Na 2O solubility in CaO–MgO–SiO 2 melts

Abstract

The sodium solubility in silicate melts in the CaO–MgO–SiO 2 (CMS) system at 1400 °C has been measured by using a closed thermochemical reactor designed to control alkali metal activity. In this reactor, Na (g) evaporation from a Na 2O– xSiO 2 melt imposes an alkali metal vapor pressure in equilibrium with the molten silicate samples. Because of equilibrium conditions in the reactor, the activity of sodium-metal oxide in the molten samples is the same as that of the source, i.e., aNa 2O (sample) = aNa 2O (source). This design also allows to determine the sodium oxide activity coefficient in the samples. Thirty-three different CMS compositions were studied. The results show that the amount of sodium entering from the gas phase (i.e., Na 2O solubility) is strongly sensitive to silica content of the melt and, to a lesser extent, the relative amounts of CaO and MgO. Despite the large range of tested melt compositions (0 < CaO and MgO < 40; 40 < SiO 2 < 100; in wt%), we found that Na 2O solubility is conveniently modeled as a linear function of the optical basicity ( Λ) calculated on a Na-free basis melt composition. In our experiments, γNa 2O (sample) ranges from 7 × 10 −7 to 5 × 10 −6, indicating a strongly non-ideal behavior of Na 2O solubility in the studied CMS melts (γNa 2O (sample) ≪ 1). In addition to showing the effect of sodium on phase relationships in the CMS system, this Na 2O solubility study brings valuable new constraints on how melt structure controls the solubility of Na in the CMS silicate melts. Our results suggest that Na 2O addition causes depolymerization of the melt by preferential breaking of Si–O–Si bonds of the most polymerized tetrahedral sites, mainly Q 4.