EQUATIONS OF STATE AND THERMODYNAMIC PROPERTIES OF ENSTATITE PYROXENES

The equations of state of end-member MgSiO3, low-clinoenstatite and or thoenstatite have been measured by single-crystal X ray diffraction to pressures in excess of 8.5 GPa. The volume variation with pressure of low-clinoenstatite (space group P2(1)/c) is described by a third-orde r Birch-Murnaghan equation of state with the parameters V-0 = 31.292(8 ) cm(3) mol(-1), K-T0= 111.1(3.3) GPa, and K-T0' 6.6(1.1). At pressure s in excess of 7 GPa the low-clinoenstatite inverts to a high-clinoens tatite phase (space group C2/c), with a volume reduction of 0.798(14) cm(3)mol(-1). This transformation is reversible but with significant h ysteresis at room temperature; the low-clinoenstatite is recovered on decompression to below 5.3 GPa. The volume variation of orthoenstatite (space group Pbca) displays a discontinuity at similar to 4 GPa which corresponds to a change in the internal mechanism of compression. The equations of state fitted to data below and above 4 GPa have the para meters: P < 4 GPa, V-0 = 31.354(8) cm(3) mol(-1), K-T0 = 95.8(3.0) GPa , and K-T0' = 14.9(2.0) and P > 4 GPa, V-0 = 31.23(10) cm(3) mol(-1), K-T0 = 122.8(16.5) GPa, and K-T0' = 5.6(2.9). From these data, and the slopes of the equilibrium phase boundaries, estimates of Delta S, Del ta H, and Delta V of the transformations between the three enstatite p olymorphs are calculated.