Mixing Properties of NaAlSI3O8Melt-H2O: New Calorimetric Data and Some Geological Implications

Abstract

We present the results of direct calorimetric determinations of the heat of mixing of $$NaAlSi_{3}O_{8}$$ and $$H_{2}O$$ to form hydrous melts and glasses. $$\Delta H_{m}$$ is small and exothermic throughout the P-T range investigated. At 298 K and 1 bar there is a minimum in $$\Delta H_{m} (-2.39 kcal/mole at X^{m}_{W} \approx 0.27$$). Results of high-pressure drop calorimetry on hydrous albite melts show that $$\Delta H_{m}$$ becomes more endothermic as T increases. The minimum in $$\Delta H_{m}$$ as a function of $$X^{m}_{W}$$ occurs in the compositional region where $$OH^{-}$$ is the dominant hydrogen-bearing species in the glass. The average partial molal heat capacity of $$H_{2}O$$ in hydrous albite melt (from 298 to 1306 K, 20 cal/mole/K) is similar to that of pure liquid water at ambient conditions. Reconnaissance studies in the system $$4Q-Ab-Or-H_{2}O$$ indicate a general similarity with the $$Ab-H_{2}O$$ system in that $$\Delta H_{m}$$ is small and exothermic. In detail, however, $$Ab-H_{2}O$$ ...