AbstractThe dissolved silica structures in quartz-saturated 0.50 and 1.50 m [mol kg H₂O^–1] Na₂CO₃ and 0.47 m NaOH 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 Na₂CO₃ and 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 Na₂CO₃ solutions. The integrated intensity ratios of the bridging oxygen band to the monomer band increased with the addition of Na₂CO₃ and 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.