Spin Transition Of Iron Research Articles

Elastic Softening of (Mg0.8Fe0.2)O Ferropericlase Across the Iron Spin Crossover Measured at Seismic Frequencies

AbstractWe experimentally determined the bulk modulus of (Mg0.8Fe0.2)O ferropericlase across the iron spin transition and in the low‐spin phase by employing a new experimental approach. In our measurements, we simulate the propagation of a compressional seismic wave (P wave) through our sample by employing a piezo‐driven dynamic diamond anvil cell that allows to oscillate pressure at seismic frequencies. During pressure oscillations, X‐ray diffraction images were continuously collected every 5–50 ms. The bulk modulus is directly calculated from these data at different pressures. Our experiments show a pronounced softening of the bulk modulus throughout the spin crossover, supporting previous single‐crystal measurements at very high frequencies and computations. Comparison of our results to previous data collected on (Mg,Fe)O with lower iron contents shows that the magnitude of softening strongly depends on iron content. Our experiments at seismic frequencies confirm that the iron spin crossover markedly affects the ratio of seismic compressional to shear wave velocities in Earth's lower mantle.

Effects of Iron Spin Transition on the Structure and Stability of Large Primordial Reservoirs in Earth's Lower Mantle

AbstractExperimental and theoretical studies have shown that the iron spin transition alters the properties of lower mantle minerals. This may have important implications for mantle dynamics. In particular, the vigor of convection is enhanced, which in turn may impact the stability of large primordial reservoirs at the base of the lower mantle. Here we performed numerical experiments of thermochemical convection in 2‐D annulus geometry including the change of density induced by iron spin transition. Our results show that this density change only slightly affects mantle dynamics by increasing the convection vigor. This, in turn, slightly increases the entrainment of primordial dense materials by plumes and leads to smaller reservoirs with sharper edges. However, this effect is small compared to those of the intrinsic density contrast between the dense primordial and regular mantle materials, which is the dominant parameter controlling the long‐term stability of the large primordial reservoirs.

Viscosity jump in the lower mantle inferred from melting curves of ferropericlase

Convection provides the mechanism behind plate tectonics, which allows oceanic lithosphere to be subducted into the mantle as “slabs” and new rock to be generated by volcanism. Stagnation of subducting slabs and deflection of rising plumes in Earth’s shallow lower mantle have been suggested to result from a viscosity increase at those depths. However, the mechanism for this increase remains elusive. Here, we examine the melting behavior in the MgO–FeO binary system at high pressures using the laser-heated diamond-anvil cell and show that the liquidus and solidus of (MgxFe1−x)O ferropericlase (x = ~0.52–0.98), exhibit a local maximum at ~40 GPa, likely caused by the spin transition of iron. We calculate the relative viscosity profiles of ferropericlase using homologous temperature scaling and find that viscosity increases 10–100 times from ~750 km to ~1000–1250 km, with a smaller decrease at deeper depths, pointing to a single mechanism for slab stagnation and plume deflection.

Spin transition of ferric iron in the calcium‐ferrite type aluminous phase

AbstractWe investigated Fe‐free and Fe‐bearing CF phases using nuclear forward scattering and X‐ray diffraction coupled with diamond anvil cells up to 80 GPa at room temperature. Octahedral Fe3+ ions in the Fe‐bearing CF phase undergo a high‐spin to low‐spin transition at 25–35 GPa, accompanied by a volume reduction of ~2.0% and a softening of bulk sound velocity up to 17.6%. Based on the results of this study and our previous studies, both the NAL and CF phases, which account for 10–30 vol % of subducted MORB in the lower mantle, are predicted to undergo a spin transition of octahedral Fe3+ at lower mantle pressures. Spin transitions in these two aluminous phases result in an increase of density of 0.24% and a pronounced softening of bulk sound velocity up to 2.3% for subducted MORB at 25–60 GPa and 300 K. The anomalous elasticity region expands and moves to 30–75 GPa at 1200 K and the maximum of the VΦ reduction decreases to ~1.8%. This anomalous elastic behavior of Fe‐bearing aluminous phases across spin transition zones may be relevant in understanding the observed seismic signatures in the lower mantle.

