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 beta angle of this high-pressure 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 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-Murnahan equations of state (EOS) using the parameters: volume of V 0 = 415.4 (5) Aa 3 , isothermal bulk modulus of K 0 = 108.5 (6.4) GPa, and its pressure derivative of K' 0 = 4.5 (1.3) for the P2 1 /c phase, and V 0 = 405.1 (1.7) Aa 3 , K 0 = 106.4 (17.4) GPa, and K' 0 = 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) (super 7/3) -(V T /V) (super 5/3) ] {1-3/4 (4-K' T )[(V T /V) (super 2/3) -1]}, where K T = K 0 +(delta K T /delta T) P (T-300), K' T = K' 0 , V T = V 0 [exp(T Sigma 300 alpha (T)dT)], where thermal expansivity alpha (T) is a 0 + a 1 T. The parameters are V 0 = 405.0 (2.6) Aa 3 , K 0 = 106.9 (25.9) GPa, K' 0 = 5.3 (3.9), a 0 = 2.01 (44)X10 (super -5) K (super -1) , a 1 = 2.10 (1.1)X10 (super -8) K (super -2) , and (delta K T /delta T ) 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 K' 0 values are slightly smaller.

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