1,721,057 research outputs found
The incorporation of water into lower-mantle perovskites: A first-principles study
No full textAbstractWe have used first principles methods to calculate the partitioning of water between perovskite and ringwoodite under lower mantle and Fe-free conditions. We find that incorporation of water into ringwoodite is more favourable than into perovskite by about 0.25eV per formula unit, or about 24kJ/mol. This translates to a ringwoodite to perovskite partition coefficient of between 10 and 13, depending on temperature. These values are in good agreement with the partitioning experiments of Inoue et al. (2010) on Fe-bearing samples, where they find a partition coefficient of about 15. We also find that water incorporates into perovskite more readily than into periclase (also under Fe-free conditions), and we predict a perovskite to periclase partition coefficient of 90 at 24GPa and 1500K. We conclude, therefore, that the lower-mantle is able to contain substantial amounts of water, perhaps as much as 1000ppm
Structural, vibrational and thermodynamic properties of Mg2SiO4 and MgSiO3 minerals from first-principles simulations
No full textStructure and elasticity of hydrous ringwoodite: A first principle investigation
No full textFirst principle calculations were performed to investigate structural, IR, and elastic properties of hydrous ringwoodite and their evolution with pressure up to 36 GPa. Hydrogen defects are introduced by creating Mg- or Si-vacancies Mg(1.875)H(0.25)SiO(4), Mg(1.75)H(0.5)SiO(4) and Mg(2)Si(0.875)H(0.5)O(4). Energy considerations imply that the Mg-vacancy coupled substitution will be the easiest to form, but, in the Earth, both vacancies will participate in the process. Calculated IR spectra, when compared with reported observations, suggest that both types of defects are abundant in synthetic samples. We find that (d ln V(S)/d ln V(P)) for lateral variations in the H content of ringwoodite will be quite small, suggesting that this quantity will be a sensitive metric for identifying the presence of dissolved water in the transition zone. The calculated bulk modulus decreases linearly with increasing water content with dK/d(C(H2O)) -7.1(GPa/wt%) at room pressure, decreasing to - 6.0(GPa/wt%) at 20 GPa. The shear modulus similarly demonstrates a decrease with increased water content given, averaged over the substitution models, by dG/d(C(H2O)) = -3.0(GPa/wt%) at room pressure, decreasing to - 1.8 (GPa/wt%) at 20GPa. Over this pressured range, the water induce variation of d ln(V(S))/d ln(V(P)) is 0.62 at 0 GPa to 0.2 at 20 GPa. Published by Elsevier B.V
The structure of iron under the conditions of the Earth's inner core
No full textThe inferred density of the solid inner core indicates that it is predominantly made of iron. In order to indicates that it is predominantly made of iron. In order to interpret the observed seismic anisotropy and understand the high pressure and temperature behaviour of the core, it is essential to establish the crystal structure of iron under core conditions. On the basis of extrapolated experimental data, a number of candidate structures for the high PIT iron phase have been proposed, namely, body-centred cubic (bcc), body-centred tetragonal (bct), hexagonal close-packed (hcp), double-hexagonal close-packed (dhcp) and an orthorhombically distorted hcp polymorph (Matsui, 1993; Stixrude and Cohen, 1995; Boehler, 1993; Saxena et al., 1996; Andrault et al., 1997). Here we present the results of the first fully ab initio free energy calculations for all of these polymorphs of iron at core pressures and temperatures. Our results show that hcp-Fe is the most stable polymorph of iron under the conditions of the Earth's inner core
Ab initio molecular dynamics study of elasticity of akimotoite MgSiO3 at mantle conditions
No full textThe thermo-elastic properties of MgSiO3 akimotoite at mantle pressure and temperature conditions are reported based on ab initio molecular dynamic simulations. A third-order Birch-Murnaghan equation at a reference temperature of 2000 K is defined by K-0 = 158 GPa, K-0'= 3.7, G(0) = 85.7 GPa, G(0)' = 4.5, V-0(2000K)=1100.54 angstrom(3), the Gruneisen parameter is determined to be gamma(V)=gamma(0)(V/V-0(2000K))(q) with gamma(0) = 1.84 and q = 1.84, with V(2000 K) = 1048.22 angstrom(3). An implied pressure correction is -7.6GPa in these parameters due to GGA overestimates the pressure. The thermal expansion is determined to be alpha/alpha(0) = (V/V-0(2000 K))(delta T) in which alpha(0) = 3.21 x 10(-5) K-1 and delta(T) = 4.6. Akimotoite may be stable above the 660 discontinuity in relatively low temperature or low aluminium environments. The high velocity and elastic anisotropy of akimotoite provide diagnostics for its presence above the 660 km discontinuity. Published by Elsevier B.V
Ab initio molecular dynamic simulation on the elasticity of Mg3Al2Si3O12 pyrope
No full textWe calculated thermo-elastic properties of pyrope (Mg3Al2Si3O12) at mantle pressures and temperatures using Ab initio molecular dynamic simulation. A third-order Birch-Murnaghan equation at a reference temperature of 2 000 K fits the calculations with bulk modulus, K (0)=159.5 GPa, K (0)'=4.3, V (0)=785.89 (3), Gruneisen parameter, gamma (0)=1.15, q=0.80, Anderson Gruneisen parameter delta (T) =3.76 and thermal expansion, alpha (0)=2.93x10(-5) K-1. Referenced to room temperature, where V (0)=750.80 (3), gamma (0) and alpha (0) become 1.11 and 2.47x10(-5) K-1. The elastic properties of pyrope are found to be nearly isotropic at transition zone conditions
