Equations of state and thermodynamic properties of enstatite pyroxenes

The equations of state of end‐member MgSiO 3 low‐clinoenstatite and orthoenstatite 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‐order 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 pressures in excess of 7 GPa the low‐clinoenstatite inverts to a high‐clinoenstatite phase (space group C2/c), with a volume reduction of 0.798(14) cm 3 mol −1 . This transformation is reversible but with significant hysteresis 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 ∼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 parameters: 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 Δ S , Δ H , and Δ V of the transformations between the three enstatite polymorphs are calculated.