107 ISSN 0016-7029, Geochemistry International, 2008, Vol. 46, No. 2, pp. 107–115. © Pleiades Publishing, Ltd., 2008. Original Russian Text © A.G. Simakin, T.P. Salova, V.O. Zavelsky, 2008, published in Geokhimiya, 2008, No. 2, pp. 131–140. INTRODUCTION The structure of melts and glasses in the Na 2 O-xSiO 2 system is generally considered in terms of Q speciation. At moderate pressures and overheating relative to liquidus, network-forming cations, such as silica and aluminum, are oxygen-coordinated in the tet- rahedral site. The degree of bonding between these tet- rahedra depends on the short-range order in the melt structure, which could be determined experimentally [1]. Completely bound tetrahedra are typically desig- nated Q 4 , completely isolated tetrahedara as Q 0 , and intermediate settings are referred to as Q 3 , Q 2 , and Q 1 . It can be easily demonstrated that strictly tetrahedral coordination of network-forming cations is described by the integral relation (1) where NBO is the number of nonbridging oxygens (belonging to network modifiers), T is the number of tetrahedral elements in the arbitrarily chosen unit vol- ume of the melt, and x i is the mole fraction of Q i spe- cies. The total content of Q i species in silicate systems could be determined by Raman spectroscopy within a wavelength range of 800–1200 cm –1 , which is assigned to various stretching vibrations of T–O [2, 3]. In melts containing 60 mol % SiO 2 ( ≥ 2), the equilibrium of the Q species is described by the disproportionation reaction [3] (2) NBO/T 4 i – ( ) x i , i 0.4 = ∑ = x SiO 2 2Q 3 Q 2 Q 4 . + = The temperature dependence of the reaction con- stant can be determined by high-temperature IR spec- troscopy. According to spectral data, the enthalpy (∆H) of this reaction is –33 kJ/mol for lithium–silicate melt (of the composition Li 2 O · 4SiO 2 ) and + 22 kJ/mol for potassium–silicate melt (K 2 O · 4SiO 2 ) [2]. Each type of Q species (short-range order, within the T–O–T bond) corresponds to the definite structure of the medium-range order (MRO): tetrahedral chains, bands, and sheets. We and some other researchers believe that the medium-range order of the melt is sim- ilar to that of some crystalline phases. This is of princi- ple significance for understanding the mechanism of water dissolution in these melts. The most reliable method for studying water disso- lution in silicate melts (glasses) is NMR spectroscopy, which allows distinguishing between different struc- tural types of OH groups and molecular water. Several types of hydroxyl groups were identified in water-bearing sodium–silicate melts: the SiOH silanol group in silica, SiOH in disilicate, and NaOH. Using 17 é NMR, Oglesby et al [4] showed that non- bridging silanol oxygen in crystalline äçSi 2 O 5 is sim- ilar to that in water-bearing disilicate glass, whereas nonbridging oxygen in water-bearing orthoclase glass has different parameters. Six compositions of water-bearing (7 wt % H 2 O) glasses of the system Na 2 O–SiO 2 were studied by MAS (high resolution magic angle spinning) 29 Si NMR and MAS 1 H NMR spectroscopy [5]. At low Na contents, water is dissolved to depolymerize the network. At high Mechanism of Water Dissolution in Sodium–Silicate Melts and Glasses: Structural Interpretation of Spectroscopic Data A. G. Simakin a , T. P. Salova a , and V. O. Zavelsky b a Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia e-mail: simakin@iem.ac.ru b Institute of Physiologically Active Compounds, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia Received June 14, 2006 Abstract—Sodium–silicate glasses with varying water contents were studied by 23 Na NMR and 1 H NMR spec- troscopy. The 23 Na NMR spectrum is made up of two Gaussian–Lorentzian components corresponding to rig- idly bound and free Na ions. The rigidly bound Na is allocated in the disilicate-like domains corresponding to Q 3 species of sodium–silicate glasses. Unbound Na is associated with Q 2 and Q 1 species. It was shown that, during water dissolution, some hydroxyls are incorporated into the disilicate unit of the structure to form NaHSi 2 O 5 , while others hydrate silica (Q 4 species). Our 23 Na NMR data are consistent with available data on Q speciation and the proportions of water species in sodium–silicate glasses in the frameworks of a proposed detailed structural scheme of water dissolution. DOI: 10.1134/S0016702908020018