ANISOTROPIC THERMAL-EXPANSION OF CALCITE AT HIGH-PRESSURES - AN IN-SITU X-RAY-DIFFRACTION STUDY IN A HYDROTHERMAL DIAMOND-ANVIL CELL

Lattice parameters of calcite were measured at simultaneous high press ures and temperatures up to 10 kbar and 500 degrees C. Samples of Soln hofen limestone and distilled, deionized water were loaded in a hydrot hermal diamond-anvil cell. In situ energy-dispersive X-ray diffraction was used to determine lattice parameters along live H2O isochores wit h densities of 1.040, 0.964, 0.760, 0.670, and 0.595 g/cm(3) from 30 t o 500 degrees C; these densities correspond to the ice-melting tempera ture of -7.7 degrees C and the liquid-vapor homogenization temperature s 92.4, 274.5, 318.9, and 344.6 degrees C, respectively. In addition, data along the P axis were collected by hydrostatic compression at roo m temperature and along the T axis by heating at atmospheric pressure. Our results show that anisotropic thermal expansion of calcite contin ues up to 10 kbar, therefore making it a good double internal X-ray st andard. Both a and c lattice parameters are fitted to second-order pol ynomials of pressure, temperature, and a cross term.