第117回ジオダイナミクスセミナー
　 　Geodynamics　Seminar

　 　　　　　　　　　　
"Dissolution mechanisms of water in depolymerized silicate melts"
　　　
　　　　　 　　　　　　　講師：Xianyu Xue
　　　　　　（岡山大学固体地球研究センター助教授）

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　
　　　　　　主催　：　愛媛大学地球深部ダイナミクス研究センター
　　　 　 　日時　：　２００５年２月４日（金）17：00〜
　　　　　　場所　：　愛媛大学理学部講義棟　２０１教室
　　　　　　　　　　　
要　旨
　Water is among the most important volatile components that significantly affect phase relations, and physical and thermodynamic properties of magmas. The effect of dissolved water on the various properties of silicate melts are known to vary significantly depending on the silicate composition. To gain an atomistic understanding of this matter, it is necessary to systematically investigate the mechanisms of water dissolution in silicate melts with composition using spectroscopic techniques. 　Most of such studies up to now have been limited to relatively polymerized silicate compositions and have suggested that silanol (SiOH) and molecular H2O are the dominant water species. We have recently carried out a systematic 1H and 29Si nuclear magnetic resonance (NMR) spectroscopic study on hydrous silicate glasses (quenched melts) of a wide range of compositions and ab initio molecular orbital calculations on representative clusters, which provided new insight into this issue (Xue and Kanzaki, 2004).
　The most prominent result from our study is the direct evidence for a substantial proportion of the dissolved water as free hydroxyl groups in quenched hydrous silicate melts. Free hydroxyls are defined here as OH groups that are only linked to metal cations (e.g. Ca, Mg), but not part of the silicate network. We have found that free hydroxyls are favored by (1) more depolymerized melts and (2) network-modifying cations of higher field strength (Z/R2: Z: charge, R: cation-oxygen bond length) in the order Mg > Ca > Na. Their formation is expected to cause an increase in the melt polymerization, contrary to the effect of SiOH formation. This water dissolution mechanism could be particularly important for ultramafic and mafic magmas, and may account for the contrasting effect of water on melt viscosity for relatively polymerized and depolymerized silicate compositions.
　In addition, our study also provided insight into the compositional dependence of hydrogen bonding strength in silicate melts and glasses. Hydrogen bond interaction could be an important factor in controlling the water solubility of silicate melts and hygroscopicity of the corresponding glasses and crystalline materials. Reference: Xue, X., and Kanzaki, M. (2004) Dissolution mechanisms of water in depolymerized silicate melts: Constraints from 1H and 29Si NMR spectroscopy and ab initio calculations. Geochimica et Cosmochimica Acta, 68(24), 5027-5057.




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