Water speciation in hydrous sodium tetrasilicate and hexasilicate melts: Constraint from high temperature NIR spectroscopy
Water speciation in hydrous sodium tetrasilicate and hexasilicate melts: Constraint from high temperature NIR spectroscopy
复制标题
水合四硅酸钠和六硅酸钠熔体中的水形态：来自高温近红外光谱的限制
DOI：
10.1016/j.chemgeo.2008.06.053
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发表时间：
2008
期刊：
影响因子：
3.9
通讯作者：
S. Yamashita
中科院分区：
文献类型：
--
作者：
H. Behrens;S. Yamashita
研究了四硅酸钠玻璃（Na 2 O·4SiO 2，1.0-5.2 wt.%）中SiOH基团在~4500 cm-1处和H2O分子在~5200 cm-1处的近红外（NIR）吸收峰在加热时的变化H2O）和六硅酸钠玻璃（Na 2 O·6SiO 2，2.3-7.0 wt.% H2O）。使用装配到傅里叶变换显微红外光谱仪的加热台，温度在25 °C和475 °C之间变化。在这种热处理过程中，溶解水在玻璃中是稳定的，如加热后记录的NIR光谱所证明的。低于200 °C时光谱的变化归因于水分子氢键的减少以及NIR组合带的摩尔吸收系数的T依赖性。在更高的温度下，观察到4500 cm-1谱带的强度迅速增加，而5200 cm-1谱带的强度则减少，这表明物种相互转化Si-O-Si+H2O=2Si-OH的开始。这些变化的起始温度随着玻璃的总水含量的降低而增加。水物种的浓度不能直接从熔融物的NIR组合带的强度确定，因为在高温下硅酸钠熔融物的摩尔吸收系数不能校准。因此，加热实验仅用于确定在室温下在玻璃上测量的水形态的假想温度（也表示为表观平衡温度）。假定准氧物种（OH基、H_2O分子、非桥氧NBO、桥氧BO）理想混合，计算了水物种互变反应的平衡常数K。熔融态形态平衡的温度依赖关系为：四硅酸钠的ln K=5.39-3427/T，六硅酸钠的ln K=5.80-3731/T。在岩浆温度下，硅酸钠熔体中水的平衡常数大于聚合铝硅酸盐熔体中的平衡常数，其系数为5或更多，这取决于熔体的组成。ln K的这种大的差异很可能是由于硅酸钠熔体中的高熔体解聚度和高碱浓度。
Changes of near-infrared (NIR) absorption peaks at ~4500 cm−1due to SiOH groups and at ~5200 cm−1due H2O molecules upon heating were investigated in sodium tetrasilicate glasses (Na2O·4SiO2, 1.0–5.2 wt.% H2O) and sodium hexasilicate glasses (Na2O·6SiO2, 2.3–7.0 wt.% H2O) at ambient pressure. Temperature was varied between 25 °C and 475 °C using a heating stage fitted to a Fourier-transform micro-infrared spectrometer. Dissolved water is stable in the glasses during such a heat treatment as evidenced by NIR spectra recorded after heating. Changes in the spectra below 200 °C are assigned to decreasing hydrogen bonding of water species and to the T-dependence of the molar absorption coefficients for the NIR combination bands. At higher temperature a rapid increase in intensity of the 4500 cm−1band on expense of the 5200 cm−1band was observed indicating the onset of species interconversion Si–O–Si+H2O=2Si–OH. The onset temperature for these changes increase with decreasing total water content of the glasses. Water species concentrations could not be determined directly from the intensities of the NIR combination bands for the melt because the molar absorption coefficients cannot be calibrated for the sodium silicate melts at high temperatures. Therefore, the heating experiments were used only to determine the fictive temperatures (also denoted as apparent equilibrium temperatures) for the water speciation measured on the glasses at room temperature. Assuming ideal mixing of quasi-oxygen species (OH group, H2O molecule, non-bridging oxygen NBO, bridging oxygen BO) the equilibrium constant K for interconversion reaction of water species was calculated. The temperature dependence of the speciation equilibrium in the melt state can be expressed by ln K=5.39–3427/T for sodium tetrasilicate and ln K=5.80–3731/T for sodium hexasilicate. At magmatic temperatures the equilibrium constant for water speciation in sodium silicate melts is larger than in polymerized aluminosilicate melts by a factor of 5 or more, depending on melt composition. This large difference of ln K is most likely owing to the high degree of melt depolymerization and the high alkali concentration in sodium silicate melts.