Spectroscopic evidence for the Fe3+ spin transition in iron-bearing δ-AlOOH at high pressure

Abstract

δ-AlOOH has emerged as a promising candidate for water storage in the lower mantle and could have delivered water into the bottom of the mantle. To date, it still remains unclear how the presence of iron affects its elastic, rheological, vibrational, and transport properties, especially across the spin crossover. Here, we conducted high-pressure X-ray emission spectroscopy experiments on a δ-(Al0.85Fe0.15) OOH sample up to 53 GPa using silicone oil as the pressure transmitting medium in a diamond-anvil cell. We also carried out laser Raman measurements on δ-(Al0.85Fe0.15)OOH and δ-(Al0.52Fe0.48)OOH up to 57 and 62 GPa, respectively, using neon as the pressure-transmitting medium. Evolution of Raman spectra of δ-(Al0.85Fe0.15)OOH with pressure shows two new bands at 226 and 632 cm−1 at 6.0 GPa, in agreement with the transition from an ordered (P21nm) to a disordered hydrogen bonding structure (Pnnm) for δ-AlOOH. Similarly, the two new Raman bands at 155 and 539 cm−1 appear in δ-(Al0.52Fe0.48)OOH between 8.5 and 15.8 GPa, indicating that the incorporation of 48 mol% FeOOH could postpone the order-disorder transition upon compression. On the other hand, the satellite peak (Kβ′) intensity of δ-(Al0.85Fe0.15)OOH starts to decrease at ~30 GPa and it disappears completely at 42 GPa. That is, δ-(Al0.85Fe0.15)OOH undergoes a gradual electronic spin-pairing transition at 30–42 GPa. Furthermore, the pressure dependence of Raman shifts of δ-(Al0.85Fe0.15)OOH discontinuously decreases at 32–37 GPa, suggesting that the improved hydrostaticity by the use of neon pressure medium could lead to a relatively narrow spin crossover. Notably, the pressure dependence of Raman shifts and optical color of δ-(Al0.52Fe0.48)OOH dramatically change at 41–45 GPa, suggesting that it probably undergoes a relatively sharp spin transition in the neon pressure medium. Together with literature data on the solid solutions between δ-AlOOH and ε-FeOOH, we found that the onset pressure of the spin transition in δ-(Al,Fe)OOH increases with increasing FeOOH content. These results shed new insights into the effects of iron on the structural evolution and vibrational properties of δ-AlOOH. The presence of FeOOH in δ-AlOOH can substantially influence its high-pressure behavior and stability at the deep mantle conditions and play an important role in the deep-water cycle.

Equation of State and Spin Crossover of (Al, Fe)‐Phase H

“…Modeling of mid-ocean ridge basalt (MORB) hydration by formation of phases in the (δ-AlOOH)-(MgSiO 2 (OH) 2 )-(ε-FeOOH) solid solution may result in increased seismic velocities of hydrous MORB relative to pyrolytic mantle just below the mantle transition zone (∼680-900 km; Satta et al, 2021). However, the ν p and ν s values constrained for δ-(Al 0.97 Fe 0.03 )OOH and used in the modeling for seismic velocities of this solid solution are significantly faster than those constrained for compositions with greater concentrations of Fe: (δ-(Al 0.87 Fe 0.13 )OOH; Ohira et al (2021), (Al 0.956 Fe 0.044 )OOH; Su et al, 2021a;Su et al, 2021b). Comparison of sound velocities predicted by these studies reveal a decrease in both ν p and ν s with increasing Fe concentration, complicating the modeled seismic signature of MORB hydration and highlighting the need for additional measurements of sound velocities of compositions in the (δ-AlOOH)-(MgSiO 2 (OH) 2 )-(ε-FeOOH) solid solution.…”

“…The onset of the spin transition at a lower pressure in δ-(Al, Fe)OOH (32-40 GPa) compared to ε-FeOOH led Ohira et al (2019) to infer that the spin transition pressure in the (δ-AlOOH)-(ε-FeOOH) solid solution would increase with increasing iron concentration, an effect confirmed via Raman spectroscopy (Su et al, 2021a;Su et al, 2021b) and also observed for varying concentrations of Fe 2+ in (Mg, Fe)O (Fei et al, 2007b;Lin et al, 2005;Solomatova et al, 2016). Nishi et al (2019) observed a spin crossover of Fe 3+ at approximately 46 GPa in δ-(Al, Fe)OOH with Fe/(Al + Fe) = 0.23 and Fe/(Al + Fe) = 0.67.…”

The iron spin transition of deep nitrogen-bearing mineral Fe3N1.2 at high pressure

“…The pressure-induced spin transition of iron greatly changes the chemical bonding of relevant minerals and thus, affects thermodynamic properties, rheology, and the thermal and electrical conductivity of iron-bearing minerals [Lin et al, 2013]. The iron spin transition occurs widely in the deep mantle for both iron-bearing oxide-hydroxides and metallic compounds [Lin et al, 2004a;Lin and Tsuchiya, 2008;Gu et al, 2014;Chen et al, 2018;Su et al, 2021]. At ambient conditions, the 3d-orbit electrons of Fe in all iron nitrides are in a high-spin ferromagnetic arrangement [Sifkovits et al, 1999].…”