Pressure dependence of the solubility of Ar and Kr in melts of the system SiO2-NaAlSi2O6

Abstract

The solubilities of Ar and Kr in supercooled melts in the system SiO2-NaAlSi2O6 have been studied at temperatures of 1200 degrees C (SiO2), 1000 and 750 degrees C (NaAlSi3O8), and 800 degrees C (NaAlSi2O6) at pressures from 200 to 6000 bar. Gas sorption experiments were performed in an internally heated pressure vessel, and noble gas concentrations in quenched samples were analyzed by absolute analytical techniques [gas chromatography (GC), and thermogravimetry (TG), gravimetry]. Comparison of the results from this study with other absolute techniques (Rutherford-backscattering, mass spectrometry) is possible for silica and exhibit excellent agreement. This agreement leads us to conclude that our silica glasses with an Ar concentration of 1.02 wt% and a Kr concentration of 0.54 wt% are suitable as standard materials. Solubility increases linearly with increasing pressure for all three compositions. In SiO2 the solubility of Ar and Kr increases with pressure with nearly parallel slopes, whereas Ar and Kr exhibit diverging slopes in NaAlSi3O8 and NaAlSi2O6. The linear relationship between fugacity and dissolved gas below 2-3 kbar, indicates that Henry's law constants (in units of 10(6) bar) for Ar in SiO2, NaAlSi3O8, and NaAlSi2O6 are 0.94, 3.15, and 5.71, respectively, and for Kr are 1.38, 5.69, and 9.28, respectively. The experimental results can be modeled by two alternative thermodynamic approaches. First, assuming mixing of noble gases with the network of the melt, partial molar volumes (in cm(3)/mol) can be calculated in SiO2, NaAlSi3O8, and NaAlSi2O6 for Ar as 25.9 +/- 1.3, 21.1 +/-. 1.6, and 21.1 +/- 0.7, respectively and for Kr as 24.3 +/- 1.8, 27.6 +/- 0.8, and 24.7 +/- 0.9, respectively. Second, assuming the volume change upon solution to be zero, a Langmuir Isotherm can be applied yielding saturation levels M (in units of 10(20) sites/cm(3) melt) in SiO2, NaAlSi3O8, and NaAlSi2O6 for Ar as 3.82 +/- 0.6, 1.33 +/- 0.2, and 0.72 +/- 0.08, respectively and for Kr as 3.49 +/- 0.7, 0.56 +/- 0.05, and 0.42 +/- 0.06, respectively. Highest solubility is observed in all three melts for Ar. Solubility decreases with increasing substitution of Si4+ by Na+ + Al3+. Th, dependence of solubility on composition supports the structural model for melts along the join SiO2-NaAlSi2O6, based on a stuffed tridymite-like structure.