Composition dependence of spin transition in (Mg,Fe)SiO3 bridgmanite

Spin transitions in (Mg,Fe)SiO3 bridgmanite have important implications for the chemistry and dynamics of Earth’s lower mantle, but have been complex to characterize in experiments. We examine the spin state of Fe in highly Fe-enriched bridgmanite synthesized from enstatites with measured compositions (Mg0.61Fe0.38Ca0.01)SiO3 and (Mg0.25Fe0.74Ca0.01)SiO3. Bridgmanite was synthesized at 78-88 GPa and 1800-2400 K and X‑ray emission spectra were measured on decompression to 1 bar (both compositions) and compression to 126 GPa [(Mg0.61Fe0.38Ca0.01)SiO3 only] without additional laser heating. Observed spectra confirm that Fe in these bridgmanites is dominantly high spin in the lower mantle. However, the total spin moment begins to decrease at ~50 GPa in the 74% FeSiO3 composition. These results support density functional theory predictions of a lower spin transition pressure in highly Fe-enriched bridgmanite and potentially explain the high solubility of FeSiO3 in bridgmanite at pressures corresponding to Earth’s deep lower mantle.

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title: EFFECTS OF THE ELECTRONIC SPIN TRANSITIONS OF IRON IN LOWER MANTLE MINERALS: IMPLICATIONS FOR DEEP MANTLE GEOPHYSICS AND GEOCHEMISTRY
abstract: We have critically reviewed and discussed currently available information regarding the spin and valence states of iron in lower mantle minerals and the associated effects of the spin transitions on physical, chemical, and transport properties of the deep Earth. A high-spin to low-spin crossover of Fe2+ in ferropericlase has been observed to occur at pressure-temperature conditions corresponding to the middle part of the lower mantle. In contrast, recent studies consistently show that Fe2+ predominantly exhibits extremely high quadrupole splitting values in the pseudo-dodecahedral site (A site) of perovskite and post-perovskite, indicative of a strong lattice distortion. Fe3+ in the A site of these structures likely remains in the high-spin state, while a high-spin to low-spin transition of Fe3+ in the octahedral site of perovskite occurs at pressures of 15–50 GPa. In post-perovskite, the octahedral-site Fe3+ remains in the low-spin state at the pressure conditions of the lowermost mantle. These changes in the spin and valence states of iron as a function of pressure and temperature have been reported to affect physical, chemical, rheological, and transport properties of the lower mantle minerals. The spin crossover of Fe2+ in ferropericlase has been documented to affect these properties and is discussed in depth here, whereas the effects of the spin transition of iron in perovskite and post-perovskite are much more complex and remain debated. The consequences of the transitions are evaluated in terms of their implications to deep Earth geophysics, geochemistry, and geodynamics including elasticity, element partitioning, fractionation and diffusion, and rheological and transport properties.

title: Iron Partitioning in Earth's Mantle: Toward a Deep Lower Mantle Discontinuity
abstract: We measured the spin state of iron in ferropericlase (Mg0.83Fe0.17)O at high pressure and found a high-spin to low-spin transition occurring in the 60- to 70-gigapascal pressure range, corresponding to depths of 2000 kilometers in Earth's lower mantle. This transition implies that the partition coefficient of iron between ferropericlase and magnesium silicate perovskite, the two main constituents of the lower mantle, may increase by several orders of magnitude, depleting the perovskite phase of its iron. The lower mantle may then be composed of two different layers. The upper layer would consist of a phase mixture with about equal partitioning of iron between magnesium silicate perovskite and ferropericlase, whereas the lower layer would consist of almost iron-free perovskite and iron-rich ferropericlase. This stratification is likely to have profound implications for the transport properties of Earth's lowermost mantle.

title: Iron spin transition in Earth's mantle.
abstract: High-pressure Mössbauer spectroscopy on several compositions across the (Mg,Fe)O magnesiowüstite solid solution confirms that ferrous iron (Fe(2+)) undergoes a high-spin to low-spin transition at pressures and for compositions relevant to the bulk of the Earth's mantle. High-resolution x-ray diffraction measurements document a volume change of 4-5% across the pressure-induced spin transition, which is thus expected to cause seismological anomalies in the lower mantle. The spin transition can lead to dissociation of Fe-bearing phases such as magnesiowüstite, and it reveals an unexpected richness in mineral properties and phase equilibria for the Earth's deep interior.

title: Spin transition in magnesiowüstite in earth's lower mantle.
abstract: Iron in the major lower mantle (LM) minerals undergoes a high spin (HS) to low spin (LS) transition at relevant pressures (23-135 GPa). Previous failures of standard first principles approaches to describe this phenomenon have hindered its investigation and the clarification of important consequences. Using a rotationally invariant formulation of LDA + U we report a successful study of this transition in low solute concentration magnesiowüstite, (Mg(1-x)Fe(x)(O), (x < 0.2) the second most abundant LM phase. We show that the HS-LS transition goes through an insulating (semiconducting) intermediate mixed spins (MS) state without discontinuous changes in properties, as seen experimentally. We show that the HS state crosses over smoothly to the LS state passing through an insulating MS state where properties change continuously, as seen experimentally.

title: Electronic Transitions in Perovskite: Possible Nonconvecting Layers in the Lower Mantle
abstract: We measured the spin state of iron in magnesium silicate perovskite (Mg0.9,Fe0.1)SiO3 at high pressure and found two electronic transitions occurring at 70 gigapascals and at 120 gigapascals, corresponding to partial and full electron pairing in iron, respectively. The proportion of iron in the low spin state thus grows with depth, increasing the transparency of the mantle in the infrared region, with a maximum at pressures consistent with the D″ layer above the core-mantle boundary. The resulting increase in radiative thermal conductivity suggests the existence of nonconvecting layers in the lowermost mantle.

title: A synchrotron Mössbauer spectroscopy study of (Mg,Fe)SiO3 perovskite up to 120 GPa
abstract: The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe0.05Mg0.95SiO3 (Pv05) and Fe0.1Mg0.9SiO3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mössbauer spectroscopy (SMS). Such investigations of extremely small and dilute 57Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the “three-doublet” model, which assumes two Fe2+-like sites and one Fe3+-like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe3+/ΣFe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe3+ remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a highspin to a low-spin state.

title: High-pressure X-ray diffraction and X-ray emission studies on iron-bearing silicate perovskite under high pressures
abstract: Iron-bearing silicate perovskite is believed to be the most abundant mineral of the Earth's lower mantle. Recent studies have shown that Fe2+ exists predominantly in the intermediate-spin state with a total spin number of 1 in silicate perovskite in the lower part of the lower mantle. Here we have measured the spin states of iron and the pressure–volume relation in silicate perovskite [(Mg0.6,Fe0.4)SiO3] at pressure conditions relevant to the lowermost mantle using in situ X-ray emission and X-ray diffraction in a diamond cell. Our results showed that the intermediate-spin Fe2+ is stable in the silicate perovskite up to ∼ 125 GPa but starts to transition to the low-spin state at approximately 135 GPa. Concurrent X-ray diffraction measurements showed a decrease of approximately 1% in the unit cell volume in the silicate perovskite [(Mg0.6,Fe0.4)SiO3], which is attributed to the intermediate-spin to the low-spin transition. The transition pressure coincides with the pressure conditions of the lowermost mantle, raising the possibility of the existence of the silicate perovskite phase with the low-spin Fe2+ across the transition from the post-perovskite to the perovskite phases in the bottom of the D″ layer.

