The Eect of Al on the Sharpness of the MgSiO3 Perovskite to Post-Perovskite Phase Transition (original) (raw)
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Effect of Al on the sharpness of the MgSiO_3 perovskite to post-perovskite phase transition
2005
1] By means of static ab-initio computations we investigate the influence of Al on the recently discovered perovskite to post-perovskite phase transition in MgSiO 3 . We examine three substitution mechanisms for Al in the two structures: MgSi ! AlAl; SiSiO ! AlAl5; and Si ! AlH. The substitutions introducing oxygen vacancies (highly unfavorable, energetically) and water (favorable) both lower the 0 Kelvin transition pressure, whereas charge coupled substitution increases it relative to 105 GPa for pure MgSiO 3 . From the transition pressures for 0, 6.25, and 100 mol% charge coupled Al 2 O 3 incorporation and simple solution theories, we estimate the phase diagram of Al-bearing MgSiO 3 at low Al concentrations. Assuming the Clapeyron slope is independent of Al concentration, we find the perovskite-to-post-perovskite transition region to span 127-140 GPa, at 6.25 mol% Al 2 O 3 . When the upper pressure limit is bounded by the core-mantle boundary, the phase coexistence region has width 150 km. Citation: Akber-Knutson, S., G. Steinle-Neumann, and P. D. Asimow , Effect of Al on the sharpness of the MgSiO 3 perovskite to post-perovskite phase transition, Geophys. Res. Lett., 32, L14303,
Mechanisms of Al 3+ incorporation in MgSiO 3 post-perovskite at high pressures
Earth and Planetary Science Letters, 2006
Aluminum is the fifth most abundant element in the Earth's mantle, yet its effect on the physical properties of the newly found MgSiO3 post-perovskite (PPv), the major mineral of the Earth's Dʺ layer, is not fully known. In this paper, large-scale ab initio simulations based on density functional theory (DFT) within the generalized gradient approximation (GGA) have been carried out
The energetics of aluminum solubility into MgSiO3 perovskite at lower mantle conditions
Earth and Planetary Science Letters, 2004
SiO 3 perovskite, commonly believed to be the most abundant mineral in the Earth, is the preferred host phase of Al 2 O 3 in the Earth's lower mantle. Aiming to better understand the effects of Al 2 O 3 on the thermoelastic properties of the lower mantle, we use atomistic models to examine the chemistry and elasticity of solid solutions within the MgSiO 3 (perovskite)^Al 2 O 3 (corundum)M gO(periclase) mineral assemblage under conditions pertinent to the lower mantle: low Al cation concentrations, P = 25^100 GPa, and T = 1000^2000 K. We assess the relative stabilities of two likely substitution mechanisms of Al into MgSiO 3 perovskite in terms of reactions involving MgSiO 3 , MgO, and Al 2 O 3 , in a manner similar to the 0 Kelvin calculations of Brodholt [J.P. Brodholt (2000) Nature 407, 620^622] and Yamamoto et al. [T. Yamamoto et al. (2003) Earth Planet. Sci. Lett. 206, 617^625]. We determine the equilibrium composition of the assemblage by examining the chemical potentials of the Al 2 O 3 and MgO components in solid solution with MgSiO 3 , as functions of concentration. We find that charge coupled substitution dominates at lower mantle pressures and temperatures. Oxygen vacancyforming substitution accounts for 3^4% of Al substitution at shallow lower mantle conditions, and less than 1% in the deep mantle. For these two pressure regimes, the corresponding adiabatic bulk moduli of aluminous perovskite are 2% and 1% lower than that of pure MgSiO 3 perovskite.
Geochimica et Cosmochimica Acta, 1994
Molecular dynamics (MD) simulations of MgSiOJ-perovskite and melt with the MATSUI ( 1988) interatomic potential are used to resolve the problem of inconsistency between modeled and experimental melting curves. Equations of state for solid and liquid MgSi03-perovskite are in agreement with experimental data and are useful for calculating densities at experimentally inaccessible temperatures and pressures. Comparison with the Preliminary Earth Model ( DZIEWONSKI and ANDERSON, 198 1) shows that the equation of state of MgSiOa-perovskite is consistent with seismic parameter for lower mantle. Two-phase MD simulations at constant pressure were also performed to calculate a melting curve of MgSiO,-perovskite in agreement with the recent experiments. Overheating does not exceed 400 K in accord with the theoretical estimate for finite systems. Extrapolation of meltings temperature to the coremantle boundary pressure ( I34 GPa) with the Simon equation gives temperature of -6400 K for MgSiOJperovskite and shows that, according to accepted estimates of temperature at core-mantle boundary, MgSiOJ-perovskite remains solid.
Post-garnet Transition in the System MgSiO3-Al2O3
THE REVIEW OF HIGH PRESSURE SCIENCE AND TECHNOLOGY, 1998
Phase relations of the MgSiO3-A12O3 system were examined with special interest of post garnet transition. High pressure experiments were performed using 6-8 type multianvil apparatus based on conventional quenching method at pressures of 26 to 37 GPa and 1600 to 1800•Ž. It was observed that pyrope dissociates into perovskite solid solution plus corundum solid solution above about 26.5 GPa, and it was suggested that this phase boundary has a very small negative slope. The stability field of single phase perovskite solid solution in the system MgSiO3-A12O3 extends as pressure increases, and perovskite solid solution isochemical with pyrope stabilizes at about 37 GPa and 1600•Ž.
