, high-field-strength element partitioning, and the physical properties of magmas are all be influenced by Ti incorporation into glasses and changes therein in response to changes in composition and temperature. In this study, we combine ²⁹Si solid state NMR and Ti K-edge XAFS spectroscopy to investigate how Ti is incorporated into quenched Na-silicate glasses, and the influence of Ti on the structure of silicate species in these glasses. ²⁹Si NMR shows that in both Ti-bearing Na₂O•4SiO₂ (NS4) and Na₂O•8SiO₂ (NS8) glasses, increasing the amount of Ti in the melt results in a shift of Si Q⁴ peak in the ²⁹Si NMR spectra reflecting Ti nearest neighbors for Si in Q⁴ speciation. The Ti XAFS results from NS8 glass indicate that Ti is primarily incorporated in [5]-fold coordination. At higher Ti content, there is a shift of the XAFS pre-edge feature suggesting mixing of [4]-fold Ti into the spectra. Combined, the ²⁹Si NMR and XAFS pre-edge data are consistent with Ti incorporation as isolated ^[5]Ti atoms and the formation of ^[5]Ti clusters at relatively low Ti concentrations, with no evidence for Ti-Na interactions as suggested by previous studies. As the Ti content increases, the Ti atoms begin to occupy 4-fold coordinated sites interacting primarily with Si in Q⁴ speciation (no significant Na-^[4] Ti bonding). The internal consistency of these two techniques provides a uniquely complete snapshot of the complexity of Ti incorporation in silicate melts and underlies the importance of understanding Ti incorporation mechanisms in natural magmatic systems.