title: Low-spin Fe2+ in silicate perovskite and a possible layer at the base of the lower mantle
abstract: We investigated the spin state of iron in Mg 0.82 Fe 0.18 SiO 3 silicate perovskite using Mossbauer spectroscopy and nuclear forward scattering (NFS) at pressures up to 130 GPa and temperatures up to 1000 K. Majorite starting material was loaded into diamond anvil cells in three separate experiments, and transformed to silicate perovskite through laser heating. We found, in agreement with previous work, the predominance of a component with high isomer shift (∼1 mm/s relative to α-Fe) and high-quadrupole splitting (QS) (>4 mm/s) in Mossbauer and NFS spectra up to 115 GPa at room temperature, and in accordance with previous work this component was assigned to intermediate-spin Fe 2+ . At higher pressures, the intensity of the high QS component in the silicate perovskite spectrum decreased, while the intensity of a new component with low isomer shift (∼0 mm/s relative to α-Fe) and low quadrupole splitting ( 2+ , and its intensity increased with both increasing pressure and increasing temperature: at 120 GPa and 1000 K all Fe 2+ was in the low-spin state. X-ray diffraction data showed well crystallised perovskite in all runs, and although the stable phase above ∼110 GPa is expected to be post-perovskite, sluggish transition kinetics likely preserved the perovskite phase in a metastable state. Our results combined with data in the literature and thermodynamic and topological considerations suggest that there may be a region where silicate perovskite containing low-spin Fe 2+ is stable, which coincides with predicted pressure–temperature conditions near the D″ layer.

title: Spin state of ferric iron in MgSiO3 perovskite and its effect on elastic properties
abstract: Recent studies have indicated that a significant amount of iron in MgSiO 3 perovskite (Pv) is Fe 3+ (Fe 3+ /ΣFe = 10–60%) due to crystal chemistry effects at high pressure ( P ) and that Fe 3+ is more likely than Fe 2+ to undergo a high-spin (HS) to low-spin (LS) transition in Pv in the mantle. We have measured synchrotron Mossbauer spectroscopy (SMS), X-ray emission spectroscopy (XES), and X-ray diffraction (XRD) of Pv with all iron in Fe 3+ in the laser-heated diamond-anvil cell to over 100 GPa. Fe 3+ increases the anisotropy of the Pv unit cell, whereas Fe 2+ decreases it. In Pv synthesized above 50 GPa, Fe 3+ enters into both the dodecahedral (A) and octahedral (B) sites approximately equally, suggesting charge coupled substitution. Combining SMS and XES, we found that the LS population in the B site gradually increases with pressure up to 50–60 GPa where all Fe 3+ in the B site becomes LS, while Fe 3+ in the A site remains HS to at least 136 GPa. Fe 3+ makes Pv more compressible than Mg-endmember below 50 GPa because of the gradual spin transition in the B site together with lattice compression. The completion of the spin transition at 50–60 GPa increases bulk modulus with no associated change in density. This elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with Fe 3+ .

title: Predominant Intermediate-Spin Ferrous Iron in Lowermost Mantle Post-Perovskite and Perovskite
abstract: The Earth’s lowermost mantle displays an important mineralogical transition from perovskite to post-perovskite but the spin state of iron in these phases remains poorly known. Experimental results suggest that iron occurs in the intermediate spin state in both of these phases, which implies that changes in physical properties of the lower mantle must be governed by factors other than spin transitions in iron.

title: Intermediate-spin ferrous iron in lower mantle perovskite
abstract: Iron has the ability to adopt different electronic configurations, and transitions in its spin state in the lower mantle can significantly influence mantle properties and dynamics. Experimental results for two lower-mantle perovskite compositions show that the intermediate spin state of iron is stable throughout the bulk of the lower mantle.

title: Effects of the Fe3 + spin transition on the properties of aluminous perovskite—New insights for lower-mantle seismic heterogeneities
abstract: We have measured the effects of the coupled substitution of Fe3 + and Al on the density and compressibility of mantle silicate perovskite (Pv) up to 95 GPa. X-ray emission spectroscopy and synchrotron Mossbauer spectroscopy reveal a rapid increase in the population of low-spin Fe3 + in Fe3 +, Al-bearing Pv over a narrow pressure range near 70 GPa, which is in sharp contrast with Al-free Fe3 +-bearing Pv, where Fe3 + undergoes a gradual spin transition, and with Al-free Fe2 +-bearing Pv, where Fe2 + does not become low spin. At low pressure, Fe3 + and Al expand the perovskite lattice. However, near the pressure range of the abrupt increase in the low-spin population, the unit-cell volume of Fe3 +, Al-bearing Pv becomes similar to that of Mg-endmember Pv, while those of Al-free Fe3 +-bearing Pv and Al-free Fe2 +-bearing Pv remain larger throughout the lower mantle. Consequently, Pv in Al-rich systems should have lower density in the shallow lower mantle but similar or greater density than Pv in pyrolite in the deep lower mantle, affecting the buoyancy and mechanical stability of heterogeneities. Although the Fe3 + spin transition in Pv is unlikely to cause a seismic discontinuity at mantle temperatures, it may result in a large change in bulk sound speed at 1200–1800 km depth, such that a vertically extending structure with an elevated amount of Fe3 + would generate slower and faster anomalies above and below the depth of the spin transition, respectively, relative to the surrounding mantle. This may have important implications for bulk sound speed anomalies observed at similar depths in seismic tomography studies.

title: The electrical resistance measurements of (Mg,Fe)SiO3 perovskite at high pressures and implications for electronic spin transition of iron
abstract: We have measured the electrical resistance of (Mg0.9Fe0.1)SiO3 perovskite with increasing pressure in a diamond-anvil cell (DAC). Results demonstrate that the electrical conductivity increased with pressure to about 70 GPa, then conversely decreased to around 85 GPa, and again increased mildly to 135 GPa. As inferred from the previous studies of (Mg,Fe)O ferropericlase, the observed reduction in the conductivity of perovskite at 70–85 GPa is most likely due to the high-spin to low-spin transition of iron; it decreases the number of unpaired electrons and therefore diminishes the conduction by electron hopping between the ferric and ferrous iron sites. Present conductivity data provide valuable implications for the pressure range of spin transition in (Mg,Fe)SiO3 perovskite, which has been highly controversial. The relatively abrupt reduction in the electrical conductivity above 70 GPa is reconciled with the spin-pairing transition in ferric iron, which may occur rather sharply at this pressure range.

title: Iron-rich perovskite in the Earth's lower mantle
abstract: The equations of state of perovskite with (Mg{sub 0.75},Fe{sub 0.25})SiO{sub 3} and MgSiO{sub 3} compositions have been investigated by synchrotron X-ray diffraction up to 130 GPa at 300 K in diamond anvil cells. Here we show that the addition of 25% Fe in MgSiO{sub 3} perovskite increases its density and bulk sound velocity (V{phi}) by 4-6% and 6-7%, respectively, at lower-mantle pressures. Based on concurrent synchrotron X-ray emission and Moessbauer spectroscopic studies of the samples, the increase in V{phi} and density can be explained by the occurrence of the low-spin Fe3+ and the extremely high-quadrupole component of Fe{sup 2+}. Combining these experimental results with thermodynamic modeling, our results indicate that iron-rich perovskite can produce an increase in density and a value of V{phi} that is compatible with seismic observations of reduced shear-wave velocity in regions interpreted as dense, stiff piles in the lower mantle. Therefore, the existence of the Fe-rich perovskite in the lower mantle may help elucidate the cause of the lower-mantle large low-shear-velocity provinces (LLSVPs) where enhanced density and V{phi} are seismically observed to anti-correlate with the reduced shear wave velocity.

title: Mössbauer modeling to interpret the spin state of iron in (Mg,Fe)SiO3 perovskite
abstract: The properties of (Mg,Fe)SiO3 perovskite at lower mantle conditions are still not well understood, and particular attention has recently been given to determining the Fe spin state. A major challenge in spin states studies is interpretation of Mossbauer spectra to determine the electronic structure of iron under extreme conditions. In this paper ab initio methods are used to predict quadrupole splitting values of high-, intermediate- and low-spin Fe2+ and Fe3+ in perovskite, as a function of pressure and composition. The calculations in (Mg0.75Fe0.25)SiO3 yield quadrupole splitting values in the range of 0.7–1.7 mm/s for all spin and valence states except high-spin Fe2+, which has two possible quadrupole splittings, 2.3 and 3.3 mm/s. The unexpected multiple quadrupole splitting values for high-spin Fe2+ are explained in terms of small changes in local structure and d-orbital occupations. The computed results are applied to interpret existing perovskite Mossbauer data for iron's spin state.

