Thermodynamic properties of H₄SiO₄ in the ideal gas state as evaluated from experimental data

• Published in: Geochimica et cosmochimica acta 2011-07, Vol.75 (13), p.3853-3865
• Main Author: Plyasunov, Andrey V
• Format: Article
• Language: English
• Subjects: alloys; ceramics; condensation; corrosion; hydroxides; silica; silicates; solubility; thermodynamics; transpiration; water vapor
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2011.04.016

Solid phases of silicon dioxide react with water vapor with the formation of hydroxides and oxyhydroxides of silica. Recent transpiration and mass-spectrometric studies convincingly demonstrate that H₄SiO₄ is the predominant form of silica in vapor phase at water pressure in excess of 10⁻²MPa. Available literature transpiration and solubility data for the reactions of solid SiO₂ phases and low-density water, extending from 424 to 1661K, are employed for the determination of ΔfG⁰, ΔfH⁰ and S⁰ of H₄SiO₄ in the ideal gas state at 298.15K, 0.1MPa. In total, there are 102 data points from seven literature sources. The resulting values of the thermodynamic functions of H₄SiO₄(g) are: ΔfG⁰=−1238.51±3.0kJmol⁻¹, ΔfH⁰=−1340.68±3.5kJmol⁻¹ and S⁰=347.78±6.2JK⁻¹mol⁻¹. These values agree quantitatively with one set of ab initio calculations. The relatively large uncertainties are mainly due to conflicting Cₚ ⁰ data for H₄SiO₄(g) from various sources, and new determinations of Cₚ ⁰ would be helpful. The thermodynamic properties of this species, H₄SiO₄(g), are necessary for realistic modeling of silica transport in a low-density water phase. Applications of this analysis may include the processes of silicates condensation in the primordial solar nebula, the precipitation of silica in steam-rich geothermal systems and the corrosion of SiO₂-containing alloys and ceramics in moist environments.