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 NaO0.5 in a series of CMAS-NaO0.5 melts relative to that in the anorthite-diopside eutectic composition at 1400 °C, using a reservoir of NaO0.5-SiO2 melt. The results show that this relative activity coefficient decreases strongly with SiO2, increases with CaO and MgO, but is insensitive to AlO0.5. This latter behavior is inconsistent with “quasi-crystalline” models of melt thermodynamics that hypothesize Na-Al species.

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