title: Spin states and hyperfine interactions of iron in (Mg,Fe)SiO$_{3}$ perovskite under pressure
abstract: With the guidance of first-principles phonon calculations, we have searched and found several metastable equilibrium sites for substitutional ferrous iron in MgSiO 3 perovskite. In the relevant energy range, there are two distinct sites for high-spin, one for low-spin, and one for intermediate-spin iron. Because of variable d-orbital occupancy across these sites, the two competing high-spin sites have different iron quadrupole splittings (QS). At low pressure, the high-spin iron with QS of 2.3–2.5 mm/s is more stable, while the high-spin iron with QS of 3.3–3.6 mm/s is more favorable at higher pressure. The crossover occurs between 4 and 24 GPa, depending on the choice of exchange-correlation functional and the inclusion of on-site Coulomb interaction (Hubbard U ). Our calculation supports the notion that the transition observed in recent Mossbauer spectra corresponds to an atomic-site change rather than a spin-state crossover. Our result also helps to explain the lack of anomaly in the compression curve of iron-bearing silicate perovskite in the presence of a large change of quadrupole splitting, and provides important guidance for future studies of thermodynamic properties of this phase.

title: Spin-state crossover and hyperfine interactions of ferric iron in MgSiO(3) perovskite.
abstract: Using density functional theory plus Hubbard U calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O(3) perovskite, the major mineral phase in Earth's lower mantle, has high-spin ferric iron (S=5/2) at both dodecahedral (A) and octahedral (B) sites. With increasing pressure, the B-site iron undergoes a spin-state crossover to the low-spin state (S=1/2) between 40 and 70 GPa, while the A-site iron remains in the high-spin state. This B-site spin-state crossover is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent x-ray diffraction and Mössbauer spectroscopy measurements. The anomalous volume reduction leads to a significant softening in bulk modulus during the crossover, suggesting a possible source of seismic-velocity anomalies in the lower mantle.

title: Spin crossover of iron in aluminous MgSiO3 perovskite and post-perovskite
abstract: Using density functional theory+Hubbard U (DFT+U) calculations, we investigate how aluminum affects the spin crossover of iron in MgSiO3 perovskite (Pv) and post-perovskite (Ppv), the major mineral phases in the Earth's lower mantle. We find that the presence of aluminum does not change the response of iron spin state to pressure: only ferric iron (Fe3+) in the octahedral (B)-site undergoes a crossover from high-spin (HS) to low-spin (LS) state, while Fe3+ in the dodecahedral (A)-site remains in the HS state, same as in Al-free cases. However, aluminum does significantly affect the placement of Fe3+ in these mineral phases. The most stable atomic configuration has all Al3+ in the B-site and all Fe3+ in the A-site (thus in the HS state). Metastable configurations with LS Fe3+ in the B-site can happen only at high pressures and high temperatures. Therefore, experimental observations of LS Fe3+ at high pressures in Al-bearing Pv require diffusion of iron from the A-site to the B-site and should be sensitive to the annealing temperature and schedule. In the Earth's lower mantle, the elastic anomalies accompanying the B-site HS-LS crossover exhibited in Al-free Pv are likely to be considerably reduced, according to the B-site Fe3+ population.

title: Solubility of FeO in (Mg,Fe)SiO3 perovskite and the post-perovskite phase transition
abstract: Phase relations in Mg 0.5 Fe 0.5 SiO 3 and Mg 0.25 Fe 0.75 SiO 3 were investigated in a pressure range from 72 to 123 GPa on the basis of synchrotron X-ray diffraction measurements in situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). Results demonstrate that Mg 0.5 Fe 0.5 SiO 3 perovskite is formed as a single phase at 85–108 GPa and 1800–2330 K, indicating a high solubility of FeO in (Mg,Fe)SiO 3 perovskite at high pressures. Post-perovskite appears coexisting with perovskite in Mg 0.5 Fe 0.5 SiO 3 above 106 GPa at 1410 K, the condition very close to the post-perovskite phase transition boundary in pure MgSiO 3 . The coexistence of perovskite and post-perovskite was observed to 123 GPa. In addition, post-perovskite was formed coexisting with perovskite also in Mg 0.25 Fe 0.75 SiO 3 bulk composition at 106–123 GPa. In contrast to earlier experimental and theoretical studies, these results show that incorporation of FeO stabilizes perovskite at higher pressures. This could be due to a larger ionic radius of Fe 2+ ion, which is incompatible with a small Mg 2+ site in the post-perovskite phase.

title: Spin transition and equations of state of (Mg, Fe)O solid solutions
abstract: We have performed a series of experiments to investigate the compositional effect on the compression behavior of (Mg, Fe)O solid solutions at high pressure. The in-situ synchrotron X-ray diffraction data revealed abnormal volume contractions at about 40, 60, and 80 GPa for (Mg0.80, Fe0.20)O, (Mg0.61, Fe0.39)O, and (Mg0.42, Fe0.58)O, respectively. The volume contractions are associated with the reported electronic transition of high-spin to low-spin in Fe2+, and caused by the reduction of Fe2+ ionic radius across the transition. A least-squares fit of the compression data to the Birch-Murnaghan equation of state yielded bulk modulus K0 (GPa) = 160 − 10XFeO for the high-spin (Mg,Fe)O and K0 = 170(3) GPa for the low-spin (Mg,Fe)O. The equations of state of (Mg,Fe)O established in this study are directly applicable to the Earth's lower mantle in composition and pressure ranges and provide essential data for modeling the density profile of the lower mantle.

title: Magnetic Collapse in Transition Metal Oxides at High Pressure: Implications for the Earth
abstract: Magnetic collapse in transition metal ions is predicted from first-principles computations at pressures reached in the Earth's lower mantle and core. Magnetic collapse would lead to marked changes in geophysically important properties, such as elasticity and conductivity, and also to different geochemical behavior, such as element partitioning, than estimated by extrapolating low-pressure data, and thus change the understanding of Earth's structure and evolution. Magnetic collapse results from band widening rather than from changes in crystal field splitting under pressure. Seismic anomalies in the outer core and the lowermost mantle may be due to magnetic collapse of ferrous iron, dissolved in iron liquid in the outer core, and in solution in magnesiowüstite in the lowermost mantle.

title: Electronic spin transitions in iron-bearing MgSiO3 perovskite
abstract: The electronic spin state of iron in perovskites with the chemical composition Mg0.9375Fe0.0625SiO3, Mg0.8750Fe0.1250SiO3 and Mg0.9375Fe0.0625Si0.9375Fe0.0625O3, have been determined at 0 K and various pressures between 40-160 GPa, using ab initio methods. The results indicate that ferric iron exhibits a wide range of spin transition pressures between about 60-160 GPa, while ferrous iron has a much narrower span, between about 130-145 GPa. In general, the lowest spin transition pressures are associated with the most energetically favorable substitution configurations, where iron atoms are closest together. The effect of spin state on calculated elastic properties is found to be small.

title: Effect of site degeneracies on the spin crossovers in (Mg, Fe)SiO3 perovskite
abstract: We investigate by first principles several aspects of the spin crossover of ferrous iron (Fe 2+ ) between the high-spin (HS), intermediate-spin (IS), and low-spin (LS) states in MgSiO3 perovskite under pressure. First, we investigate the effect of site degeneracy intrinsic to LS and IS states of iron. We show that the entropy owing to this effect (site entropy) significantly increases the fraction of LS irons and reduces the crossover onset pressure, especially at high temperatures. Even including the effect of site entropy, the fraction of IS irons in the lower mantle or in the laboratory is predicted to be negligible. We also show that the effect of the crossover on equation-of-state parameters is probably undetectable at any temperature. Although we address only ferrous iron in only one atomic configuration and results are specific to this situation, our results suggest that the effect of the crossover on the bulk modulus derived from direct sound speed measurements could perhaps be more noticeable below room temperature. However, at mantle conditions the bulk sound speed should hardly be affected by the crossover.

title: Pressure-Induced High-Spin to Low-Spin Transition in FeS Evidenced by X-Ray Emission Spectroscopy
abstract: We report the observation of the pressure-induced high-spin to low-spin transition in FeS using new high-pressure synchrotron x-ray emission spectroscopy techniques. The transition is evidenced by the disappearance of the low-energy satellite in the Fe Kb emission spectrum of FeS. Moreover, the phase transition is reversible and closely related to the structural phase transition from a manganese phosphidelike phase to a monoclinic phase. The study opens new opportunities for investigating the electronic properties of materials under pressure.

