High-pressure and high-temperature Raman spectroscopy of carbonate ions in aqueous solution

Two aqueous solutions of sodium carbonate (0.5 and 2 m) have been studied by in situ Raman spectroscopy in the 1–30 GPa, 25–400 °C pressure–temperature (P–T) range, in an externally heated diamond anvil cell. Neither bicarbonate ions nor CO2 species were observed; carbonate ions were found to be the major species in these solutions over the entire P–T range investigated, which proved to be an important constraint for further modelling of deep H2O–CO2 fluids. It was shown that sodium carbonate in ice VII is not present as segregated Na2CO3 but rather as stable or metastable solid solution or as a hydrate. In order to provide thermodynamical data on carbonate-bearing fluids and natural fluid inclusions, the pressure dependence of the frequency mode of the CO32− symmetric stretching mode was measured at different temperatures in carbonate-bearing liquid water and in ice VII. The pressure-induced frequency shifts of this mode are significantly greater in liquid water than in ice VII and solid carbonates. From these data and using appropriate equation of state (EOS) for volume calculations, the Grüneisen parameters and intrinsic anharmonicity parameters of carbonate ions in water and in ice VII were calculated at different temperatures. Grüneisen parameters of the CO32− symmetric stretching mode are significantly smaller in liquid water than in crystalline carbonates. Intrinsic anharmonicity parameters of this mode are similar in liquid water and crystalline carbonate and close to 0, suggesting that internal modes of CO32− groups behave quasi-harmonically in liquid water.