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

Abstract

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 29 Si 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. 29 Si NMR shows that in both Ti-bearing Na 2 O•4SiO 2 (NS4) and Na 2 O•8SiO 2 (NS8) glasses, increasing the amount of Ti in the melt results in a shift of Si Q 4 peak in the 29 Si NMR spectra reflecting Ti nearest neighbors for Si in Q 4 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 29 Si 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 Q 4 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.

Structural changes in shocked tektite and their implications to impact-induced glass formation

“…The trend (i) can be explained simply by coordination increase of Ti 4+ . The latter trend (ii) cannot be explained by only Ti 4+ and is indistinguishable in the plot of energy-intensity for Ti K pre-edge peaks (Ackerson et al, 2020). The present maximum experimental pressure was 35 GPa and achieved by the multiple shock reflection.…”

“…The D1 bands of TK3, TK4, and TKP2 at high pressures are clear relative to those of the others. 3) as characterized by the pre-edge features (Ackerson et al, 2020). All the patterns in Fig.…”

“…When we take an average distance of Ti-O in silicate glass, the ionic radii of Ti with CN=6 are 0.605 Å, 0.670 Å, and 0.86 Å for Ti 4+ , Ti 3+ , and Ti 2+ (Shannon, 1976), respectively. The previous Ti K edge XANES and EXAFS studies on some typical Ti-bearing crystals (Berry et al, 2007;Ackerson et al, 2020) reported the relationship between the coordination of Ti 4+ and the average Ti-O distance. They summarized that the Ti 4+ -O distances for CN=6 and CN=4 are 1.96-2.085 Å and 1.804-1.835 Å, respectively.…”

“…Previous studies have investigated characteristic glass structures by various methods such as X-ray diffraction (XRD), Raman, X-ray absorption fine structure (XAFS) (e.g. Masleraro and Zanotto, 2018), Infrared spectroscopy (IR) (Morlok et al, 2016), neutron diffraction (Yarker et al, 1986, Mössbauer spectroscopy (Cottrell et al, 2009), nuclear magnetic resonance (NMR) (Ackerson et al, 2020), cathode luminescence (CL) (Gucsik et al, 2004), electron spin resonance (ESR), electron paramagnetic resonance (EPR) (Lombert et al, 2011), atomic force microscopy (Golubev et al, 2018), etc. In particular, Raman spectra measurements on silicate glasses have been used widely to provide information on the local structures of short range and intermediate range around Si atoms (Hemley et al, 1986;McMillan et al,…”

Gas-mediated trace element incorporation into rhyolite-hosted topaz: A synchrotron microbeam XAS study

“…In late-stage rhyolite melt Ti 4+ may be in 5-coordinated complexes with 4 bridging and one non-bridging O bond (e.g., Ackerson et al 2020) that are tightly bound and likely show little solubility in the exsolving gas. This may account for the observation that Ti abundance is low in the gas phase (e.g., Symonds and Reed 1993;Shevko et al 2018).…”