A method for controlling alkali-metal oxide activities in one-atmosphere experiments and its application to measuring the relative activity coefficients of NaO_0.5 in silicate melts

The activity of alkali metal oxides can be controlled in one-atmosphere wire-loop experiments at high temperature by suspending a crucible containing alkali silicate melt beneath the samples. The method has been applied to measuring the activity coefficient of NaO_0.5 in a series of CMAS-NaO_0.5 melts relative to that in the anorthite-diopside eutectic composition at 1400 °C, using a reservoir of NaO_0.5-SiO₂ melt. The results show that this relative activity coefficient decreases strongly with SiO ₂, increases with CaO and MgO, but is insensitive to AlO_0.5. This latter behavior is inconsistent with " quasi-crystalline" models of melt thermodynamics that hypothesize Na-Al species.