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The density and compressibility of KAlSi₃O₈ liquid to 6.5 GPa

Department of Geological Sciences, 1100 N. University Avenue, University of Michigan, Ann Arbor, Michigan 48109-1005, U.S.A.

Correspondence: ^* E-mail: becky{at}umich.edu

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 °C (H_Tf = 63.0 kJ/mol, 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 (K₀ ’ = dK₀/dP, where K₀ = 1/ß₀) 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 °C. Determination of the liquid K₀ ’ allows the density and compressibility of KAlSi₃O₈ liquid to be calculated to 6.5 GPa (2.709 ± 0.014 g/cm³ at 1600 °C). The uncertainty in K₀ ’ of ±1.0 leads to an error in melt density at 6.5 GPa of ±0.52%. With a K₀ ’ = 12.2, the relatively high compressibility of KAlSi₃O₈ liquid at 1600 °C (K₀ = 15.8 GPa) drops rapidly with increasing pressure. The dominant mechanism of compression for KAlSi₃O₈ 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 K₀ ’ values that exceed 12 (at pressures 6.5 GPa).

Key Words: Sanidine • fusion curve • enthalpy of fusion • topology • equation of state

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