Solubility behavior of alkali aluminosilicate components in aqueous fluids and silicate melts at high pressure and temperature

Abstract

The solubility behavior of K 2O, Na 2O, Al 2O 3, and SiO 2 in silicate-saturated aqueous fluid and coexisting H 2O-saturated silicate melts in the systems K 2O-Al 2O 3-SiO 2-H 2O and Na 2O-Al 2O 3-SiO 2-H 2O has been examined in the 1- to 2-GPa pressure range at 1100°C. Glasses of Na- and K-tetrasilicate compositions with 0, 3, and 6 mol% Al 2O 3 were used as starting materials. In both systems, the oxides dissolve incongruently in aqueous fluid and silicate melt. When recalculated to an anhydrous basis, the aqueous fluids are enriched in alkalis and depleted in silica and alumina relative to their proportions in the starting materials. The extent of incongruency is more pronounced in the Na 2O-Al 2O 3-SiO 2-H 2O system than in the K 2O-Al 2O 3-SiO 2-H 2O system. The partition coefficients of the oxides, D oxide fluid/melt, are linear and positive functions of the oxide concentration in the fluid for each composition. There is a slight dependence of the partition coefficients on bulk composition. No effect of pressure could be discerned. For alkali metals, the fluid/melt partition coefficients range from 0.06 to 0.8. For Al 2O 3 this range is 0.01 to 0.2, and for SiO 2, it is 0.01 to 0.32. For all compositions, D K 2O fluid/melt∼D Na 2O fluid/melt>D SiO 2 fluid/melt>D Al 2O 3 fluid/melt for the same oxide concentration in the fluid. D K 2O fluid/melt, D Na 2O fluid/melt, and D SiO 2 fluid/ melt correlate negatively with the Al 2O 3 content of the systems. This correlation is consistent with a solubility model of alkalis that involve associated KOH°, NaOH°, silicate, and aluminate complexes.