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

Abstract

The 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.

Electronic origins of the giant volume collapse in the pyrite mineral MnS2

“…Here, the position of the Fe K-edge pre-peak was shown to track the collapse in Fe-O bond lengths extracted from EXAFS analysis. Although only the average pre-peak position was extracted throughout the entire pressure range, a doublet structure can also clearly been seen in the low-pressure phase, with a splitting of ∼2.5 eV, which is remarkably close to the crystal-field splitting of 2.75 eV extracted for Fe 3+ by optical means 45 . If we assume that the splitting we observe in MnS 2 reflects the crystal field, then this immediately rules out simple toy models for the spin-state transition.…”

Anomalous thermal properties and spin crossover of ferromagnesite (Mg,Fe)CO3

“…In addition, volume and elastic anomalies accompanying SCO have been observed via x-ray diffraction [16,17,[25][26][27][28][29] and Brillouin scattering [30,31] experiments, respectively. In the above-mentioned room-temperature (T = 300 K) experiments, SCO typically starts at [40][41][42][43][44][45][46][47][48][49] GPa and finishes at 46-56 GPa; the typical width of the SCO region is 5-10 GPa. An exception is observed via Mössbauer spectroscopy, indicating an SCO region of 52-61 GPa [15].…”

“…Further geophysical and geochemical effects of SCO have been anticipated [35,36]. In addition to ferropericlase, bridgmanite, and ferromagnesite, a few more minerals of potential geophysical and geochemical importance have also been reported to undergo SCO, including Fe-bearing new hexagonal aluminous (NAL) phase NaMg 2 (Si,Al) 6 O 12 [39][40][41], calcium-ferrite aluminous (CF) phase (Na,Mg)(Si,Al) 2 O 4 [42], and pyrite-type FeO 2 H y (0 ≤ y ≤ 1) [43,44]. Despite a variety of mantle minerals are subject to SCO, studies for their thermal properties during SCO at high P -T conditions have been scarce.…”

Radiometric Temperature Measurements in Nongray Ferropericlase With Pressure‐ Spin‐ and Temperature‐Dependent Optical Properties

“…Wavelength‐dependent emissivity of mantle minerals has never been characterized at appropriate P‐T conditions due to associated experimental challenges, hence the ubiquity of the graybody assumption. Iron‐bearing mantle minerals, however, are nongray semitransparent solids and may show strongly wavelength‐dependent optical properties (e.g., emissivity) at 600–900 nm (Dubrovinsky et al, ; Goncharov et al, ; Goncharov et al, ; Keppler et al, ; Keppler et al, ; Keppler & Smyth, ; Lobanov et al, ; Lobanov et al, ; Thomas et al, ; Ullrich et al, ), which is a common spectral range used for radiometric measurements in LH DACs. The application of Kirchhoff's law of radiation to a partially transparent body entails that the emissive and absorptive powers of the body at a given wavelength are equal if light reflections are negligible (McMahon, ), a condition that is to first order satisfied in LH DACs on mantle minerals (Goncharov et al, ).…”