A HIGH P-T SINGLE-CRYSTAL X-RAY-DIFFRACTION STUDY OF THERMOELASTICITYOF MGSIO3 ORTHOENSTATITE

P-V-T data of MgSiO3 orthoenstatite have been measured by single-cryst al X-ray diffraction at simultaneous high pressures (in excess of 4.5 GPa) and temperatures (up to 1000 K). The new P-V-T data of the orthoe nstatite, together with previous compression data and thermal expansio n data, are described by a modified Birch-Murnaghan equation of state for diverse temperatures. The fitted thermoelastic parameters for MgSi O3 orthoenstatite are: thermal expansion alpha(nu) = a + bT with value s of a = 2.86(29) x 10(-5) K-1 and b = 0.72(16) x 10(-8) K-2; isotherm al bulk modulus K-TO = 102.8(2) GPa; pressure derivative of bulk modul us K' = partial derivative K/partial derivative P = 10.2(1.2); and tem perature derivative of bulk modulus K = partial derivative K/partial d erivative T = -0.037(5) GPa/K. The derived thermal Gruneisen parameter is gamma(th) = 1.05 for ambient conditions; Anderson-Gruneisen parame ter is delta(TO) = 11.6, and the pressure derivative of thermal expans ion is partial derivative alpha/partial derivative P = -3.5 x 10(-6) K -1 GPa(-1) From the P-V-T data and the thermoelastic equation of state , thermal expansions at two constant pressures of 1.5 GPa and 4.0 GPa are calculated. The resulting pressure dependence of thermal expansion is Delta alpha/partial derivative P = -3.2(1) x 10(-6) K-1 GPa(-1). T he significantly large values of K', K, delta(T) and partial derivativ e alpha/partial derivative P indicate that compression/expansion of Mg SiO3 orthoenstatite is very sensitive to changes of pressure and tempe rature.