In situ X-ray diffraction study of enstatite up to 12 GPa and 1473 K and equations of state
In situ X-ray diffraction study of enstatite up to 12 GPa and 1473 K and equations of state
American Mineralogist (October 1999) 84 (10): 1588-1594
Crystal structures and phase transitions of enstatite (MgSiO (sub 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 (sub 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 (sub 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 (sub 0) = 415.4 (5) Aa (super 3) , isothermal bulk modulus of K (sub 0) = 108.5 (6.4) GPa, and its pressure derivative of K' (sub 0) = 4.5 (1.3) for the P2 (sub 1) /c phase, and V (sub 0) = 405.1 (1.7) Aa (super 3) , K (sub 0) = 106.4 (17.4) GPa, and K' (sub 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 (sub T) [V (sub T) /V) (super 7/3) -(V (sub T) /V) (super 5/3) ] {1-3/4 (4-K' (sub T) )[(V (sub T) /V) (super 2/3) -1]}, where K (sub T) = K (sub 0) +(delta K (sub T) /delta T) (sub P) (T-300), K' (sub T) = K' (sub 0) , V (sub T) = V (sub 0) [exp( (super T) Sigma (sub 300) alpha (T)dT)], where thermal expansivity alpha (T) is a (sub 0) + a (sub 1) T. The parameters are V (sub 0) = 405.0 (2.6) Aa (super 3) , K (sub 0) = 106.9 (25.9) GPa, K' (sub 0) = 5.3 (3.9), a (sub 0) = 2.01 (44)X10 (super -5) K (super -1) , a (sub 1) = 2.10 (1.1)X10 (super -8) K (super -2) , and (delta K (sub T) /delta (sub T) ) (sub P) = -0.021 (10) GPa/K. Although the K (sub 0) values are nearly the same with those of previous studies for both the P2 (sub 1) /c and C2/c phases, the K' (sub 0) values are slightly smaller.
