MgSiO3 Perovskite Research Articles

Theoretical determination of the Raman spectra of MgSiO3 perovskite and post‐perovskite at high pressure

We use the density functional perturbation theory to determine for the first time the pressure evolution of the Raman intensities for a mineral, the two high‐pressure structures of MgSiO3 perovskite and post‐perovskite. At high pressures, the Raman powder spectra reveals three main peaks for the perovskite structure and one main peak for the post‐perovskite structure. Due to the large differences in the spectra of the two phases Raman spectroscopy can be used as a good experimental indication of the phase transition.

Estimation of polyhedral compressibilities and structural evolution of GdFeO3-type perovskites at high pressures

A new approach based on the bond-valence matching relation is developed to predict the detailed structural evolution of GdFeO(3)-type perovskites at high pressure from knowledge of the room-pressure structure and the high-pressure unit-cell parameters alone. The evolution of perovskite structures estimated in this way is in good agreement with the structure refinements available from high-pressure single-crystal diffraction measurements of a number of perovskites. The method is then extended to predict the structure of MgSiO(3) perovskite at pressures for which no single-crystal structural data are available and the results are compared to ab initio quantum calculations of MgSiO(3) perovskite up to 120 GPa.

Carbon solubility in mantle minerals

The solubility of carbon in olivine, enstatite, diopside, pyrope, MgAl2O4 spinel, wadsleyite, ringwoodite, MgSiO3–ilmenite and MgSiO3–perovskite has been quantified. Carbon-saturated crystals were grown from carbonatite melts at 900–1400 °C and 1.5 to ∼ 26 GPa in piston cylinder or multi-anvil presses using carbon enriched to >99% in the 13C isotope. In upper mantle silicates, carbon solubility increases as a function of pressure to a maximum of ∼ 12 ppm by weight in olivine at 11 GPa. No clear dependence of carbon solubility on temperature, oxygen fugacity or iron content was observed. The observation that carbon solubility in olivine is insensitive to oxygen fugacity implies that the oxidation state of carbon in the carbonatite melt and in olivine is the same, i.e., carbon dissolves as C4+ in olivine. Carbon solubility in spinel MgAl2O4, transition zone minerals (wadsleyite and ringwoodite), MgSiO3–ilmenite and MgSiO3–perovskite are below the limit of detection of our SIMS-based analytical technique (i.e., below 30–200 ppb by weight). The differences in carbon solubilities between the various minerals studied appear to correlate with the polyhedral volume of the Si4+ site, consistent with a direct substitution of C4+ for Si4+. These results show that other, minor carbon-rich phases, rather than major, nominally volatile-free minerals, dominate the carbon budget within the bulk Earth's mantle. A significant fraction of total carbon could only be stored in silicates in a thin zone in the lowermost upper mantle, just above the transition zone, and only if the bulk carbon content is at the lower limit of published estimates. The carbon budget of the remaining mantle is dominated by carbonates and possibly diamond. The low melting point of carbonates and the high mobility of carbonate melts suggest that carbon distribution in the mantle may be highly heterogeneous, including the possibility of massive carbon enrichments on a local scale, particularly in the shallow subcontinental mantle.

Reinvestigation of the MgSiO3 perovskite structure at high pressure

High-pressure single-crystal X-ray diffraction experiments of MgSiO 3 perovskite have been carried out up to 15 GPa in a diamond-anvil cell using synchrotron radiation. Precise crystal structural parameters, including the anisotropic displacement parameters of every atom in MgSiO 3 , are determined under high pressure. In the pressure range up to 15 GPa, the most important responses of the structure are the compressions of SiO 6 and MgO 8 polyhedra and an increase in tilting of SiO 6 octahedra represented by the decrease in angles between octahedra (both Si-O2-Si angle in the a-b plane and Si-O1-Si angle in the b-c plane decrease). The degree of the change in both angles in the a-b and b-c planes is the same. The amplitude of mean square displacement for the Mg atom has the largest value in the structures and its thermal vibration is significantly anisotropic at ambient pressure. Under high pressure, all atoms in the structure have obvious anisotropy of thermal vibration and the largest amplitudes of thermal vibration for Mg, Si, and O2 atoms are directed toward vacant space in the structure. Anisotropy of the structure increases with pressure.