title: Spin selective x‐ray absorption spectroscopy: Demonstration using high resolution Fe Kβ fluorescence
abstract: In this letter it is shown that high resolution Fe Kβ excitation spectra can be used to separately probe empty spin‐up and spin‐down final states. Spin‐selective x‐ray absorption spectra were obtained by selectively monitoring different regions of the Kβ emission. The fluorescence was excited with monochromatized synchrotron radiation and analyzed using a spherically bent Ge(620) crystal. Spin–polarization was demonstrated by showing that the 1s→3d transition at the Fe K edge is seen with Kβ1,3 detection, but missing in the excitation spectrum using Kβ’ detection. The spin–polarization is also confirmed by ligand field atomic multiplet calculations that reproduce the Kβ spectra. Calculations are presented showing the applicability of spin–polarized Kβ detection to nearly all first transition metal ions.

title: Metal-ligand interplay in strongly correlated oxides: a parametrized phase diagram for pressure-induced spin transitions.
abstract: We investigate the magnetic properties of archetypal transition-metal oxides MnO, FeO, CoO, and NiO under very high pressure by x-ray emission spectroscopy at the Kbeta line. We observe a strong modification of the magnetism in the megabar range in all the samples except NiO. The results are analyzed within a multiplet approach including charge-transfer effects. The spectral changes are well accounted for by changes of the ligand field acting on the d electrons and allows us to extract the d-hybridization strength, O-2p bandwidth and ionic crystal field across the magnetic transition. This approach allows first-hand insight into the mechanism of the pressure-induced spin transition.

title: Probing the 3d spin momentum with X-ray emission spectroscopy: the case of molecular-spin transitions.
abstract: We report X-ray emission spectra of Fe(III), Fe(II), and Co(II) spin-crossover compounds in their high-spin and low-spin forms. It is shown that all X-ray emission features are sensitive to the spin state. Variations of the Kbeta and the Kalpha emission line shapes, which are in agreement with theory, can be used as quantitative probes of the spin state; it is suggested that with appropriate reference experiments one can extract the spin momentum for a general case. Resonant X-ray emission spectra unveil details of the redistribution of electrons on the 3d levels associated with the spin-state change by revealing features at the X-ray absorption preedge not accessible through standard absorption measurements.

title: Pressure calibration of diamond anvil Raman gauge to 310GPa
abstract: In order to develop an optical method for pressure determination in the multimegabar region, the first-order Raman spectra of diamond anvils were investigated at pressures up to 310GPa. The high-frequency edge of the Raman band, which corresponds to the Raman shift of the anvil culet due to the normal stress, was calibrated against the sample pressure derived from the equation of state of Pt. The obtained pressure dependence of the edge frequency demonstrates the reliability of this diamond anvil Raman gauge. Up to the maximum pressure of this study, the relation between Raman frequency and normal stress at the diamond anvil culet is formally similar to the equation of state of a hydrostatically compressed isotropic elastic body having a bulk modulus of K0=547(11)GPa and a pressure derivative of the bulk modulus K0′=3.75(20).

title: 4f electron delocalization and volume collapse in praseodymium metal
abstract: We study the pressure evolution of the 4f electrons in elemental praseodymium metal compressed through several crystallographic phases, including the large volume-collapse transition at 20 GPa. Using resonant x-ray emission, we directly and quantitatively measure the development of multiple electronic configurations with differing 4f occupation numbers, the key quantum observable related to the delocalization of the strongly correlated 4f electrons. These results provide a high-fidelity test of prior predictions by dynamical mean-field theory, and support the hypothesis of a strong connection between electronic and structural degrees of freedom at the volume-collapse transition.

title: The GALAXIES beamline at the SOLEIL synchrotron: inelastic X-ray scattering and photoelectron spectroscopy in the hard X-ray range.
abstract: The GALAXIES beamline at the SOLEIL synchrotron is dedicated to inelastic X-ray scattering (IXS) and photoelectron spectroscopy (HAXPES) in the 2.3-12 keV hard X-ray range. These two techniques offer powerful complementary methods of characterization of materials with bulk sensitivity, chemical and orbital selectivity, resonant enhancement and high resolving power. After a description of the beamline components and endstations, the beamline capabilities are demonstrated through a selection of recent works both in the solid and gas phases and using either IXS or HAXPES approaches. Prospects for studies on liquids are discussed.

title: Resonant X-ray emission study of the lower-mantle ferropericlase at high pressures
abstract: Electronic states of iron in Earth’s mantle minerals including ferropericlase, silicate perovskite, and post-perovskite have been previously investigated at high pressures and/or temperatures using various experimental techniques, including X-ray emission and Mössbauer spectroscopies. Although such methods have been used to infer changes in the electronic spin and valence states of iron in lower mantle minerals, they do not directly probe the 3d electronic states quantitatively. Here we use 1s2p resonant X-ray emission spectroscopy (RXES) at the Fe K pre-edge to directly probe and assess the 3d electronic states and the crystal-field splittings of Fe2+ in the lower-mantle ferropericlase [(Mg0.75,Fe0.25)O] at pressures up to 90 GPa. The pre-edge features from X-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) and RXES results explicitly show three excited states for high-spin Fe2+ (a lower-energy 4T1g state, a 4T2g state, and a higher-energy 4T1g state) and a single 2Eg state for low-spin Fe2+, attributed to the (t2g)0(eg)3 excited configuration. This latter feature begins to appear at 48 GPa and grows with pressure, while the peaks related to high-spin Fe2+ vanish above 80 GPa. The observed pre-edge features are consistent with purely quadrupolar transitions resulting from the centrosymmetric character of the Fe2+ site. The K pre-edge RXES spectra at the incident energy of 7112 eV, which are similar to the Fe L-edge spectra, are also used successfully to quantitatively obtain consistent results on the spin transition of Fe2+ in ferropericlase under high pressures. Owing to the superior sensitivity of the RXES technique, the observed electronic states and their energy separations provide direct information on the local electronic structures and crystal-field splitting energies of the 3d electronic shells of Fe2+ in ferropericlase at relevant pressures of the Earth’s lower mantle.

title: Electronic spin and valence states of Fe in CaIrO3-type silicate post-perovskite in the Earth's lowermost mantle
abstract: The electronic spin and valence states of Fe in post-perovskite ((Mg{sub 0.75}Fe{sub 0.25})SiO{sub 3}) have been investigated by synchrotron X-ray diffraction, Moessbauer and X-ray emission spectroscopy at 142 GPa and 300 K. Rietveld refinement of the X-ray diffraction patterns revealed that our sample was dominated by CaIrO{sub 3}-type post-perovskite. Combined Moessbauer and X-ray emission results show that Fe in post-perovskite is predominantly Fe{sup 2+} (70%) in the intermediate-spin state with extremely high quadrupole splitting of 3.77(25) mm/s. The remaining 30% Fe can be assigned to two sites. Compared with recent studies, our results indicate that the intermediate-spin Fe{sup 2+} is stabilized in CaIrO{sub 3}-type post-perovskite over a wide range of Fe content, whereas the low-spin Fe{sup 3+} is more dominant in the 2 x 1 kinked post-perovskite structure. The characterization of these structural and compositional effects on the spin and valence states of Fe in post-perovskite can help in understanding the geochemical and geophysical behavior of the core-mantle region.

title: Electronic spin states of ferric and ferrous iron in the lower-mantle silicate perovskite
abstract: The electronic spin and valence states of iron in lower-mantle silicate perovskite have been previously investigated at high pressures using various experimental and theoretical techniques. However, experimental results and their interpretation remain highly debated. Here we have studied a wellcharacterized silicate perovskite starting sample [(Mg0.9,Fe0.1)SiO3] in a chemically inert Ne pressure medium at pressures up to 120 GPa using synchrotron Mössbauer spectra. Analyses of the Mössbauer spectra explicitly show a high-spin to low-spin transition of the octahedral-site Fe3+ occurring at ~13-24 GPa, as evidenced from a significant increase in the hyperfine quadrupole splitting. Two quadrupole doublets of the A-site Fe2+, with extremely high-QS values of 4.1 and 3.1 mm/s, occur simultaneously with the spin transition of the octahedral-site Fe3+ and continue to develop to 120 GPa. It is conceivable that the spin-pairing transition of the octahedral-site Fe3+ causes a volume reduction and a change in the local atomic-site configurations that result in a significant increase of the quadrupole splitting in the dodecahedral-site Fe2+ at 13-24 GPa. Our results here provide a coherent explanation for recent experimental and theoretical results on the spin and valence states of iron in perovskite, and assist in comprehending the effects of the spin and valence states of iron on the properties of the lower-mantle minerals.

