In situ X-ray diffraction study of enstatite up to 12 GPa and 1473 K and equations of state

Crystal structures and phase transitions of enstatite (MgSiO3) were studied by in situ X-ray diffraction experiments using synchrotron radiation and a multi-anvil high-pressure apparatus at pressures to 12 Gpa and temperature s to 1473 K. Low clinoenstatite with space group P2(1)/c transforms to high -pressure C2/c clinoenstatite at high pressures and high temperatures, acco mpanied by a volume reduction of about 2.5%. The beta angle of this high-pr essure C2/c phase ranges from 101.4 degrees to 101.7 degrees, shows almost no variation with pressure and temperature, and is about 8 degrees smaller than that of the high-temperature C2/c phase previously reported. This conf irms the suggestion (Hugh-Jones et al. 1994) that these two clinoenstatite phases differ at high pressures and high temperatures. The pressure-volume- temperature data for P2(1)/c and high pressure C2/c clinoenstatite were fit to room-temperature third-order Birch-Murnahan equations of state (EOS) us ing the parameters: volume of V-o = 415.4 (5) Angstrom(3), isothermal bulk modulus of K-o = 108.5 (6.4) Gpa, and its pressure derivative of K-o' = 4.5 (1.3) for the P2(1)/c phase, and V-o = 405.1 (1.7) Angstrom(3), K-o = 106. 4 (17.4) Gpa, and K-o' = 5.4 (2.7) for the C2/ c phase. These values are at ambient conditions. For the C2/c phase, we determined the high-temperature EOS, expressed as P = 3/2 K-T [(V-T/V)(7/3)-(V-T/V)(5/3)]{1-3/4 (4-K-T')[( V-T/V)(2/3)-I]}, where K-T = K-0+(partial derivative K-T/partial derivative T)(P) (T-300), K-T(t) = K-o(t), V-T = V-o [exp(alpha(T)dT)], where thermal expansivity alpha(T) is a(o) + a(1) T. The parameters are V-o = 405.0 (2.6 ) Angstrom(3), K-o = 106.9 (25.9) GPa, K-o' = 5.3 (3.9), a(o) =-2.01 (44) x 10(-s) K-1, a(1) = 2.10 (1.1) x 10(-8) K-2, and (partial derivative K-T/pa rtial derivative T)(P) = -0.021 (10) GPa/K. Although the K-o values are nea rly the same with those of previous studies for both the P2(1)/c and C2/c p hases, the K-o(t) values are slightly smaller.