Equations of state of CaSiO3 Perovskite: a molecular dynamics study

The molar volumes and bulk moduli of CaSiO3 perovskite are calculated in the temperature range from 300 to 2,800 K and the pressure range from 0 to 143 GPa using molecular dynamics simulations that employ the breathing shell model for oxygen and the quantum correction in addition to the conventional pairwise interatomic potential models. The performance of five equations of state, i.e., the Keane, the generalized-Rydberg, the Holzapfel, the Stacey–Rydberg, and the third-order Birch–Murnaghan equations of state are examined using these data. The third-order Birch–Murnaghan equation of state is found to have a clear tendency to overestimate the bulk modulus at very high pressures. The Stacey–Rydberg equation of state degrades slightly at very high pressures along the low-temperature isotherms. In comparison, the Keane and the Holzapfel equations of state remain accurate in the whole temperature and pressure range considered in the present study. K 0′ derived from the Holzapfel equation of state also agrees best with that calculated independently from molecular dynamics simulations. The adiabatic bulk moduli of CaSiO3 perovskite along lower mantle geotherms are further calculated using the Keane and the Mie-Gruneisen–Debye equations of state. They are found to be constantly higher than those of the PREM by ~5%, and also very similar to those of the MgSiO3 perovskite. Our results support the view that CaSiO3 perovskite remains invisible in the Earth’s lower mantle.

Simulated Equations of State of MgSiO3 Perovskite

The equation of state of MgSiO3 perovskite under high pressure and high temperature is simulated using the molecular dynamics method. It was found that the molecular dynamics simulation is very successful in accurately reproducing the measured molar volumes of MgSiO3 perovskite over a wide range of temperatures and pressures. The simulated equation of state of MgSiO3 perovskite matched experimental data at up to 140 GPa at 300 K, as well as the fitting data of others and results from the first-principles simulation based on the local density approximation. The simulated equations of state of MgSiO3 perovskite at higher temperatures and higher pressures also correspond to the other calculations. In addition, the volume compression data of MgSiO3 perovskite is simulated up to 120 GPa at 300, 900, 2000 and 3000 K, respectively.

Crystal structure and equation of state of MgSiO3 perovskite

We have re‐examined the crystal structure and equation of state of MgSiO3 perovskite by single‐crystal X‐ray diffraction up to 10 GPa. We have determined an isothermal bulk modulus K0 = 253(1) GPa with K′0 = 4, which is in excellent agreement with the adiabatic bulk moduli reported in recent studies by Brillouin spectroscopy and ultrasonic interferometry. We confirm that the compressibility of perovskite is anisotropic with the b‐axis being the least compressible direction. On compression, the bond lengths and angles on Mg‐site and Si‐site decrease continuously, indicating that the structure gets more distorted as pressure increases. This is supported by a general model for perovskites that predicts that the octahedra become more tilted at lower mantle pressures.

Molecular dynamics of MgSiO 3 perovskite melting

The melting curve of MgSiO3 perovskite is simulated using molecular dynamics simulations method at high pressure. It is shown that the simulated equation of state of MgSiO3 perovskite is very successful in reproducing accurately the experimental data. The pressure dependence of the simulated melting temperature of MgSiO3 perovskite reproduces the stability of the orthorhombic perovskite phase up to high pressure of 130GPa at ambient temperature, consistent with the theoretical data of the other calculations. It is shown that its transformation to the cubic phase and melting at high pressure and high temperature are in agreement with recent experiments.

Efficacy of the post-perovskite phase as an explanation for lowermost-mantle seismic properties

Constraining the chemical, rheological and electromagnetic properties of the lowermost mantle (D'') is important to understand the formation and dynamics of the Earth's mantle and core. To explain the origin of the variety of characteristics of this layer observed with seismology, a number of theories have been proposed, including core-mantle interaction, the presence of remnants of subducted material and that D'' is the site of a mineral phase transformation. This final possibility has been rejuvenated by recent evidence for a phase change in MgSiO3 perovskite (thought to be the most prevalent phase in the lower mantle) at near core-mantle boundary temperature and pressure conditions. Here we explore the efficacy of this 'post-perovskite' phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post-perovskite polymorphs of MgSiO3, performed at lowermost-mantle temperatures and pressures. We show that a post-perovskite model can explain the topography and location of the D'' discontinuity, apparent differences in compressional- and shear-wave models and the observation of a deeper, weaker discontinuity. Furthermore, our calculations show that the regional variations in lower-mantle shear-wave anisotropy are consistent with the proposed phase change in MgSiO3 perovskite.

High‐pressure sound velocities and elasticity of aluminous MgSiO3 perovskite to 45 GPa: Implications for lateral heterogeneity in Earth's lower mantle

Brillouin scattering measurements on aluminous magnesium silicate perovskite, arguably the most abundant phase in Earth, have been performed to 45 GPa in a diamond anvil cell at room temperature, using methanol‐ethanol‐water and neon as pressure transmitting media. The experiments were performed on a polycrystalline sample of aluminous MgSiO3 perovskite containing 5.1 ± 0.2 wt.% Al2O3. The pressure derivatives of the adiabatic bulk (K0S) and shear (μ0S) moduli are 3.7 ± 0.3 and 1.7 ± 0.2, respectively. These measurements allow us to evaluate whether the observed lateral variations of seismic wave speeds in Earth's lower mantle are due at least in part to a chemical origin. Our results indicate that a difference in the aluminum content of silicate perovskite, reflecting a variation in overall chemistry, is a plausible candidate for such seismic heterogeneity.

