“…3). This pressure is higher and lower than previous experimental ($40 GPa) (Kubo and Akaogi, 2000) and theoretical ($80 GPa) (Panero et al, 2006;Tsuchiya and Tsuchiya, 2008) results, respectively. The experimental phase boundary was based on high-pressure X-ray diffraction measurements by Ito et al (1998), where peaks associated with Cor in a diffraction pattern do not clearly vanish even at 40 GPa.…”
Section: Resultscontrasting
confidence: 61%
“…On the other hand, we fixed chemical compositions of the solid solutions in the present estimation, though they change with pressure. Present modeling should therefore underestimate the pressure where pyropic Pv stabilizes, compared to more careful evaluations of solid solution thermodynamics (Panero et al, 2006;Tsuchiya and Tsuchiya, 2008).…”
“…3). This pressure is higher and lower than previous experimental ($40 GPa) (Kubo and Akaogi, 2000) and theoretical ($80 GPa) (Panero et al, 2006;Tsuchiya and Tsuchiya, 2008) results, respectively. The experimental phase boundary was based on high-pressure X-ray diffraction measurements by Ito et al (1998), where peaks associated with Cor in a diffraction pattern do not clearly vanish even at 40 GPa.…”
Section: Resultscontrasting
confidence: 61%
“…On the other hand, we fixed chemical compositions of the solid solutions in the present estimation, though they change with pressure. Present modeling should therefore underestimate the pressure where pyropic Pv stabilizes, compared to more careful evaluations of solid solution thermodynamics (Panero et al, 2006;Tsuchiya and Tsuchiya, 2008).…”
“…Panero et al . 16 ) due to balancing defect charges over longer distances. Given the Cr 2 O 3 concentrations considered here, the difference in configurational entropy is less than 2 kJ/mol and therefore unlikely to be a significant contributor to further stabilization.Figure 2Crystal structure.…”
Section: Resultsmentioning
confidence: 99%
“…For the supercell considered, a single substitution represents 6.25 mol% Cr 2 O 3 in ringwoodite. While nearest-neighbor defects are considered here, non-locally balanced charges in the case of non-nearest neighbor Tschermak defects have an additional defect enthalpy of ~15% in ilmenite and bridgmanite 16 .…”
Tibetan ophiolites are shallow mantle material and crustal slabs that were subducted as deep as the mantle transition zone, a conclusion supported by the discovery of high-pressure phases like inverse ringwoodite in these sequences. Ringwoodite, Mg2SiO4, exhibits the normal spinel structure, with Mg in the octahedral A site and Si in the tetrahedral B site. Through A and B site-disorder, the inverse spinel has four-coordinated A cations and the six-coordinated site hosts a mixture of A and B cations. This process affects the density and impedance contrasts across the boundaries in the transition zone and seismic-wave velocities in this portion of the Earth. We report the first synthesis at high pressure (20 GPa) and high temperature (1600 °C) of a Cr-bearing ringwoodite with a completely inverse-spinel structure. Chemical, structural, and computational analysis confirm the stability of inverse ringwoodite and add further constraints to the subduction history of the Luobusa peridotite of the Tibetan ophiolites.
“…Such work would provide a better understanding of the variation of shear modulus within solid solutions, and more generally the effect of deviatoric stresses on the thermodynamics and elasticity of natural rocks (Hobbs and Ord 2016). (Thomson et al 1996;Brodholt 2000;Caracas and Cohen 2005;Panero et al 2006). Experimental data are shown as red closed symbols Ito et al 1998;Kubo et al 2000;Walter et al 2004).…”
Non-ideality in mineral solid solutions affects their elastic and thermodynamic properties, their thermobaric stability, and the equilibrium phase relations in multiphase assemblages. At a given composition and state of order, non-ideality in minerals is typically modelled via excesses in Gibbs free energy which are either constant or linear with respect to pressure and temperature. This approach has been extremely successful when modelling near-ideal solutions. However, when the lattice parameters of the solution endmembers differ significantly, extrapolations of thermodynamic properties to high pressures using these models may result in significant errors. In this paper, I investigate the effect of parameterising solution models in terms of the Helmholtz free energy, treating volume (or lattice parameters) rather than pressure as an independent variable. This approach has been previously applied to models of order-disorder, but the implications for the thermodynamics and elasticity of solid solutions have not been fully explored. Solid solution models based on the Helmholtz free energy are intuitive at a microscopic level, as they automatically include the energetic contribution from elastic deformation of the endmember lattices. A chemical contribution must also be included in such models, which arises from atomic exchange within the solution. Derivations are provided for the thermodynamic properties of n-endmember solutions. Examples of the use of the elastic model are presented for the alkali halides, pyroxene, garnet, and bridgmanite solid solutions. Elastic theory provides insights into the microscopic origins of non-ideality in a range of solutions, and can make accurate predictions of excess enthalpies, entropies, and volumes as a function of volume and temperature. In solutions where experimental data are sparse or contradictory, the Helmholtz free energy approach can be used to assess the magnitude of excess properties and their variation as a function of pressure and temperature. The formulation is expected to be useful for geochemical and geophysical studies of the Earth and other planetary bodies.
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