Earth and Planetary Science Letters, 2004
We have investigated the effect of Al 3 + on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO 3 -AlO 1.5 join with up to 25 mol% AlO 1.5 . Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of f 30 -100 GPa at temperatures of f 2000 to 2600 K. Lattice parameters for orthorhombic (Pbnm) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al 3 + into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions (V 0 ). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al 3 + causes a mild increase in compressibility, with the bulk modulus (K 0 ) decreasing at a rate of À67 F 35 GPa/X Al . This decrease in K 0 is consistent with recent theoretical calculations if essentially all Al 3 + substitutes equally into the six-and eight-fold sites by charge-coupled substitution with Mg 2 + and Si 4 + . In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al 3 + into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO 3 -AlO 1.5 -SiO 2 indicate that a stability field of ternary defect Mgperovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO 3 -AlO 1.5 -FeO 1.5 -SiO 2 based on recent experimental results indicates the existence of a complex polyhedral volume of Mgperovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al 3 + and Fe 3 + substitutions. Primitive mantle with about 5 mol% AlO 1.5 and an Fe 3+ /(Fe 3+ +Fe 2+ ) ratio of f 0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al 3 + and Fe 3 + defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, 0012-821X/$ -see front matter D address: M.J.Walter@bristol.ac.uk (M.J. Walter). www.elsevier.com/locate/epsl Earth and Planetary Science Letters 222 (2004) 501 -516 defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth. D
Earth and Planetary Science Letters, 2006
Experimentally determined phase relations in the system MgO-AlO 1.5 -SiO 2 at pressures relevant to the upper part of the lower mantle indicate that Mg-silicate perovskite incorporates aluminum into its structure almost exclusively by a charge-coupled reaction. MgSiO 3 -rich bulk compositions along the joins MgSiO 3 -MgAlO 2.5 and MgSiO 3 -MgAl 2 O 4 crystallize assemblages of perovskite coexisting with periclase. MgO-saturated perovskites along these joins have ambient unit cell volumes consistent with those measured and calculated for aluminous perovskite along the charge-coupled substitution join, MgSiO 3 -AlO 1.5 . The compressibility of aluminous perovskite along the MgO-saturated joins is not anomalously low as predicted for oxygen-defect perovskites. The bulk moduli, however, are consistent with previous measurements made for aluminous perovskites along the charge-coupled substitution join. These results agree with first-principles calculations showing very limited stability of O-defects in Mg-perovskite at pressures and temperatures corresponding to lower mantle conditions, but are inconsistent with earlier experimental results showing unusually compressive aluminous perovskite. The maximum solubility of alumina in perovskite is ∼25 mol% along the MgSiO 3 -AlO 1.5 join within the ternary MAS-system (i.e. pyrope composition), and the join is apparently binary. Although primitive mantle peridotite compositions are MgO-saturated and fall nearly on the oxygen vacancy join, alumina substitution into perovskite is expected to occur primarily by charge-coupled substitution throughout the lower mantle. The compressibility of aluminous perovskite in primitive mantle is expected to be only a few percent lower than for end member MgSiO 3 perovskite.
Al, Fe substitution in the MgSiO3 perovskite structure: A single-crystal X-ray diffraction study
Physics of the Earth and Planetary Interiors, 2006
We have determined by single-crystal X-ray diffraction the crystal structure of three Fe-Al-MgSiO 3 perovksite samples containing up to 9.5 wt% of Al 2 O 3 and 19 wt% of FeO. We find that there is no evidence for Fe (Fe 3+ or Fe 2+) on the octahedral site. Therefore, we deduce that the two dominant substitution mechanisms for the combined substitution of Al and Fe into the perovskite structure are: (i) Mg A 2+ + Si B 4+ ⇔ Fe A 3+ + Al B 3+ , where the excess of Fe is accommodated by (ii) Mg A 2+ ⇔ Fe A 2+. This is in agreement with all past theoretical and experimental work and solves the long-debated issue of Fe 3+ occupancy in the perovskite structure.
In situ Observation of ilmenite-perovskite phase transition in MgSiO 3 using synchrotron radiation
Geophysical Research Letters, 2001
In situ observations of the ilmenite-perovskite transition in MgSiO3 were carried out in a multianvil high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300-1600 K was determined to be P (GPa) = 28.4(ñ0.4) -0.0029(ñ0.0020)T (K) based on Jamieson's equation of state of gold [Jamieson et al., 1982] and P (GPa)= 27.3(ñ0.4)-0.0035(ñ0.0024)T (K) based on Anderson's equation of state of gold [Anderson et al., 1989]. The consistency of our results, using Jamieson's equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Anderson's equation of state implies that the depth of the 660-km seismic discontinuity does not match the pressure of this transition. 1.