title: Spin and valence states of iron in Al-bearing silicate glass at high pressures studied by synchrotron Mössbauer and X-ray emission spectroscopy
abstract: High-pressure synchrotron Mössbauer (SMS) and X-ray emission (XES) spectroscopic measurements were conducted to investigate the spin and valence states of iron in (Al,Fe)-bearing magnesium silicate glass (Mg0.79Fe0.10Al0.10Si0.96O3) up to 126 GPa and 300 K. By analyzing the Fe Kβ emission spectra using the integrated relative difference (IRD) method, which accounts for the spectral broadening effects, the derived total spin momentum (S) of the iron in the glass shows no observable changes with pressure within the experimental uncertainties. A two-doublet fitting model representing two diverse local iron atomic environments was used to satisfactorily simulate the high-pressure SMS spectra of iron in the glass. The doublet with an averaged quadrupole splitting (QS) value of 1.94(±0.25) mm/s and chemical shift (CS) of 1.02(±0.25) mm/s at ambient conditions was assigned to be high-spin Fe2+, whereas the second doublet with QS = 0.83(±0.25) mm/s and CS = 0.49(±0.25) mm/s was assigned to be high-spin Fe3+. Increasing pressure continuously elevates the QS of Fe2+ from ~2 mm/s at ambient pressure to 3.5 mm/s at 126 GPa, while Fe3+ only exhibits a slight increase in the QS to 1.34(±0.25) mm/s. Comparing with previous experimental and theoretical studies on the local geometries and hyperfine parameters of silicate glasses and minerals, we conclude that the occurrence of the extremely high QS of Fe2+ in our glass above ~40-50 GPa can be associated with the enhanced density and diverse distortions and geometries of the local Fe2+ environments. Our combined XES and SMS results show that both Fe2+ and Fe3+ ions in Al-bearing silicate remain in the high-spin state, rather than undergoing a spin-pairing transition as proposed previously. Assuming that the silicate glass results can be used as an analog for understanding silicate melts, our results here indicate that iron ions likely experience significant changes in the local environments yet remain overall in the high-spin state in silicate melts at the extreme pressure and temperature conditions of the deep mantle.

title: Spin and valence states of iron in (Mg 0.8 Fe 0.2 )SiO 3 perovskite
abstract: The spin and valence states of iron in (Mg0.8Fe0.2)SiO3 perovskite were measured between 0 and 65 GPa using synchrotron Mössbauer spectroscopy. Samples were synthesized in situ in the laser-heated diamond cell under reducing conditions. The dominant spin state of iron in perovskite is high spin at pressures below 50 GPa. Above 50 GPa, the spectra shows severe changes which can be explained by appearance of two distinct iron sites with similar site weightings. One site has Mössbauer parameters consistent with high spin Fe, while the other has the parameters previously interpreted as intermediate spin. The latter intermediate-spin assignment is not unique, as similar Mössbauer parameters have been reported for high spin Fe in almandine at ambient pressure. However, our data do not rule out the existence of low-spin iron, which may exist with a smaller fraction and explain the observation of lower spin moments in the X-ray emission spectroscopy of perovskite at high pressure. From these considerations, our preferred interpretation is that iron in perovskite is mixed or high spin to at least 2000 km depths in the mantle, consistent with computational results. Our study also reveals that reducing conditions do not inhibit the formation of Fe in perovskite at deep-mantle pressures.  Grocholski, B., S.-H. Shim, W. Sturhahn, J. Zhao, Y. Xiao, and P. C. Chow (2009), Spin and valence states of iron in (Mg0.8Fe0.2)SiO3 perovskite,

title: Spin transition of iron in magnesiowüstite in the Earth's lower mantle
abstract: Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by ∼35 per cent and bulk sound velocity increases by ∼15 per cent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition.

title: Mössbauer Spectroscopy of Earth and Planetary Materials
abstract: The field of M ¨ ossbauer spectroscopy (MS) has recently enjoyed renewed visibility in the diverse geoscience communities as a result of the inclusion of M ¨ ossbauer

title: Electronic, magnetic and structural properties of the RFe03 antiferromagnetic-perovskites at very high pressures
abstract: At ambient pressure the orthorhombic perovskites R-orthoferrites (R {triple_bond}Lu, Eu, Y, Pr, and La) exhibit very large optical gaps. These large-gap Mott insulators in which the 3d{sup 5} high-spin ferric ions carry large local moments and magnetically order at T{sub N} > 600 K, undergo a sluggish structural first-order phase transition in the 30-50 GPa range, with the exception of the LuFeO{sub 3} which undergoes an isostructural volume reduction resulting from a high to low-spin crossover. High-pressure methods to 170 GPa using Moessbauer spectroscopy, resistance, and synchrotron-based XRD in diamond anvil cells were applied. Following the quasi-isostructural volume reduction (3-5%) the new phase the magnetic-ordering temperature is drastically reduced, to {approx}100 K, the direct and super-exchange interactions are drastically weakened, and the charge-transfer gap is substantially reduced. The high-pressure (HP) phases of the La and Pr oxides, at their inception, are composed of high- and low-spin Fe{sup 3+} magnetic sublattices, the abundance of the latter increasing with pressure but HP phases of the Eu, Y, and Lu oxides consist solely of low-spin Fe{sup 3+}. Resistance and Moessbauer studies in La and Pr orthoferrites reveal the onset of a metallic state with moments starting at P > 120 GPa. Based on the magnetic and electrical data of the latter species, a Mott phase diagram was established.

title: Spin transition of ferric iron in Al-bearing Mg–perovskite up to 200 GPa and its implication for the lower mantle
abstract: There are still large discrepancies among the previous reports on the spin transition of iron in Mg–perovskite (Pv). To alleviate this problem, we examined the spin state of Fe 3 + in Mg 0.85 Fe 3 + 0.15 Al 0.15 Si 0.85 O 3 Pv up to 200 GPa by X-ray emission spectroscopy (XES) and X-ray diffraction (XRD). The gradual decrease of the high spin (HS) ratio of Fe 3 + by low temperature annealing of the samples above ~ 60 GPa in XES and the change of the trend of unit cell volumes with pressure by annealing at 50–60 GPa in XRD indicate that Fe 3 + occupies the A-site and is HS below ~ 50 GPa but above 50–60 GPa it gradually replaces Al at the B-site through cation exchange reaction by annealing and becomes low spin (LS), while Fe 3 + remaining at the A-site is HS up to 200 GPa. This means that the spin state of Fe 3 + depends on Fe 3 + occupancies between the A- and B-sites and these Fe 3 + occupancies are strongly controlled by the synthesis condition and annealing temperature of the samples through the cation exchange reaction. The present results combined with the previous reports indicate that in Al-bearing Mg–Pv in the lower mantle Fe 2 + occupies the A-site and remains HS for the whole lower mantle, while Fe 3 + occupies the A-site and is HS below ~ 50 GPa but above 50–60 GPa it replaces Al at the B-site and becomes LS, on the assumption that spin transition pressure of Fe 2 + at the A-site is higher than that of Fe 3 + at the same site.

title: Electronic spin state of iron in lower mantle perovskite.
abstract: The electronic spin state of iron in lower mantle perovskite is one of the fundamental parameters that governs the physics and chemistry of the most voluminous and massive shell in the Earth. We present experimental evidence for spin-pairing transition in aluminum-bearing silicate perovskite (Mg,Fe)(Si,Al)O(3) under the lower mantle pressures. Our results demonstrate that as pressure increases, iron in perovskite transforms gradually from the initial high-spin state toward the final low-spin state. At 100 GPa, both aluminum-free and aluminum-bearing samples exhibit a mixed spin state. The residual magnetic moment in the aluminum-bearing perovskite is significantly higher than that in its aluminum-free counterpart. The observed spin evolution with pressure can be explained by the presence of multiple iron species and the occurrence of partial spin-paring transitions in the perovskite. Pressure-induced spin-pairing transitions in the perovskite would have important bearing on the magnetic, thermoelastic, and transport properties of the lower mantle, and on the distribution of iron in the Earth's interior.