Compression of oxygen vacancy type Al-bearing MgSiO3perovskite

MgSiO3 perovskite is believed to be a dominant constituent of the Earth’s lower mantle. Experimental results on the effect of Al on the compressibility of MgSiO3 perovskite have been pretty controversial. Two kinds of the Al substitution mechanisms are expected: 2Al = Mg + S and 2Al = 2Si + (as a vacancy site) O [1]. Theory predicts that the latter mechanism significantly increases the compressibility [2]. Recently, Kojitani et al (2005) demonstrated the structural differences between these two types of Al-bearing MgSiO3 perovskite on the basis of Rietveld analyses. In this work, the volume compression measurements were performed on the oxygen vacancy type Al-MgSiO3 perovskite by using synchrotron radiation x-rays. Data were collected under hydrostatic conditions using helium pressure transmitting medium. Preliminary results show that the isothermal bulk modulus is reduced due to the incorporation of Al2O3 in perovskite with oxygen vacancy.

Phase relations in Mg3Al2Si3O12 to 180 GPa: Effect of Al on post‐perovskite phase transition

Phase relations in Mg3Al2Si3O12 (MgSiO3 + 25 mol% Al2O3; pyrope garnet composition) were investigated on the basis of in‐situ synchrotron X‐ray diffraction measurements at high‐pressure and ‐temperature in a laser‐heated diamond anvil cell (LHDAC). Results demonstrate that perovskite is solely stable up to 140 GPa and 2200 K, and perovskite and CaIrO3‐type post‐perovskite phase (space group: Cmcm) coexist above 140 GPa and 2200 K. Post‐perovskite is formed as a single phase above 150–170 GPa. Previous study has shown that pure MgSiO3 perovskite transforms to post‐perovskite phase above 125 GPa and 2500 K based on the same pressure standard. These results indicate that addition of Al2O3 expands the stability of MgSiO3 perovskite relative to post‐perovskite.

Clapeyron slope of the post‐perovskite phase transition in CaIrO3

The Clapeyron slope of the perovskite to post‐perovskite phase transition has not been well determined experimentally. Crystal structure of CaIrO3 is equivalent to the MgSiO3 post‐perovskite phase (space group: Cmcm) at ambient condition. We have found that perovskite phase (Pbnm) of CaIrO3, which is isostructural with MgSiO3 perovskite, is stable at 1–3 GPa and 1400–1550°C. The perovskite to post‐perovskite phase transition in CaIrO3 was determined by using piston‐cylinder apparatus at 0.9–3.0 GPa and 1350–1550°C. Results show that phase transition pressure is strongly temperature‐sensitive with a Clapeyron slope of +17(±3) MPa/°C. These results, obtained for a suitable analogue material, suggest that post‐perovskite phase transition in MgSiO3 also has a large positive value of the Clapeyron slope. Such a pronounced exothermic reaction in the Earth's lowermost mantle will promote the upwelling of high‐temperature plumes from the core‐mantle boundary region.

Ab initio thermodynamics of MgSiO3 perovskite at high pressures and temperatures

Using quantum-mechanical simulations based on density-functional perturbation theory, we address the problem of stability of MgSiO3 perovskite to decomposition into MgO and SiO2 at pressures and temperatures of the Earth's lower mantle. We show that MgSiO3 perovskite (and its post-perovskite phase) is more stable than the mixture of oxides throughout the pressure-temperature regime of the Earth's mantle. Structural stability and lattice dynamics of phases in the system MgO-SiO2 are discussed.

Prediction of a new phase transition in Al2O3 at high pressures

We use density functional theory to investigate several high‐pressure polymorphs of Al2O3 and predict a new stable polymorph at high pressures with post‐perovskite structure. The ambient pressure Rc corundum phase transforms to the Pbcn Rh2O3(II) structure at about 104 GPa. At about 156 GPa, the Rh2O3(II) structure transforms further to the Cmcm post‐perovskite structure. The Al2O3 Pbnm perovskite structure is metastable at all pressures with respect to corundum, Rh2O3(II) and post‐perovskite. The metastable perovskite to post‐perovskite transition takes place at about 120 GPa, which is above the same transition in MgSiO3. Thus the presence of Al2O3 in the lower mantle, dissolved in MgSiO3 perovskite, will increase the transition pressure to post‐perovskite. Our calculations also show that Al2O3 is most likely dissolved in the post‐perovskite phase of MgSiO3, increasing its stability at high pressures.

