SIMULTANEOUS HIGH-P, HIGH-T X-RAY-DIFFRACTION STUDY OF BETA-(MG,FE)2SIO4 TO 26-GPA AND 900-K

The lattice parameters of beta phase [(Mg0.84Fe0.16)2SiO4] have been determined by X ray diffraction using synchrotron radiation under simultaneous high-pressure and high-temperature conditions. The experiments were conducted up to a pressure of 26 GPa and a temperature of 900 K. High pressures were generated in a Mao-Bell type diamond anvil cell using neon gas as pressure medium. The sample was heated with an external Ni80Cr20 wire heater. Gold was used as an internal high-pressure calibrant at high temperatures. Using 4.0 for (partial derivative K(T)/partial derivative P)T, we found that the 300-K isothermal bulk modulus, K(T0), and its temperature derivative at constant pressure, (partial derivative K(T)/partial derivative T)P, are 174(+/- 3) GPa and -2.7(+/- 0.3) x 10(-2) GPa/K, respectively. The value of the Anderson-Gruneisen parameter delta(T) of the beta phase is 5.1(+/- 0.8) above the Debye temperature. The experimental results provide direct measurements of the pressure effect on the thermal expansivity and the temperature dependence of bulk modulus for the beta phase.