title: Pressure effect on the electronic structure of iron in (Mg,Fe)(Si,Al)O3 perovskite: a combined synchrotron Mössbauer and X-ray emission spectroscopy study up to 100 GPa
abstract: We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg0.88Fe0.09)(Si0.94Al0.10)O3 between 1 bar and 100 GPa and at 300 K, using diamond cells and synchrotron Mössbauer spectroscopy techniques. At pressures below 12 GPa, our Mössbauer spectra can be sufficiently fitted by a “two-doublet” model, which assumes one ferrous Fe2+-like site and one ferric Fe3+-like site with distinct hyperfine parameters. The simplest interpretation that is consistent with both the Mössbauer data and previous X-ray emission data on the same sample is that the Fe2+-like site is high-spin Fe2+, and the Fe3+-like site is high-spin Fe3+. At 12 GPa and higher pressures, a “three-doublet” model is necessary and sufficient to fit the Mössbauer spectra. This model assumes two Fe2+-like sites and one Fe3+-like site distinguished by their hyperfine parameters. Between 12 and 20 GPa, the fraction of the Fe3+-like site, Fe3+/∑Fe, changes abruptly from about 50 to 70%, possibly due to a spin crossover in six-coordinate Fe2+. At pressures above 20 GPa, the fractions of all three sites remain unchanged to the highest pressure, indicating a fixed valence state of iron within this pressure range. From 20 to 100 GPa, the isomer shift between the Fe3+-like and Fe2+-like sites increases slightly, while the values and widths of the quadruple splitting of all three sites remain essentially constant. In conjunction with the previous X-ray emission data, the Mössbauer data suggest that Fe2+ alone, or concurrently with Fe3+, undergoes pressure-induced spin crossover between 20 and 100 GPa.

title: Spin transition, substitution, and partitioning of iron in lower mantle minerals
abstract: Spin transition and substitution of Fe3+ in Fe3+AlO3-bearing MgSiO3 perovskite (Pv) and post-perovskite (PPv) were examined up to 200 and 165 GPa, respectively, at room temperature by X-ray emission spectroscopy (XES) and XRD. The results of XES and XRD indicate that in Pv high spin (HS) Fe3+ at the dodecahedral (A) site replaces Al at the octahedral (B) site and becomes low spin (LS) between 50 and 70 GPa with pressure, while in PPv LS Fe3+ occupies the B-site and Al occupies the A-site above 80–100 GPa. The Fe3+–Al coupled substitution seems to be at work in both Pv and PPv. Combining these results on Fe3+ with the recent first-principles calculations on Fe2+ in Pv and PPv, the spin transition and substitution of iron in pyrolitic lower mantle minerals are proposed. Further, their effects on iron-partitioning among the lower mantle minerals are discussed.

title: Effect of iron oxidation state on the electrical conductivity of the Earth's lower mantle.
abstract: Iron can adopt different spin states in the lower mantle. Previous studies indicate that the dominant lower-mantle phase, magnesium silicate perovskite (which contains at least half of its iron as Fe(3+)), undergoes a Fe(3+) high-spin to low-spin transition that has been suggested to cause seismic velocity anomalies and a drop in laboratory-measured electrical conductivity. Here we apply a new synchrotron-based method of Mössbauer spectroscopy and show that Fe(3+) remains in the high-spin state in lower-mantle perovskite at conditions throughout the lower mantle. Electrical conductivity measurements show no conductivity drop in samples with high Fe(3+), suggesting that the conductivity drop observed previously on samples with high Fe(2+) is due to a transition of Fe(2+) to the intermediate-spin state. Correlation of transport and elastic properties of lower-mantle perovskite with electromagnetic and seismic data may provide a new probe of heterogeneity in the lower mantle.

title: The effect of Al 2 O 3 on Fe-Mg partitioning between magnesiowüstite and magnesium silicate perovskite
abstract: We have measured the partitioning of Fe2+, Fe3+ and Mg2+ between magnesiowustite and magnesium silicate perovskite in both Al2O3-bearing and Al2O3-free systems. Starting compositions with varying Fe/(Fe+Mg) molar ratios were equilibrated in a multianvil apparatus between 24 and 25 GPa and 1650–1900°C. A time study indicated that run durations of at least 8 h were required for the partitioning to reach equilibrium at 1650°C. Multi-chamber rhenium metal capsules were employed that provided relatively oxidising conditions and allowed up to eight starting materials to be equilibrated in a single experiment. In the Al2O3-free system Fe partitions preferentially into magnesiowustite. In the Al2O3-bearing system, however, the proportion of Fe that partitions into perovskite increases with the perovskite Al2O3 content. The Fe–Mg distribution between the phases has been parameterised as a function of the perovskite Al2O3 concentration and we observe the Al2O3 dependence to be non-linear. The Fe3+/ΣFe ratios of coexisting perovskite and magnesiowustite crystals were measured using electron energy-loss near-edge structure (ELNES) spectroscopy. In the Al2O3-bearing system perovskite Fe3+/ΣFe ratios range between 60 and 80%, which is over three times greater than that measured for Al2O3-free perovskite. The results imply that increasing the Al2O3 content of perovskite only increases the Fe3+ solubility and has no measurable effect on the Fe2+ partitioning between magnesiowustite and perovskite. The variation in the Fe3+ solubility with the Al3+ content of perovskite is non-linear. We propose that this results from substitution of Fe3+ onto the perovskite six-fold coordinated site for low Al3+ concentrations but substitution of a FeAlO3 component at higher Al3+ contents, where Fe3+ is on the eight-fold site charge balanced by Al on the six-fold coordinated site. A comparison with our experimental data suggests that the Fe partitioning between some (Mg,Fe)SiO3 and (Mg,Fe)O inclusions found in diamonds is quite consistent with their origin as perovskite and magnesiowustite in the lower mantle. The partitioning between some (Mg,Fe)O and (Mg,Fe,Al)(Al,Si)O3 diamond inclusions, however, is inconsistent with our results.

title: Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle
abstract: The oxidation state recorded by rocks from the Earth's upper mantle can be calculated from measurements of the distribution of Fe3+ and Fe2+ between the constituent minerals. The capacity for minerals to incorporate Fe3+ may also be a significant factor controlling the oxidation state of the mantle, and high-pressure experimental measurements of this property might provide important insights into the redox state of the more inaccessible deeper mantle. Here we show experimentally that the Fe3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe3+/total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle. Consequently, the lower mantle must either be enriched in Fe3+ or Fe3+ must form by the disproportionation of Fe2+ to produce Fe3+ plus iron metal. We argue that the lower mantle contains approximately 1 wt% of a metallic iron-rich alloy. The mantle's oxidation state and siderophile element budget have probably been influenced by the presence of this alloy.

title: Valence state and spin transitions of iron in Earth's mantle silicates
abstract: Using ab initio simulations, we investigated the valence and spin states of iron impurities in the perovskite (Pv) and postperovskite (PPv) polymorphs of MgSiO3. In agreement with the previous experimental work, we find a valence disproportionation reaction: 3Fe 2+ →2Fe 3+ Fe 0 metal. This exothermic reaction results in the predominance of Fe 3+ impurities in lower mantle silicates and produces free metallic iron. It occurs both in Pv and PPv, Al-free and Al-rich, at all lower mantle pressures. This reaction provides a possible mechanism for the growth of the Earth's core and core-mantle chemical equilibration. In the presence of Al 3+ , iron forms Fe 3+ –Al 3+ coupled substitutions in Pv, but separate Fe 3+ -Fe 3+ and Al 3+ -Al 3+ substitutions in PPv. Only the high-spin state is found for Fe 2+ impurities at all mantle pressures, while Fe 3+ impurities on the Si-site are low-spin at all pressures in both phases. Fe 3+ impurities on the Mg-site are in the high-spin state in PPv at all mantle pressures, but in Pv we predict a high-spin– low-spin transition. The pressure at which this transition occurs strongly depends on the Al 3+ content and according to our calculations increases from 76 GPa for Al-free to 134 GPa for aluminous Pv; this reconciles many of the previous experimental results. Our findings have implications for the chemical evolution of the Earth and for the radiative conductivity and dynamics of the D” layer.