Fine-scale structure of the mid-mantle characterised by global stacks of PP precursors

Subduction zones are likely a major source of compositional heterogeneities in the mantle, which may preserve a record of the subduction history and mantle convection processes. The fine-scale structure associated with mantle heterogeneities can be studied using the scattered seismic wavefield that arrives as coda to or as energy preceding many body wave arrivals. In this study we analyse precursors to PP by creating stacks recorded at globally distributed stations. We create stacks aligned on the PP arrival in 5° distance bins (with range 70–120°) from 600 earthquakes recorded at 193 stations stacking a total of 7320 seismic records. As the energy trailing the direct P arrival, the P coda, interferes with the PP precursors, we suppress the P coda by subtracting a best fitting exponential curve to this energy. The resultant stacks show that PP precursors related to scattering from heterogeneities in the mantle are present for all distances. Lateral variations are explored by producing two regional stacks across the Atlantic and Pacific hemispheres, but we find only negligible differences in the precursory signature between these two regions. The similarity of these two regions suggests that well mixed subducted material can survive at upper and mid-mantle depth. To describe the scattered wavefield in the mantle, we compare the global stacks to synthetic seismograms generated using a Monte Carlo phonon scattering technique. We propose a best-fitting layered heterogeneity model, BRT2017, characterised by a three layer mantle with a background heterogeneity strength (ϵ=0.8%) and a depth-interval of increased heterogeneity strength (ϵ=1%) between 1000 km and 1800 km. The scalelength of heterogeneity is found to be 8 km throughout the mantle. Since mantle heterogeneity of 8 km scale may be linked to subducted oceanic crust, the detection of increased heterogeneity at mid-mantle depths could be associated with stalled slabs due to increases in viscosity, supporting recent observations of mantle viscosity increases due to the iron spin transition at depths of ∼1000 km.

Optical signatures of low spin Fe3+ in NAL at high pressure

AbstractThe iron spin transition directly affects properties of lower mantle minerals and can thus alter geophysical and geochemical characteristics of the deep Earth. While the spin transition in ferropericlase has been documented at P ~60 GPa and 300 K, experimental evidence for spin transitions in other rock‐forming minerals, such as bridgmanite and post‐perovskite, remains controversial. Multiple valence, spin, and coordination states of iron in bridgmanite and post‐perovskite are difficult to resolve with conventional spin probing techniques. Optical spectroscopy, on the other hand, can discriminate between high and low spin and between ferrous and ferric iron at different sites. Here we establish the optical signature of low spin Fe3+O6, a plausible low spin unit in bridgmanite and post‐perovskite, by optical absorption experiments in diamond anvil cells. We show that the optical absorption of Fe3+O6 in new aluminous phase (NAL) is very sensitive to the iron spin state and may represent a model behavior of bridgmanite and post‐perovskite across the spin transition. Specifically, an absorption band centered at ~19,000 cm−1 is characteristic of the 2T2g → 2T1g (2A2g) transition in low spin Fe3+ in NAL at 40 GPa, constraining the crystal field splitting energy of low spin Fe3+ to ~22,200 cm−1, which we independently confirm by first‐principles calculations. Together with available information on the electronic structure of Fe3+O6 compounds, we show that the spin‐pairing energy of Fe3+ in an octahedral field is ~20,000–23,000 cm−1. This implies that octahedrally coordinated Fe3+ in bridgmanite is low spin at P > ~40 GPa.

Raman spectroscopy of siderite at high pressure: Evidence for a sharp spin transition

We have measured high-pressure Raman spectra of both siderite single-crystalline and polycrystalline powder samples in diamond-anvil cell experiments across the pressure-induced high-spin (HS) to low-spin (LS) transition of Fe 2+ . Between 43.3 and 45.5 GPa, we observed a color change from transparent to green, which is associated to the spin transition. Furthermore, we calibrated the position of the Raman active ν 1 mode with pressure. In a second diamond-anvil cell experiment, we observed the color change from transparent to green in the form of a transition front passing through the single crystal and collected Raman spectra across the transition front. We were able to constrain the stress variation across this transition front to about 0.2 GPa, well below the resolution of our Raman-based pressure/stress calibration. In contrast to the single crystal, the powder sample shows the spin transition over a pressure range of 5 GPa, which we attribute to intergranular stresses. We conclude that within the resolution of our stress/pressure calibration the spin transition of iron in single-crystalline siderite is sharp.