First-principles modelling of Earth and planetary materials at high pressures and temperatures
No full textAtomic-scale materials modelling based on first-principles quantum mechanics is playing an important role in the science of the Earth and the other planets. We outline the basic theory of this kind of modelling and explain how it can be applied in a variety of different ways to probe the thermodynamics, structure and transport properties of both solids and liquids under extreme conditions. After a summary of the density functional formulation of quantum mechanics and its practical implementation through pseudopotentials, we outline the simplestway of applying first-principles modelling, namely static zero-temperature calculations. We show how calculations of this kind can be compared with static compression experiments to demonstrate the accuracy of first-principles modelling at pressures reached in planetary interiors. Noting that virtually all problems concerning planetary interiors require an understanding of materials at high temperatures as well as high pressures, we then describe how first-principles lattice dynamics gives a powerful way of investigating solids at temperatures not too close to the melting line. We show how such calculations have contributed to important progress, including the recent discovery of the post-perovskite phase of MgSiO3 in the D '' layer at the base of the Earth's mantle. A range of applications of first-principles molecular dynamics are then reviewed, including the properties of metallic hydrogen in Jupiter and Saturn, of water, ammonia and methane in Uranus and Neptune, and of oxides and silicates and solid and liquid iron and its alloys in the Earth's deep interior. Recognizing the importance of phase equilibria throughout the planetary sciences, we review recently developed techniques for the first- principles calculation of solid and liquid free energies, melting curves and chemical potentials of alloys. We show how such calculations have contributed to an improved understanding of the temperature distribution and the chemical composition throughout the Earth's interior. The review concludes with a summary of the present state of the field and with some ideas for future developments
The ab initio simulation of the Earth's core
No full textThe Earth has a liquid outer and solid inner core. It is predominantly composed of Fe, alloyed with small amounts of light elements, such as S, 0 and Si. The detailed chemical and thermal structure of the core is poorly constrained, and it is difficult to perform experiments to establish the properties of core-forming phases at the pressures (ca. 300 GPa) and temperatures (ca. 5000-6000 K) to be found in the core. Here we present some major advances that have been made in using quantum mechanical methods to simulate the high-P/T properties of Fe alloys, which have been made possible by recent developments in high-performance computing. Specifically, we outline how we have calculated the Gibbs free energies of the crystalline and liquid forms of Fe alloys, and so conclude that the inner core of the Earth is composed of hexagonal close packed Fe containing ca. 8.5% S (or Si) and 0.2% O in equilibrium at 5600 K at the boundary between the inner and outer cores with a liquid Fe containing ca. 10% S (or Si) and 8% O
Ab initio free energy calculations on the polymorphs of iron at core conditions
No full textIn order to predict the stable polymorph of iron under core conditions, calculations have been performed on all the candidate phases proposed for inner core conditions, namely, body-centred cubic (bcc), body-centred tetragonal (bct), hexagonal close-packed (hcp), double-hexagonal close-packed (dhcp) and an orthorhombically distorted hcp polymorph. Our simulations are ab initio free energy electronic structure calculations, based upon density functional theory, within the generalised gradient approximation; we use Vanderbilt ultrasoft non-normconserving pseudopotentials to describe the core interactions, and the frozen phonon technique to obtain the vibrational characteristics of the candidate structures. Our results show that under conditions of hydrostatic stress, the orthorhombic, bce and bet structures are mechanically unstable. The relative free energies of the remaining phases indicate that dhcp and fee Fe are thermodynamically less stable than hcp Fe, therefore, we predict that the stable phase of iron at core conditions is hcp-Fe. (C) 2000 Elsevier Science B.V. All rights reserved
Elasticity of Mg2SiO4 ringwoodite at mantle conditions
No full textThe themoelastic properties of Mg2SiO4 ringwoodite at mantle pressure and temperature conditions are reported based on ab initio molecular dynamic simulations. A third-order Birch-Murnaghan equation at a reference temperature of 2000 K is defined by K-0 = 138 GPa, K-0' = 5.2, and V-0(2000 K) = 560 angstrom(3). The Gruneisen parameter is determined to be gamma(V) = gamma(0)(V/V-0(298 K))(q) with gamma(0) = 1.22 and q = 1.44(5), with V-0(298 K) = 524.56 angstrom(3). The thermal expansion is determined to be (alpha/alpha(0)) = (V/V-0(298 K))(delta T) in which alpha(0) = 2.74 x 10(-5) K-1 and delta(T) = 5.2(1). The bulk modulus is temperature independent at constant volume, while the shear moduli vary with temperature at constant volume. Elastic anisotropy decreases with both pressure and temperature becoming isotropic by the bottom of the upper mantle. (c) 2006 Elsevier B.V. All rights reserved
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