title: MossA: a program for analyzing energy-domain Mössbauer spectra from conventional and synchrotron sources
abstract: The program MossA provides a straightforward approach to the fitting of 57Fe conventional and synchrotron energy-domain Mossbauer spectra. Sites can be defined simply by mouse clicks and hyperfine parameters can be constrained to constant values, within specific ranges, and can be coupled linearly between different subspectra. The program includes a full transmission integral fit with Lorentzian line shape (conventional source) or Lorentzian-squared line shape (synchrotron source). The fitting process is graphically displayed in real time while fitting and can be interrupted at any time. Gaussian-shaped quadrupole splitting distributions for analyzing nonmagnetic amorphous materials are included. MossA is designed especially for the rapid and comprehensive analysis of complex Mossbauer spectra, made possible by its native graphical user input.

title: Al, Fe substitution in the MgSiO3 perovskite structure: A single-crystal X-ray diffraction study
abstract: We have determined by single-crystal X-ray diffraction the crystal structure of three Fe–Al–MgSiO3 perovksite samples containing up to 9.5 wt% of Al2O3 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) MgA 2+ +S iB 4+ ⇔ FeA 3+ +A lB 3+ , where the excess of Fe is accommodated by (ii) MgA 2+ ⇔ FeA 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.

title: Phase relations in the system MgO‐FeO‐SiO2 to 50 GPa and 2000°C: An application of experimental techniques using multianvil apparatus with sintered diamond anvils
abstract: Phase relations in the system MgO-FeO-SiO2 were investigated between 22 and 47 GPa at 1500°C and 2000°C using multianvil apparatus with sintered diamond anvils. Synthesized samples were analyzed with electron microprobe, analytic transmission electron microscopy, and X-ray diffraction using synchrotron radiation. Univariant compositions of (Mg,Fe)SiO3 perovskite and (Mg,Fe)O magnesiowustite coexisting with SiO2 stishovite were determined as functions of pressure and temperature. The maximum iron solubility in perovskite corresponding to the univariant composition gradually increases with increasing pressure and temperature to be more than 30 mol % at 2000°C and pressures above 40 GPa, and a significant pressure effect was found in Fe-Mg partitioning between perovskite and magnesiowustite in pressures between 22 and 35 GPa. The iron content of magnesiowustite dramatically increases from 50 to greater than 90 mol % with increasing pressure, and the Fe-Mg distribution coefficients between perovskite and magnesiowustite, KD = (XFePv/XMgPv)/(XFeMw/XMgMw), decrease to less than 0.05. This significant pressure effect in Fe-Mg partitioning causes strong concentration of ferrous iron in magnesiowustite with increasing depth in the lower mantle.

title: Effect of Fe on the equation of state of mantle silicate perovskite over 1 Mbar
abstract: In order to investigate the effect of Fe on the equation of state (EOS), the volume of the perovskite (Pv) phases, (Mg1−xFex)SiO3, with different Fe contents ( x = 0 , 0.09, 0.15), has been measured at high pressures up to 1 Mbar in an Ar or NaCl medium. A bulk modulus ( K 0 ) ranging between 255 and 261 GPa is obtained when the Au scales consistent with the Pt scale are used and the data are fit to the second order Birch–Murnaghan (BM) equation. When the third order BM equation is used, we obtain K 0  = 250–264 GPa and K 0 ′  = 3.7–4.5, which is consistent with previous low-pressure X-ray diffraction and Brillouin measurements. Within experimental uncertainties (0.5% in volume and 6% in bulk modulus), we do not resolve differences in the EOS between Fe-bearing (up to 15%) and Fe-free Pv to 1 Mbar (2600-km depth in the mantle). This is in contrast with the much larger effect of the Fe spin transition for the EOS of ferropericlase (Fp) reported recently. This is perhaps due to much more diverse states of Fe (e.g., oxidation, coordination, and spin states) in Pv than in Fp.

title: Effect of Fe-enrichment on seismic properties of perovskite and post-perovskite in the deep lower mantle
abstract: Recent experimental measurements of the equation of state of perovskites and post-perovskites in the (Mg,Fe)SiO3 and (Mg,Fe,Al)(Fe,Al,Si)O-3 systems over a wide range of iron contents are used to constrain the effects of Fe and Al on density and bulk modulus of these phases at deep mantle pressures. The density of Fe-bearing perovskite follows a linear relationship with Fe-content at a representative mid-mantle depth of 1850 km (80 GPa): (80) (g cm(- 3)) = 5.054(1) + 1.270(3)X-Fe. The bulk modulus of silicate perovskite is not sensitive to Fe-content and follows the relationship, K-80 (GPa) = 546(2) + 12(25)X-Fe. The velocity heterogeneity parameter, partial derivative ln V-B/partial derivative X-Fe, determined by experimental values for the bulk sound speed is 0.10(1), in agreement with theory and the behaviour of other Fe-bearing silicates. Near the core-mantle boundary, Fe-rich post-perovskite is observed to be more compressible than the Mg-end-member, in contrast to theoretical predictions. From experimental data, the densities of perovskite and post-perovskite at 125 GPa (2700 km depth) are (125,Pv) (g cm(-3)) = 5.426(11) + 1.38(4)X-Fe and (125,pPv) (g cm(-3)) = 5.548(1) + 1.41(3)X-Fe. The density contrast across the post-perovskite transition is similar to 2 per cent, irrespective of Fe-content, but the contrast in bulk sound speed increases with Fe-content. Al-rich silicates exhibit no significant differences in density or compressibility relative to Al-free silicates, but may be responsible for seismic heterogeneities due to differences in the depth and width of the post-perovskite transition. Observations of increased densities in large low shear velocity provinces and ultra-low-velocity zones may be consistent with local iron enrichment from Mg#90 to Mg# 78-88 and Mg# < 50, respectively.

title: Iron Partitioning and Density Changes of Pyrolite in Earth’s Lower Mantle
abstract: Spin Transitions and Mantle Phases The extreme temperatures and pressures at depth can cause mantle minerals to undergo phase transformations. For example, it is thought that an electronic spin transition that occurs at high pressures in the lower mantle leads to the partitioning of various iron mineral phases. Using a multianvil apparatus to study a synthetic material similar in composition to the lower mantle, Irifune et al. (p. 193, see the Perspective by Hirose; published online 3 December) demonstrate that the spin transition occurs at lower pressures, and thus shallower depth, than suggested by earlier studies based on simpler compositions. As verification of the material's applicability in lower mantle studies, its density profile with pressure overlaps very well with those predicted by seismological models. Increasing the compositional complexity of mantle samples causes an electronic spin transition to occur at lower pressures. Phase transitions and the chemical composition of minerals in Earth’s interior influence geophysical interpretations of its deep structure and dynamics. A pressure-induced spin transition in olivine has been suggested to influence iron partitioning and depletion, resulting in a distinct layered structure in Earth’s lower mantle. For a more realistic mantle composition (pyrolite), we observed a considerable change in the iron-magnesium partition coefficient at about 40 gigapascals that is explained by a spin transition at much lower pressures. However, only a small depletion of iron is observed in the major high-pressure phase (magnesium silicate perovskite), which may be explained by preferential retention of the iron ion Fe3+. Changes in mineral proportions or density are not associated with the change in partition coefficient. The observed density profile agrees well with seismological models, which suggests that pyrolite is a good model composition for the upper to middle parts of the lower mantle.

title: Lattice thermal conductivity of MgSiO3 perovskite and post-perovskite at the core–mantle boundary
abstract: Thermal conductivity is essential for controlling the rate of core heat loss and long-term thermal evolution of the Earth, but it has been poorly constrained at the high pressures of Earth's lowermost mantle. We measured the lattice component of thermal diffusivity, heat transport by scattering of phonons, of both MgSiO 3 perovskite (Pv) and post-perovskite (PPv) at high pressures of up to 144 GPa and at room temperature. Lattice thermal conductivity of Pv-dominant lowermost mantle assemblage obtained in this study is about 11 W/m/K, while PPv-bearing rocks exhibit ∼60% higher conductivity. Since such Pv value is comparable to the conventionally assumed lowermost mantle conductivity, our findings do not significantly alter but support the recent notion of high core–mantle boundary heat flow along with a young inner core and high temperatures in the early deep Earth.

