Phase Equilibria in the Granite—H2O—HF System and Effect of Fluorine on Granitic Melt Structure

摘要

Experiments carried out in the system granite-H2O-HF at 0.1 GPa show that the crystal-liquid equilibrium 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 equilibrium 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 a AbL than aOrL.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 complexes with Na than K.The comparison between effects of F and H2O on phase equilibrium suggests that the maximum difference affecting melt structure between F and OH is F complexing without Si and OH complexing with Si in granitic melt.