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 (MgSiO 3 ) 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 temperatures to 1473 K. Low clinoenstatite with space group P2 1 /c transforms to high-pressure C2/c clinoenstatite at high pressures and high temperatures, accompanied by a volume reduction of about 2.5%. The ß angle of this high-pressure C2/c phase ranges from 101.4° to 101.7°, shows almost no variation with pressure and temperature, and is about 8° smaller than that of the high-temperature C2/c phase previously reported. This confirms 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-Mumahan equations of state (EOS) using the parameters: volume of V 0 = 415.4 (5) Å 3 , isothermal bulk modulus of K 0 = 108.5 (6.4)GPa, and its pressure derivative of Kʹ 0 =4.5 (1.3) for the P2 1 /c phase, and F 0 = 405.1 (1.7)Å 3 ,K 0 = 106.4 (17.4) GPa, and Kʹ 0 =5.4(2.7)forthe 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-Kʹ T [(V T /V) 2/3 -1]}, where K t = K 0 + (მK T /მT) P (T-300), Kʹ T =Kʹ 0 , V t = V 0 [exp(ʃ T 300 α(T)dT)], where thermal expansivity α(T) is a 0 + a 1 T. The parameters are V 0 = 405.0 (2.6) Å 3 , K 0 = 106.9 (25.9) GPa, Kʹ 0 = 5.3 (3.9), a 0 = 2.01 (44) × 10 -5 K -1 , a 1 = 2.10 (1.1) × 10 -8 K -2 , and (მK T /მT) P = -0.021 (10) GPa/K. Although the K 0 values are nearly the same with those of previous studies for both the P2 1 /c and C2/c phases, the Kʹ 0 values are slightly smaller.