Mid-mantle heterogeneities and iron spin transition in the lower mantle: Implications for mid-mantle slab stagnation

Recent high pressure experimental results reveal that the elastic and transport properties of mantle materials are impacted by the electronic spin transition in iron under lower mantle pressure and temperature conditions. The electronic transition in ferropericlase (Fp), the second major constituent mineral of the lower mantle material, is associated with a smooth increase in density starting from the mid-mantle depth to the core–mantle boundary (CMB). The transition also yields softening in the elastic moduli and an increase in the thermal expansivity over the transition zone in the lower mantle. Although there is not yet robust experimental evidence for spin-transition induced density change in the perovskite (Pv) phase (the major constituent mineral in the lower mantle), the spin transition in the octahedral (B) site in Al-free perovskite causes a bulk modulus hardening (increase in the bulk modulus) in the mineral. We have incorporated these physical processes into high resolution 3D-spherical control volume models for mantle convection. A series of numerical experiments explore how the electronic spin transition in iron modifies the mantle flow, and in particular the fate of sinking cold slabs. Such mid-mantle stagnations are prevalent globally in seismic tomographic inversions, but previous explanations for their existence are not satisfactory. Employing density anomalies from the iron spin transition in ferropericlase and density anomaly models for perovskite, we study the influence of the spin transition in the minerals of the lower mantle on mantle flow. Our model results reveal that while the spin transition-induced property variations in ferropericlase enhance mixing in the lower depths of the mantle, the density anomaly arising from the hardening in the bulk modulus of Al-free perovskite can be effective in slowing the descent of slabs and may cause stagnation at mid-mantle levels. A viscosity hill in the lower mantle may further enhance the stagnation effect. Cold descending slabs can stall in the mid mantle for tens of million years or even longer before penetrating to the lower mantle.

Spawning superplumes from the midmantle: The impact of spin transitions in the mantle

AbstractThe formation of large‐scale upwellings with lateral extents of several hundreds of kilometers, reaching up to ∼10,000 km or more, still remains a hotly debated topic. Some seismic imaging studies based on high‐resolution data suggest that the main superplumes underneath Africa and South‐central Pacific are clusters, composed of several individual plumes rather than being a single large mantle upwelling. The iron spin transition in the lower mantle minerals may present a new idea on the origin and the formation of such superplumes, notably sourcing such features in the midmantle. Stagnation of both cold sinking slabs and hot rising plumes can be caused by density and viscosity variation due to the spin transition in iron in ferropericlase (Fp) and a possible spin‐dependent bulk modulus hardening in bridgmanite silicate perovskite (Pv). This process produces intermittent downward spin transition‐induced midmantle avalanches (SIMMA) of the cold sinking flow as well as upward spin transition‐induced midmantle superplume avalanches (SIMMSA) of the rising hot plumes, triggered at the spin transition‐induced thermal boundary layer at around 1600 km depth. Our high‐resolution axi‐symmetric models reveal that the hot upwellings, trapped below ∼1600 km depth, can suddenly penetrate into the upper levels in the mantle and spread laterally for hundreds of kilometres. Owing to the upward penetration of the midmantle‐rooted superplumes, as broad as ∼1500 km across, a large amount of heat can be delivered to the upper mantle and base of the lithosphere with implications for large volcanic episodes.

Two‐stage spin transition of iron in FeAl‐bearing phase D at lower mantle

AbstractHydrous magnesium silicate phase D plays a key role in the transport of water from the upper to the lower mantle via subducted slabs. Here we report pressure dependence hyperfine and lattice parameters of FeAl‐bearing phase D up to megabar pressures using synchrotron nuclear forward scattering and X‐ray diffraction in a diamond anvil cell at room temperature. FeAl‐bearing phase D undergoes a two‐stage high‐spin to low‐spin transition of iron for Fe2+ at 37–41 GPa and for Fe3+ at 64–68 GPa. These transitions are accompanied by an increase in density and a significant softening in the bulk modulus and bulk velocity at their respective pressure range. The occurrence of the dense low‐spin FeAl‐bearing phase D with relatively high velocity anisotropies in deep‐subducted slabs can potentially contribute to small‐scale seismic heterogeneities in the middle‐lower mantle beneath the circum‐Pacific area.

