The density and compressibility of KAlSi3O8 liquid to 6.5 GPa

The thermodynamic properties of crystalline and liquid KAlSi3O8 are used to calculate the fusion curve of sanidine to 6.5 GPa. New values for the enthalpy and entropy of fusion of sanidine at one bar and 1200 degrees C (Delta H-Tf = 63.0 kJ/mol, Delta S-Tf = 42.8 J/mol-K) are recommended on the basis of improved heat-capacity equations for KAlSi3O8 crystal, glass, and liquid. On the basis of phase-equilibrium experiments on the congruent melting reaction between 2 and 6.5 GPa, the pressure dependence of the liquid compressibility (K-0' = dK(0)/dP, where K-0 = 1/beta(0)) is constrained to be 12.2 +/- 1.0 in a third-order Birch-Murnaghan equation of state (EOS). The metastable, one-bar melting temperature (T-f) is additionally constrained to be 1203 +/- 26 degrees C. Determination of the liquid K-0' allows the density and compressibility of KAlSi3O8 liquid to be calculated to 6.5 GPa (2.709 +/- 0.014 g/cm(3) at 1600 degrees C). The uncertainty in K-0' of +/- 1.0 leads to an error in melt density at 6.5 GPa of 0.52%. With a K-0' = 12.2, the relatively high compressibility of KAlSi3O8 liquid at 1600 degrees C (K-0 = 15.8 GPa) drops rapidly with increasing pressure. The dominant mechanism of compression for KAIS'A liquid between 0 and 6.5 GPa most likely involves topological changes and increases in network connectivity with pressure. It is probable that highly compressible liquids, such as hydrous, silica-rich liquids formed by partial melting of a subducted slab, may have K-0' values that exceed 12 (at pressures <= 56.5 GPa).