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 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.
Highlights
Titanium, typically a minor element in natural magmatic systems, does play a major role in the evolution of igneous and metamorphic rocks
Sodium silicate compositions are favored for structure vs. composition studies is because the key structural elements of silicate glasses and melts (the so called “Qn” species (Lippmaa et al 2002), see discussion below) are clearly resolved using 29Si solid state NMR
In the case of the sodium silicate compositions explored here, given the relatively high silica content, the 29Si solid state NMR reveals only Q3 and Q4 species well resolved from each other
Summary
Typically a minor element in natural magmatic systems, does play a major role in the evolution of igneous and metamorphic rocks. Results from Ti pre-edge XAFS and 29Si NMR provide three critical observations that aid in interpreting structural changes in the glasses with changes in bulk TiO2 content: 1) At low TiO2 content, Ti is primarily in [5]-fold coordination.
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