Elasticity of single‐crystal NAL phase at high pressure: A potential source of the seismic anisotropy in the lower mantle

AbstractThe new hexagonal aluminous phase, named the NAL phase, is expected to be stable at depths of <1200 km in subducted slabs and believed to constitute 10~30 wt% of subducted mid‐ocean ridge basalt together with the CaFe2O4‐type aluminous phase. Here elasticity of the single‐crystal NAL phase is investigated using Brillouin light scattering coupled with diamond anvil cells up to 20 GPa at room temperature. Analysis of the results shows that the substitution of iron lowers the shear modulus of the NAL phase by ~5% (~6 GPa) but does not significantly affect the adiabatic bulk modulus. The NAL phase exhibits high‐velocity anisotropies with AVP = 14.7% and AVS = 15.12% for the Fe‐bearing phase at ambient conditions. The high AVS of the NAL phase mainly results from the high anisotropy of the faster VS1 (13.9~15.8%), while the slower VS2 appears almost isotropic (0.1~2.8%) at ambient and high pressures. The AVP and AVS of the NAL phase decrease with increasing pressure but still have large values with AVP = 11.4% and AVS = 14.12% for the Fe‐bearing sample at 20.4 GPa. The extrapolated AVP and AVS of the Fe‐free and Fe‐bearing NAL phases at 40 GPa are larger than those of bridgmanite at the same pressure. Together with its spin transition of iron and structural transition to the CF phase, the presence of the NAL phase with high‐velocity anisotropies may contribute to the observed seismic anisotropy around subducted slabs in the uppermost lower mantle.

Guest-Dependent Spin-Transition Behavior of Porous Coordination Polymers.

The host-guest composites of Hofmann-type iron(II) spin-transition (ST) porous coordination polymers incorporating guest molecules show guest-dependent ST behavior in accordance with the respective guest species, which may be a gas, solvent, halogen, or organic molecule. The guest also works as a chemical stimulant to switch the spin state of the host between high and low spin at room temperature. In this review, we discuss guest properties including size, shape, flexibility, chemical properties, and pore loading content, which impact the spin states of the host framework and the ST behavior exhibited by the host-guest composites.

Abrupt Spin Transition and Chiral Hydrogen-Bonded One-Dimensional Structure of Iron(III) Complex [FeIII(Him)2(hapen)]SbF6 (Him = imidazole, H2hapen = N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine)

Solvent-free spin crossover (SCO) iron(III) complex, [FeIII(Him)2(hapen)]SbF6 (Him = imidazole, H2hapen = N,N′-bis(2-hydroxyacetophenylidene)ethylenediamine), is synthesized. The FeIII ion has an octahedral coordination geometry, with N2O2 donor atoms of hapen and N2 atoms of two imidazoles at the axial positions. The saturated five-membered chelate ring of hapen moiety assumes a gauche-type δ- or λ-conformation to give chiral species of δ-[FeIII(Him)2(hapen)]+ or λ-[FeIII(Him)2(hapen)]+. One imidazole is hydrogen-bonded to phenoxo oxygen atom of hapen of the adjacent unit to give a hydrogen-bonded chiral one-dimensional structure, {δ-[FeIII(Him)2(hapen)]+}1∞ or {λ-[FeIII(Him)2(hapen)]+}1∞. The adjacent chains with the opposite chiralities are arrayed alternately. The temperature dependences of the magnetic susceptibilities revealed an abrupt one-step spin transition between high-spin (S = 5/2) and low-spin (S = 1/2) states at the spin transition temperature of T1/2 = 105 K. The crystal structures were determined at 296 and 100 K, where the populations of HS:LS of high- and low-spin ratio are evaluated to be 1:0 and 0.3:0.7, respectively, based on magnetic measurements. During the spin transition from 296 K to 100 K, the average Fe–N distance and O–Fe–O angle decrease to a regular octahedron by 0.16 Å and 13.4°, respectively. The structural change in the coordination environment is transmitted to the adjacent spin crossover (SCO) sites along the chiral 1D chain through hydrogen-bonds. The abrupt SCO profile and the spin transition temperature for the isomorphous compounds [FeIII(Him)2(hapen)]Y (Y = PF6, AsF6, SbF6) are ascribed to the chiral hydrogen-bonded 1D structure and chain-anion interaction.

