Phase Equilibria in the Granite-H_2O-HF System and Effect of Fluorine on Granitic Melt Structure

Experiments carried out in the system granites-H2O-HF at 0. 1 GPa show that the crystal-liquid equllibrium temperature of quartz rises and that of alkali-feldspar goes down with increasing F content. The calculated results of quartz and alkali feldspar crystal-liquid equilibri-um show that the activity of SiO2 in melt increases and the activities of NaAlSi3O8 (Ab) and KAlSi3O8(Or) decrease, with a greater decreasing extent for than . These systematic changes are believed to be caused by F complexing with Al, Na, K, but not Si in the melt,and are consistent with F decomposing AlO2 tetrahedra and more preferentially forming com-plexes with Na than K. The comarison between effects of F and H2O on phase equilibrium suggests that the maximum difference affecting melt structure between F and OH is F complex-ing without Si and OH complexing with Si in granitic melt.