Compressional behavior and spin state of δ-(Al,Fe)OOH at high pressures

Itaru Ohira, Jennifer M. Jackson, Natalia V. Solomatova, Wolfgang Sturhahn, Gregory J. Finkelstein, Seiji Kamada, Takaaki Kawazoe, Fumiya Maeda, Naohisa Hirao, Satoshi Nakano, Thomas S. Toellner, Akio Suzuki, Eiji Ohtani

Research output: Contribution to journalArticlepeer-review

22 Citations (Scopus)


Hydrogen transport from the surface to the deep interior and distribution in the mantle are important in the evolution and dynamics of the Earth. An aluminum oxy-hydroxide, δ-AlOOH, might influence hydrogen transport in the deep mantle because of its high stability extending to lower mantle conditions. The compressional behavior and spin states of δ-(Al,Fe3+)OOH phases were investigated with synchrotron Xray diffraction and Mössbauer spectroscopy under high pressure and room temperature. Pressure-volume (P-V) profiles of the δ-(Al0.908(9)57Fe0.045(1))OOH1.14(3) [Fe/(Al+Fe) = 0.047(10), δ-Fe5] and the δ-(Al0.832(5)57Fe0.117(1))OOH1.15(3) [Fe/(Al+Fe) = 0.123(2), δ-Fe12] show that these hydrous phases undergo two distinct structural transitions involving changes in hydrogen bonding environments and a high- to low-spin crossover in Fe3+. A change of axial compressibility accompanied by a transition from an ordered (P21nm) to disordered hydrogen bond (Pnnm) occurs near 10 GPa for both δ-Fe5 and δ-Fe12 samples. Through this transition, the crystallographic a and b axes become stiffer, whereas the c axis does not show such a change, as observed in pure δ-AlOOH. A volume collapse due to a transition from high- to low-spin states in the Fe3+ ions is complete below 32-40 GPa in δ-Fe5 and δ-Fe12, which is ~10 GPa lower than that reported for pure e-FeOOH. Evaluation of the Mössbauer spectra of δ-(Al0.824(10)57Fe0.126(4))OOH1.15(4) [Fe/(Al+Fe) = 0.133(3), δ-Fe13] also indicate a spin transition between 32-45 GPa. Phases in the δ-(Al,Fe)OOH solid solution with similar iron concentrations as those studied here could cause an anomalously high r/vF ratio (bulk sound velocity, defined as K/ρ ) $\sqrt{\left. K/\rho \right)}$at depths corresponding to the spin crossover region (~900 to ~1000 km depth), whereas outside the spin crossover region a low r/vF anomaly would be expected. These results suggest that the δ-(Al,Fe)OOH solid solution may play an important role in understanding the heterogeneous structure of the deep Earth.

Original languageEnglish
Pages (from-to)1273-1284
Number of pages12
JournalAmerican Mineralogist
Issue number9
Publication statusPublished - 2019 Sept 25


  • Fe)OOH
  • Mössbauer spectroscopy
  • Xray diffraction
  • diamond-anvil cell
  • high-pressure
  • hydrous minerals
  • synchrotron
  • water transport in the deep mantle
  • δ-(Al
  • δ-AlOOH


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