Spin transition of ferric iron in the NAL phase: Implications for the seismic heterogeneities of subducted slabs in the lower mantle

Al-rich phases (NAL: new hexagonal aluminous phase and CF: calcium–ferrite phase) are believed to constitute 10∼30 wt% of subducted mid-ocean ridge basalt (MORB) in the Earth's lower mantle. In order to understand the effects of iron on compressibility and elastic properties of the NAL phase, we have studied two single-crystal samples (Fe-free Na1.14Mg1.83Al4.74Si1.23O12 and Fe-bearing Na0.71Mg2.05Al4.62Si1.16Fe2+0.09Fe3+0.17O12) using synchrotron nuclear forward scattering (NFS) and X-ray diffraction (XRD) combined with diamond anvil cells up to 86 GPa at room temperature. A pressure-induced high-spin (HS) to low-spin (LS) transition of the octahedral Fe3+ in the Fe-bearing NAL is observed at approximately 30 GPa by NFS. Compared to the Fe-free NAL, the Fe-bearing NAL undergoes a volume reduction of 1.0% (∼1.2 Å3) at 33∼47 GPa as supported by XRD, which is associated with the spin transition of the octahedral Fe3+. The fits of Birch–Murnaghan equation of state (B–M EoS) to P–V data yield unit-cell volume at zero pressure V0=183.1(1) Å3 and isothermal bulk modulus KT0=233(6) GPa with a pressure derivative KT0′=3.7(2) for the Fe-free NAL; V0-HS=184.76(6) Å3 and KT0-HS=238(1) GPa with KT0-HS′=4 (fixed) for the Fe-bearing NAL. The bulk sound velocities (VΦ) of the Fe-free and Fe-bearing NAL phase are approximately 6% larger than those of Al, Fe-bearing bridgmanite and calcium silicate perovskite in the lower mantle, except for the spin transition region where a notable softening of VΦ with a maximum reduction of 9.4% occurs in the Fe-bearing NAL at 41 GPa. Considering the high volume proportion of the NAL phase in subducted MORB, the distinct elastic properties of the Fe-bearing NAL phase across the spin transition reported here may provide an alternative plausible explanation for the observed seismic heterogeneities of subducted slabs in the lower mantle at depths below 1200 km.

The impacts of mantle phase transitions and the iron spin crossover in ferropericlase on convective mixing—is the evidence for compositional convection definitive? New results from a Yin‐Yang overset grid‐based control volume model

AbstractHigh‐resolution seismic tomographic images from several subduction zones provide evidence for the inhibition of the downwelling of subducting slabs at the level of the 660 km depth seismic discontinuity. Furthermore, the inference of old (~140 Myr) sinking slabs below fossil subduction zones in the lower mantle has yet to be explained. We employ a control volume methodology to develop a new anelastically compressible model of three‐dimensional thermal convection in the “mantle” of a terrestrial planet that fully incorporates the influence of large variations in material properties. The model also incorporates the influence of (1) multiple solid‐solid pressure‐induced phase transitions, (2) transformational superplasticity at 660 km depth, and (3) the high spin‐low spin iron spin transition in ferropericlase at midmantle pressures. The message passing interface‐parallelized code is successfully tested against previously published benchmark results. The high‐resolution control volume models exhibit the same degree of radial layering as previously shown to be characteristic of otherwise identical 2‐D axisymmetric spherical models. The layering is enhanced by the presence of moderate transformational superplasticity, and in the presence of the spin crossover in ferropericlase, stagnation of cold downwellings occurs in the range of spin crossover depths (~1700 km). Although this electronic spin transition has been suggested to be invisible seismically, recent high‐pressure ab initio calculations suggest it to have a clear signature in body wave velocities which could provide an isochemical explanation of a seismological signature involving the onset of decorrelation between Vp and Vs that has come to be interpreted as requiring compositional layering.

Iron-spin transition controls structure and stability of LLSVPs in the lower mantle

Seismic tomography models have revealed that there exist two large low shear velocity provinces (LLSVPs) beneath Africa and the Pacific Ocean in the Earth's lower mantle. Waveform modeling results suggest both LLSVPs have steep sharp side boundaries, which imply that they probably are compositionally heterogeneous from the ambient mantle. When applying the surface plate motion history in the last a few hundred million years (Ma) as the driving mechanism, numerical modeling has successfully reproduced the geographical distribution of the two LLSVPs in thermochemical mantle convection. However, two prominent seismic features of the LLSVPs, the steep side boundaries and the high elevation, can hardly be obtained in previous geodynamic models. Here, we include in our mantle convection model the effects of iron-spin transition of ferropericlase which substantially change physical properties of the mantle. Our results show that iron-spin transition plays a dominant role in controlling the structure and stability of LLSVPs. Large chemical blocks with steep side boundaries and high elevations up to ∼1200 km above core–mantle boundary (CMB) emerge in our models with the volume content of ferropericlase ∼20%. Such blocks cause a shear wave velocity decrease of ∼3.5% which is consistent with the seismic observations. Our results also show that these LLSVPs are transient structures in the lower mantle, which can typically last for a few hundred million years before destroyed by large-scale mantle motion.

