Structure of volatile-containing silicate melts by multi-nuclear and multi-quantum NMR spectroscopy

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1.Structure of hydrous glasses : In order to investigate water dissolution mechanisms in silicate melts, we have studied hydrous melts in CaO-MgO-SiO_2 system using-^<29>Si and ^1H MAS NMR and ^1H-^<29>Si CP MAS NMR. We found free OH as well as SiOH and H_2O in these melts. Free OH was not observed in previous studies which are conducted on silica-rich melts. There is systematic changes in abundance of free OH with Mg/Ca and M/Si ratio. From these results, we suggest that free OH could be important in natural ultramafic magmas.2.Structure of sulfur-bearing glasses : In order to study how sulfur dissolves in silicate melts, S-bearing glasses were studied by ^<29>Si MAS NMR and micro-Raman spectroscopy. We synthesized sodium silicate glasses containing sulfur of different valence states (S^0〜S^<6+>). When native S is added, melt structure was polymerized, whereas slightly less polymerized when sulfate is added. No detectable Si-S linkages are found.3.Ab initio calculation of chemical shifts : Using molecular orbital theory, we have calculated wide varieties, of silicate, clusters relevant to melt or glass structures with volatile components. Calculated shifts of^1H and ^<29>Si NMR in hydrous systems support our assignment of free OH observed in the NMR spectra. Similar calculations for NMR and also vibrational spectra help us to identify sulfur species in the sulfur-containing melts.