Vibrational and thermodynamic properties of MgSiO3 postperovskite

Phonon dispersions and vibrational density of states of MgSiO3 postperovskite, the new high‐pressure phase of MgSiO3 perovskite, are calculated as a function of pressure up to 180 GPa using density‐functional perturbation theory. The calculated frequencies are then used to determine the thermal contribution to the Helmholtz free energy within the quasi‐harmonic approximation. The equation of state and several thermodynamic properties of interest are derived and compared with those of perovskite. The overall thermodynamic properties of postperovskite are almost the same as those of perovskite under its stability conditions.

Crystal morphology and surface structures of orthorhombic MgSiO3 perovskite

Orthorhombic MgSiO3 perovskite is thought to be the most abundant mineral in the mantle of the Earth. Its bulk properties have been widely studied, but many geophysical and rheological processes are also likely to depend upon its surface and grain boundary properties. As a first step towards modelling these geophysical properties, we present here an investigation of the structures and energetics of the surfaces of MgSiO3-perovskite, employing both shell-model atomistic effective-potential simulations, and density-functional-theory (DFT) calculations. Our shell-model calculations predict the {001} surfaces to be the energetically most stable surfaces: the calculated value of the surface energy being 2.2 J/m(2) for the MgO-terminated surface, which is favoured over the SiO2-terminated surface (2.7 J/m(2)). Also for the polar surfaces {111}, {101} and {011} the MgO-terminated surfaces are energetically more stable than the Si-terminated surfaces. In addition we report the predicted morphology of the MgSiO3 perovskite structure, which is dominated by the energetically most stable {001} and {110} surfaces, and which appears to agree well with the shape of grown single crystals.

The effect of temperature on the seismic anisotropy of the perovskite and post-perovskite polymorphs of MgSiO3

Using elastic constants determined by ab initio molecular dynamics over a range of temperatures, we show that the seismic anisotropy of MgSiO3 perovskite is significantly temperature dependent. At 90 GPa, the direction of greatest shear wave splitting changes from [001] at 0 K to [010] at 3500 K. In addition, there is no shear wave splitting in the [100] direction at 0 K, but a significant amount of splitting is exhibited at 3500 K. We have also calculated the first set of high-temperature elastic constants of the new post-perovskite phase by ab initio molecular dynamics methods. We find that, in contrast to MgSiO3 perovskite, temperature has little effect on the acoustic anisotropy of this phase. We determine ∂ln VS/∂ln VP for the post-perovskite phase and find it to be very low, about 0.9. This may be an important parameter for mapping out the post-perovskite phase in the lowermost mantle.

Effect of aluminium on the compressibility of silicate perovskite

Volume measurements for aluminous MgSiO3 perovskite containing 5 mol% Al2O3 were carried out up to pressures of 40 GPa at ambient temperature, using monochromatic synchrotron X‐ray diffraction. A least‐squares refinement of the data to the Birch‐Murnaghan equation of state yields the following parameters V0 = 163.234(8) Å3, KT0 = 251.5(13) GPa, K′0 = 4. Within uncertainties, the presence of 5 mol% Al2O3 in MgSiO3 perovskite induces a decrease of the bulk modulus in the range of 0% to 1.8%. Thus, KT of perovskite is affected little if at all by the presence of Al3+. This result is in excellent agreement with the values deduced from sound velocity measurements on the same sample [Jackson et al., 2004]. We discuss the possible origin of discrepancies among the different bulk moduli reported to date for aluminous perovskite. In light of recent calculations, our results are consistent with aluminium being dissolved in MgSiO3 perovskite through a coupled substitution mechanism involving the replacement of both Mg2+ and Si4+ in the dodecahedral and octahedral sites by 2 Al3+. Moreover, any slight reduction in the bulk modulus of MgSiO3 perovskite induced by the dissolution of 5 mol% Al2O3, indicates that the relative proportions of the minerals characteristic of the lower mantle, as inferred from seismological models, should not be significantly altered by the introduction of Al in the system.

Phase transition in MgSiO 3 perovskite in the earth's lower mantle

A new polymorph of MgSiO 3 more stable than the Pbnm-perovskite phase has been identified by first-principles computations. It has the CaIrO 3 structure with Cmcm symmetry and consists of SiO 3 layers intercalated by eightfold-coordinated Mg ions. High-temperature calculations within the quasiharmonic approximation give a volume change of ∼1.5% and a Clapeyron slope of ∼7.5±0.3 MPa/K at ∼2750 K and ∼125 GPa. These pressure–temperature ( P– T) conditions are close to those in which a phase transition in MgSiO 3-perovskite has been observed by in situ angle dispersive X-ray diffraction measurements. This transition appears to be associated with the Dʺ discontinuity.