The MgSiO3 orthoenstatite‐clinoenstatite (OEn‐CEn) phase boundary has been reversed between pressures of 70 and 110 kbar and temperatures of 900°C and 1700°C with a split‐sphere anvil apparatus (USSA‐2000). Starting materials contained PbO‐PbF2 (1∶1) flux to promote equilibration of the charges and eliminate a potential effect of deviatoric stresses on the phase boundary. The phase boundary separating orthoenstatite from clinoenstatite at high pressures and temperatures can be described by the equation P (kbar) = 0.031 T (°C)+50. Our results are consistent with previous unreversed determinations of the boundary at these high‐pressure, high‐temperature conditions. However, the dP/dT slope determined in the present study is much smaller than that implied by earlier experimental studies of the orthoenstatite/low clinoenstatite boundary at low pressures and temperatures. We propose that the clinoenstatite observed at high pressures and temperatures is a new high‐pressure clinoenstatite phase which is possibly an analogue of MgGeO3 clinopyroxene.

Unit cell volumes of the beta ([•) and spinel (7) phases (Mg2SiO4) have been measured under simultaneous high pressure and high temperature using synchrotron X ray radiation, in a cubic anvil apparatus. With volume-temperature data at constant pressure, we determine the average volume thermal expansion coefficients of the [• phase from 724 to 872 K at 7.6 GPa to be 2.28(_0.45)x10-5/K and of the 7 phase from 759 to 962 K at 9.8 GPa to be 1.71(_0.14)x10-5/K. Thermodynamic relations are used to constrain the temperature derivative of the isothermal bulk modulus (Kr) from the high-pressure thermal expansion data:/)K•/i)T)• is found to be -2.7(_0.5)x10 -2 GPa/K for the [• phase and -2.8(_0.3)x10 -2 GPa/K for the 7 phase. Unit cell volumes of the olivine (o0 phase, back-transformed from the [• and 7 phases at high temperatures, have been measured under pressure at temperatures above the Debye temperature; using thermal pressure equations, we find (/)K•/i)T)• for the ot phase to be -2.1(_0.2)x10 -2 GPa/K. These new data on the temperature derivatives of the bulk modulus for the three phases are consistent with an upper mantle containing 60 to 65% olivine and the absence of a velocity signature for the [• to 7 phase transition near 520 km depth. [1967], Anderson et al. [1990, 1992a], and Chopelas and Boehler [1989,1992], there has been relatively little experimental work in this field [e.g., Bridgman, 1935, 1940; Yagi, 1978; Yagi et al., 1987; Boehler and Kennedy, 1980a, 1980b]. In the absence of experimental P-V-T data, (3Kvt3T)•, has been calculated mostly from the ambient pressure measurement of the adiabatic bulk modulus (Ks) as a function of temperature. For high-pressure phases of mantle minerals, such elasticity measurements at high temperature are very limited due to the small crystal size that causes experimental difficulties. X ray P-V-T experiments, therefore, are needed to constrain (3Kvt3T)•,. For [•-(Mgo. 84, Feo. 16)28iO4, (3K•3T)•, has been derived by Fei et al. [1992] from unit cell volume data under simultaneous high P-T conditions in a diamond anvil cell. Since in their experiments, the pressure decreased systematically as the temperature was increased, (3Kvt3T)•, was obtained by fitting all the P-V-T data to a thermal pressure model. Our experiments, using a cubic anvil apparatus (SAM-85) with synchrotron X ray radiation, enable us to measure the lattice parameters accurately under relatively constant pressure over certain temperature ranges and, consequently, to directly determine the thermal expansion at high pressure and high temperature; our technique is complementary to the diamond anvil cell, which can achieve higher pressure. The purpose of this paper is to present for the first time the results of high P-T thermal expansion measurements on the [5 and 7 phases of Mg2SiO4. As both phases transformed back to the olivine structure at high temperatures, we also measured the unit cell volumes of the back-transformed t• phase under high pressures and above the Debye temperature up to 1569 K. With the...

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The lattice distortion and structural phase transition of NaMgFa perovskite (Neighborite) have been studied using synchrotron X ray powder diffraction at high pressure and temperature. Changes in the unit cell dimensions of the perovskite are determined by conventional peak indexing and least squares routines. The stress field within the high‐pressure cell assembly is analyzed, and the yield strength of the NaMgF3 perovskite is determined at high P and T. The pressure‐ and temperature‐induced dimensional changes of the NaMgF3 perovskite structure are expressed empirically as a combination of compression/expansion of the [Mg‐F] bond length and tilting of the MgF6 octahedral framework. The linear thermal expansions of the NaMgF3 perovskite observed at different pressures show significant anisotropy with αa > αc > αb which reflects the decrease of structural distortion and the development of a phase transition in the perovskite with increasing temperature. The tilting angle of the MgF6 octahedral framework is observed to decrease rapidly toward zero, in a manner expected for a ferroelastic phase transition, as the temperature approaches the transition point Tc. The apparent [Mg‐F] bond lengths of the MgF6 octahedra experience drastic shrinkage with increasing temperature just prior to the transition. Despite a 12% change in volume due to compression, the experimental results on NaMgF3 perovskite show that the thermal expansivity is independent of pressure, i.e., dα/dP ≈ 0, and, compatibly, that the compressibility is independent of temperature, i.e., dβ/dT 0. However, the dominant compression mechanism is the compression of the octahedral bond length, whereas the dominant mechanism for thermal expansion is the diminishing of octahedral tilting. The Earth's mantle may be isochemical if the thermal expansion of MgSiO3 perovskite at high pressure behaves like NaMgF3, of which the Anderson‐Grüneisen parameter is near zero, i.e., δs ≈ 0. It is observed that crystal structure of the NaMgF3 perovskite transforms directly from the orthorhombic Pbnm phase to the cubic Pm3"m phase at all pressures. The transition temperature is observed to increase with increasing pressure with a positive slope of 45 K/GPa.