HR: 1340h
AN: MR13A-0064    [Abstracts]
TI: The NaCl Fusion Curve: New Experiments to 5 GPa and Constraints on the Liquid Equation of State
AU: * Lange, R A
EM: becky@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1005 United States
AU: Chen, J
EM: jiuhua.chen@stonybrook.edu
AF: Stony Brook University, Mineral Physics Institute Department of Geosciences, Stony Brook, NY 11794-2100 United States
AB: The fusion curve of NaCl to high pressure is of interest from several perspectives. First, if sufficiently well characterized, it can be used for pressure calibrations in a piston-cylinder apparatus over a wide range of pressure. Second, thermodynamic analysis of this fusion curve permits the only unknown thermodynamic property for NaCl to be derived, namely the pressure dependence of the liquid compressibility. All other thermodynamic data are known from the literature. This will enable the compression of this ionic liquid to be compared and contrasted to its crystalline equivalent as well as to covalent silicate liquids. Here, we use the 3rd-order Birch-Murnaghan equation of state for both crystalline and liquid NaCl. Prior to this study, the most reliable determinations of the melting reaction were performed by differential thermal analysis in an internally heated gas vessel between 0-1.8 GPa by Clarke (1959). In addition, Bohlen (1984) reports the fusion temperature in an internally heated gas vessel at 0.5 GPa, which is within experimental error of that given by Clarke (1959). At pressures < 0.5 GPa, the effect of varying the liquid K₀' on calculations of the fusion curve is negligible. Our thermodynamic calculation of the fusion curve is in excellent agreement (± 4 degrees) with the experimental determinations at < 0.5 GPa. The gas vessel experiments, with uncertainties in temperature of ± 5 °C to 1.8 GPa are consistent with a liquid K₀' of ~14.5 ± 1. Experiments at higher pressure provide tighter constraints on the K₀' value for liquid NaCl. Accordingly, an experiment was performed at 5 GPa using in-situ x-ray diffraction and radiography in a multi-anvil device at the National Synchrotron Light Source (NSLS). Two samples were held in the high-pressure cell; one was pure NaCl (99.998% purity) and the other was NaCl+BN (1:2 in volume) for pressure calculation based on x-ray diffraction of NaCl (Decker, 1971). A W3%Re-W25%Re thermocouple was used to measure temperature. A WC sphere was placed at the top of the pure crystalline NaCl sample. The samples were compressed to 6 GPa and then heated. The pressure decreased to 5 GPa as the samples approached 1400 °C, at which point the x-ray diffraction peaks disappeared. However, the x-ray radiograph showed that the sphere had not yet fallen. The temperature was increased until the sphere finally dropped at 1457 ± 10°C. The pressure at this temperature is estimated to be 4.95 ± 0.20 GPa. This experiment places a tight constraint on the K₀' value for liquid NaCl of 13.9 ± 0.3. This, in turn, allows the density of NaCl liquid to be calculated at 5 GPa and 1450 °C (1.871 ± .007 g/cm³), with an uncertainty of ± 0.4 %. Additional experiments are planned at 6, 7 and 8 GPa to further tighten the constraints on the liquid K₀'.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3630 Experimental mineralogy and petrology
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005