Equations of state and thermodynamic properties of enstatite pyroxenes
Abstract
The equations of state of endmember MgSiO_{3} lowclinoenstatite and orthoenstatite have been measured by singlecrystal X ray diffraction to pressures in excess of 8.5 GPa. The volume variation with pressure of lowclinoenstatite (space group P2_{1}/c) is described by a thirdorder BirchMurnaghan 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 lowclinoenstatite inverts to a highclinoenstatite 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 lowclinoenstatite 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 ∆S, ∆H, and ∆V of the transformations between the three enstatite polymorphs are calculated.
 Publication:

Journal of Geophysical Research
 Pub Date:
 October 1994
 DOI:
 10.1029/94JB01750
 Bibcode:
 1994JGR....9919777A
 Keywords:

 Enstatite;
 Equations Of State;
 Pressure Reduction;
 Pyroxenes;
 Thermodynamic Properties;
 Transition Pressure;
 Volume;
 Earth Mantle;
 Pressure Effects;
 X Ray Diffraction;
 Geophysics;
 Mineral Physics: Equations of state;
 Mineral Physics: Elasticity and anelasticity;
 Mineral Physics: Highpressure behavior;
 Mineral Physics: X ray;
 neutron;
 and electron spectroscopy and diffraction