title: Element partitioning between magnesium silicate perovskite and ferropericlase: New insights into bulk lower-mantle geochemistry
abstract: n this study, we investigated iron–magnesium exchange and transition-metal trace-element partitioning between magnesium silicate perovskite (Mg,Fe)SiO3 and ferropericlase (Mg,Fe)O synthetised under lower-mantle conditions (up to 115 GPa and 2200 K) in a laser-heated diamond anvil cell. Recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy (ATEM) and nanometer-scale secondary ion mass spectroscopy (nanoSIMS). Iron concentrations in both phases were obtained from X-ray energy dispersive spectroscopy measurements and nanoSIMS. Our results are the first to show that recently reported spin-state and phase transitions in the lower mantle directly affect the evolution of Fe–Mg exchange between both phases. Mg-perovskite becomes increasingly iron-depleted above 70–80 GPa possibly due to the high spin–low spin transition of iron in ferropericlase. Conversely, the perovskite to post-perovskite transition is accompanied by a strong iron enrichment of the silicate phase, ferropericlase remaining in the Fe-rich phase though. Nanoparticles of metallic iron were observed in the perovskite-bearing runs, suggesting the disproportionation of ferrous iron oxide, but were not observed when the post-perovskite phase was present. Implications on the oxidation state of the Earth and core segregation will be discussed. Transition trace-element (Ni, Mn) concentrations (determined with the nanoSIMS) show similar trends and could thus be used to trace the origin of diamonds generated at depth. This study provides new results likely to improve the geochemical and geophysical models of the Earth's deep interiors.

title: Iron partitioning between perovskite and post-perovskite: A transmission electron microscope study
abstract: The effect of iron on the post-perovskite phase transition has been controversial. We have performed direct chemical analyses of co-existing perovskite and post-perovskite that were synthesized from an (Mg0.91Fe0.09)SiO3 bulk composition using a laser-heated diamond anvil cell at pressures above 100 GPa and temperatures of 1700–1800 K. Analysis on quenched samples was carried out using the transmission electron microscope (TEM). The results demonstrate that crystalline perovskite grains are enriched in iron compared to adjacent amorphous parts presumably converted from post-perovskite. This indicates that ferrous iron stabilizes perovskite to higher pressures. The ferrous and ferric irons are likely to have competing effects on the post-perovskite phase transition, and therefore the effect of iron may be controlled by aluminum.

title: Partitioning of iron between perovskite/postperovskite and ferropericlase in the lower mantle
abstract: The Fe2+-Mg distribution coefficients (KD) between perovskite/postperovskite and ferropericlase were determined in a (Mg0.9Fe0.1)2SiO4 bulk composition. High-pressure and high-temperature experiments were carried out between 36 and 128 GPa and 1760–2170 K in a laser-heated diamond anvil cell (LHDAC). Chemical analyses were made on recovered samples with transmission electron microscope (TEM) and field-emission-type scanning electron microscope (FE-SEM). All the samples exhibited a strong heterogeneity in iron content caused by relatively large temperature gradient during laser-heating, being depleted at the center of the hot spot and enriched at the margin. The KD values measured from coexisting perovskite/postperovskite and ferropericlase showed apparently very large variation in each sample, which could be the source of considerable discrepancy among previous measurements of KD using the LHDAC techniques. Nevertheless, the data obtained from a specific portion of the sample that retained the original bulk composition may represent the equilibrium partitioning. Present results demonstrate that the iron partitioning between perovskite/postperovskite and ferropericlase changes little at a given temperature throughout the lower mantle pressures. Both electronic transition of iron and postperovskite phase transition have small effects on iron partitioning.

title: Fe-Mg partitioning between perovskite and ferropericlase in the lower mantle
abstract: Fe-Mg partitioning between perovskite and ferropericlase in the MgO-FeO-SiO2 system has been studied up to about 100 GPa at around 2000 K using a laser-heated diamond anvil cell (LHDAC). The compositions of both phases were determined by using analytical transmission electron microscopy (ATEM) on the recovered samples. Present results reveal that the Fe-Mg apparent partition coefficient between perovskite and ferropericlase [KDPv/Fp = (XFePv XMgFp)/(XMgPv XFeFp)] decreases with increasing pressure for a constant FeO of the system, and it decreases with increasing FeO content of ferropericlase. The gradual decrease of KDPv/Fp with increasing pressure is consistent with the spin transition in ferropericlase occurring in the broad pressure range from 50 to 100 GPa at around 2000 K.

title: Fe–Mg partitioning between post-perovskite and ferropericlase in the lowermost mantle
abstract: Fe–Mg partitioning between post-perovskite and ferropericlase has been studied using a laser-heated diamond anvil cell at pressures up to 154 GPa and 2,010 K which corresponds to the conditions in the lowermost mantle. The composition of the phases in the recovered samples was determined using analytical transmission electron microscopy. Our results reveal that the Fe–Mg partition coefficient between post-perovskite and ferropericlase (KDPPv/Fp) increases with decreasing bulk iron content. The compositional dependence of KDPPv/Fp on the bulk iron content explains the inconsistency in previous studies, and the effect of the bulk iron content is the most dominant factor compared to other factors, such as temperature and aluminum content. Iron prefers ferropericlase compared to post-perovskite over a wide compositional range, whereas the iron content of post-perovskite (XFePPv, the mole fraction) does not exceed a value of 0.10. The iron-rich ferropericlase phase may have significant influence on the physical properties, such as the seismic velocity and electrical conductivity at the core–mantle boundary region.

title: Disproportionation of (Mg,Fe)SiO3 perovskite in Earth’s deep lower mantle
abstract: The mineralogical constitution of the Earth’s mantle dictates the geophysical and geochemical properties of this region. Previous models of a perovskite-dominant lower mantle have been built on the assumption that the entire lower mantle down to the top of the D″ layer contains ferromagnesian silicate [(Mg,Fe)SiO3] with nominally 10 mole percent Fe. On the basis of experiments in laser-heated diamond anvil cells, at pressures of 95 to 101 gigapascals and temperatures of 2200 to 2400 kelvin, we found that such perovskite is unstable; it loses its Fe and disproportionates to a nearly Fe-free MgSiO3 perovskite phase and an Fe-rich phase with a hexagonal structure. This observation has implications for enigmatic seismic features beyond ~2000 kilometers depth and suggests that the lower mantle may contain previously unidentified major phases. A presumed dominant mineral in Earth’s interior can disassociate into two phases at lower mantle conditions [Also see Perspective by Williams] Delving deeper into the lower mantle Earth's lower mantle is an enigmatic region, a transition zone between slowly churning solids and a liquid outer core. Large seismic structures and discontinuities in this region are probably due to sharp gradients in temperature, composition, or mineralogy. Teasing apart the precise effects of these factors requires experiments at lower mantle temperatures and pressures (see the Perspective by Williams). Zhang et al. found that the major mineral phase of the lower mantle decomposes into two minerals. Andrault et al. show how the melting of subducted basalt from the oceanic crust will form pile-like structures on top of the core/mantle boundary. Science, this issue p. 877, p. 892; see also p. 800.

title: Structural Polymorphism in Al-bearing Magnesiumsilicate Post-perovskite
abstract: aB s T rac T In the present study, we summarize indications for the existence of kinked post-perovskite structures in the MAS system. X-ray diffraction data and Raman spectra of aluminous magnesium metasilicate post-perovskite are inconsistent with the CaIrO3 structure. Instead the observations are consistent with structures intermediate between the perovskite and the CaIrO3 structure. Ab initio calculations show that the enthalpies of the kinked structures are slightly higher than the CaIrO3 structure at 0 K. Finite temperature, minor element chemistry, kinetics of phase transformation, and actual stress regime are plausible reasons for the observed differences between the present and the previously reported postperovskite phases.