The density and compressibility of KAlSi3O8 liquid to 6.5 GPa

Abstract

The thermodynamic properties of crystalline and liquid KAlSi 3 O 8 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 °C (ΔH Tf = 63.0 kJ/mol, ΔS Tf = 42.8 J/mol-K) are recommended on the basis of improved heat-capacity equations for KAlSi 3 O 8 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/β 0 ) is constrained to be 12.2 ± 1.0 in a thirdorder Birch-Murnaghan equation of state (EOS). The metastable, one-bar melting temperature (T f ) is additionally constrained to be 1203 ± 26 °C. Determination of the liquid K0’ allows the density and compressibility of KAlSi 3 O 8 liquid to be calculated to 6.5 GPa (2.709 ± 0.014 g/cm 3 at 1600 °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 KAlSi 3 O 8 liquid at 1600 °C (K 0 = 15.8 GPa) drops rapidly with increasing pressure. The dominant mechanism of compression for KAlSi 3 O 8 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 ≤6.5 GPa).