In-situ Raman spectroscopic analysis of dissolved silica structures in Na2CO3 and NaOH solutions at high pressure and temperature
In-situ Raman spectroscopic analysis of dissolved silica structures in Na2CO3 and NaOH solutions at high pressure and temperature
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高压高温下 Na2CO3 和 NaOH 溶液中溶解二氧化硅结构的原位拉曼光谱分析
DOI：
10.1007/s00410-022-01892-y
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发表时间：
2022
影响因子：
3.5
通讯作者：
Nakamura Michihiko
中科院分区：
文献类型：
--
作者：
Takahashi Naoko;Tsujimori Tatsuki;Kamada Seiji;Nakamura Michihiko
利用Bassett型水热金刚石对顶砧装置，在750 °C和1.50 m [mol kg H2O-1] Na 2CO 3和0.47 mNaOH溶液中，用原位拉曼光谱研究了SiO2的溶解结构。石英在溶液中的溶解度通过现场观察颗粒的完全溶解来确定。在高温高压条件下，石英饱和Na_2CO_3和NaOH溶液的拉曼光谱显示了SiO_2单体的四面体对称伸缩带。较低的频率和较宽的频带比那些在纯水中表明中性和去质子化的单体的存在。此外，我们还新确认了Na_2CO_3溶液光谱中的强桥氧带和Q_1（硅酸盐中心有一个桥氧原子）的四面体对称伸缩带.随着向流体中添加Na 2CO 3和NaOH，桥氧谱带与单体谱带的积分强度比增加，对应于测量的石英溶解度的升高。这些观察结果表明，除了中性和去质子化单体之外，二氧化硅低聚物的形成解释了溶液中高溶解二氧化硅浓度。实验条件下的去质子化单体的存在表明，去质子化的低聚物存在于溶液中，因为后者的生产更显着地降低吉布斯自由能。在碱性硅酸盐流体中形成的阴离子二氧化硅物种和低聚物可以作为深俯冲带中某些金属离子或络合物的有效配体。
The dissolved silica structures in quartz-saturated 0.50 and 1.50m[mol kg H2O–1] Na2CO3and 0.47mNaOH solutions at up to 750 °C and 1.5 GPa were investigated by in-situ Raman spectroscopy using a Bassett-type hydrothermal diamond anvil cell. The solubility of quartz in the solutions was determined by in-situ observations of the complete dissolution of the grain. The Raman spectra of the quartz-saturated Na2CO3and NaOH solutions at high pressures and temperatures exhibited the tetrahedral symmetric stretching band of silica monomers. The lower frequency and broader width of the band than those in pure H2O indicated the presence of both neutral and deprotonated monomers. In addition, we newly confirmed the intense bridging oxygen band and the tetrahedral symmetric stretching band of Q1(silicate center having a single bridging oxygen atom) in the spectra of the Na2CO3solutions. The integrated intensity ratios of the bridging oxygen band to the monomer band increased with the addition of Na2CO3and NaOH to fluids, corresponding to an elevation of the measured quartz solubilities. These observations indicate that the formation of silica oligomers in addition to neutral and deprotonated monomers explains the high dissolved silica concentrations in the solutions. The presence of deprotonated monomers under the experimental conditions suggests that deprotonated oligomers exist in the solutions, because the production of the latter more significantly reduces the Gibbs free energy. The anionic silica species and oligomers formed in alkaline silicate fluids may act as effective ligands for certain metal ions or complexes in deep subduction zones.