Unraveling the structure of high-pressure hydrous silicate melts by multi-dimensional NMR spectroscopy and ab intio calculations

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The purpose of this study was to unravel the structure of hydrous (alumino)silicate melts (glasses) at high pressure in order to understand the behavior of natural magmas. The main achievements are outlined below:1.Development of multi-nuclear, multi-dimensional NMR techniques. Multi-dimensional NMR techniques are particularly powerful structural tools because of the direct information they provide about atomic connectivities. We have succeeded in a variety of NMR measurements including ^1H DQ (double quantum) MAS NMR, ^<27>Al 3Q (triple quantum) MAS NMR, and ^<27>Al-^1H HETCOR (heteronuclear correlation) and 3QMAS/HETCOR NMR.2.Settling a long-term controversy concerning the water dissolution mechanism in polymerized aluminosilicate melts. By using above techniques, we have confirmed that the dissolution of water in polymerized aluminosilicate melts results in the depolymerization of the network structure through formation of SiOH/AlOH.3.Unraveling the compositional dependency of water … More dissolution mechanisms in aluminosilicate melts. Our systematic study over a range of compositions, coupled with ab intio calculations, has revealed that the dissolution of water in (alumino)silicate melts/glasses involves two competing processes : (1) formation of SiOH/AlOH and depolymerization of the network structure, and (2) formation of free OH and polymerization of the network structure, with the latter favored by depolymerized compositions with cations of large field strength. Four-, five-and six-coordinate Al were found to coexist in hydrous Ca-and Mg-aluminosilicate glasses.4.NMR study of high-pressure hydrous minerals. A similar study was also carried out on high-pressure hydrous minerals. This is not only important for understanding the structures of mantle minerals themselves, but also provided insights into the high-pressure behavior of hydrous melts/glasses.5.Development of synthesis method for ^<17>O-enriched oxides. We have succeeded in synthesizing ^<17>O-enriched oxides with high yield. This is essential for ^<17>O NMR measurements on high-pressure materials of limited quantity. Less