Viscosity undulations in the lower mantle: The dynamical role of iron spin transition

A proper determination of the lower-mantle viscosity profile is fundamental to understanding Earth geodynamics. Based on results coming from different sources, several models have been proposed to constrain the variations of viscosity as a function of pressure, stress and temperature. While some models have proposed a relatively modest viscosity variation across the lower mantle, others have proposed variations of several orders of magnitude. Here, we have determined the viscosity of ferropericlase, a major mantle mineral, and explored the role of the iron high-to-low spin transition. Viscosity was described within the elastic strain energy model, in which the activation parameters are obtained from the bulk and shear wave velocities. Those velocities were computed combining first principles total energy calculations and the quasi-harmonic approximation. As a result of a strong elasticity softening across the spin transition, there is a large reduction in the activation free energies of the materials creep properties, leading to viscosity undulations. These results suggest that the variations of the viscosity across the lower mantle, resulting from geoid inversion and postglacial rebound studies, may be caused by the iron spin transition in mantle minerals. Implications of the undulated lower mantle viscosity profile exist for both, down- and up-wellings in the mantle. We find that a viscosity profile characterized by an activation free energy of G⁎(z0)∼300–400 kJ/mol based on diffusion creep and dilation factor δ=0.5 better fits the observed high velocity layer at mid mantle depths, which can be explained by the stagnation and mixing of mantle material. Our model also accounts for the growth of mantle plume heads up to the size necessary to explain the Large Igneous Provinces that characterize the start of most plume tracks.

High-pressure orthorhombic ferromagnesite as a potential deep-mantle carbon carrier.

Knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite [(Mg,Fe)CO3] at high pressure and temperature of the deep mantle is necessary for our understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO3 system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO2, the occurrence of the tetrahedrally-coordinated carbonates based on CO4 structural units, and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO3 carbonates up to relevant lower-mantle conditions of approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO3 phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth.

Theoretical modeling of two-step spin-crossover transitions in FeII dinuclear systems

Calculations using the hybrid meta-GGA TPSSh exchange–correlation functional have been employed to model the spin-transition in dinuclear iron(ii) systems.

Optical properties of (Mg0.97, Fe0.03)O ferropericlase under the pressure of the Earth’s lower mantle

The optical-absorption and refractive-index properties of (Mg0.97, Fe0.03)O ferropericlase crystals without and with Mg and O ionic divacancy point-defect under the pressure of the Earth’s lower mantle are investigated using the first-principles calculations. Optical-absorption data show that the perfect-crystal results are similar to the predictions from the crystal-field theory:the pressure-induced spin transition of iron in ferropericlase causes a large blue-shift in its optical-absorption spectrum, leaving the near-infrared region transparent. However, when there are point defects in ferropericlase, the calculated optical-absorption results are completely inconsistent with predictions from the crystal-field theory, the spin transition causes the enhancement in the optical absorption in the near-infrared region. Refractive-index data of defect crystal indicate that the effects of pressure, wavenumber, and spin-transition on the high-pressure refractive-index of (Mg0.97, Fe0.03)O ferropericlase are obvious, but perfect-crystal results show that those effects should be relatively weak. The ～15%-20% iron-bearing ferropericlase is currently considered as an important mineral in the Earth’s lower mantle. Due to similar characteristics of the observed high-pressure optical-absorption spectrum in ferropericlase with different iron content, we suggest that:(1) the above-mentioned calculated results is conducive to the understanding of high-pressure optical properties of lower-mantle ferropericlase and the exploring of the origin of discrepancies in its high-pressure optical-absorption spectrum between experiment and crystal-field theory; (2) the high-pressure optical-absorption spectrum measurements may be a good approach for probing iron spin state.