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is O1.5- in cubic iron dioxide. Advanced density-functional theory and dynamical mean-field theory calculations have confirmed the reduced valence of oxygen in these materials (Figure 2).

Generalising observations on cubic HP-PdF2-structured FeO2 and FeO2Hx phases, one may conclude that at pressures above ~50 GPa, the valence of oxygen may vary significantly. If pronounced in other materials, such a Fe-O bonding feature is therefore extremely important for modelling the interiors of Earth and other rocky planets.

PRINCIPAL PUBLICATION AND AUTHORS

Revealing the Complex Nature of Bonding in the Binary High-Pressure Compound FeO2, E. Koemets (a,b), I. Leonov (c,d,e), M. Bykov (a), E. Bykova (a,f), S. Chariton (a), G. Aprilis (g,h), T. Fedotenko (g), S. Clément (i), J. Rouquette (b), J. Haines (b), V. Cerantola (h), K. Glazyrin (j), C. McCammon (a), V.B. Prakapenka (k), M. Hanfland (h), H.-P. Liermann (j), V. Svitlyk (h), R. Torchio (h), A.D. Rosa (h), T. Irifune (l), A.V. Ponomareva (d), I.A. Abrikosov (m), N. Dubrovinskaia (g,m), L. Dubrovinsky (a), Phys. Rev. Letts. 126(10), 1-7 (2021); https:/doi.org/10.1103/PhysRevLett.126.106001 (a) Bayerisches Geoinstitut, University of Bayreuth (Germany) (b) Institut Charles Gerhardt Montpellier (UMR CNRS 5253), Université de Montpellier (France) (c) Institute of Metal Physics (Russia)

(d) Materials Modeling and Development Laboratory, NUST MISIS (Russia) (e) Ural Federal University (Russia) (f) Carnegie Institution of Washington, Earth and Planets Laboratory (USA) (g) Laboratory of Crystallography, University of Bayreuth (Germany) (h) ESRF (i) Laboratoire Charles Coulomb (L2C), Université de Montpellier (France) (j) Photon Science, Deutsches Elektronen-Synchrotron (Germany) (k) Center for Advanced Radiation Sources, University of Chicago (USA) (l) Geodynamics Research Center, Ehime University (Japan) (m) Department of Physics, Chemistry and Biology (IFM), Linköping University (Sweden)

REFERENCES

[1] E. Bykova et al., Nat. Commun. 7, 10661 (2016). [2] Q. Hu et al., Nature 534, 241-244 (2016).

Platinum hitches a ride with sulfur in the Earth s crust

In-situ X-ray absorption spectroscopy measurements combined with molecular simulations have revealed that a particular chemical form of sulfur, the trisulfur radical ion [S3 ]−, formed highly stable soluble complexes with platinum. These molecular vehicles are capable of massively transporting the metal by hydrothermal fluids in the Earth s crust.

Platinum-group elements (PGE: Pt, Pd, Rh, Ru, Ir, and Os) are among the most valued metals required for new technologies; in particular, for the car industry, catalytic nanomaterials or medicine, with current prices for some of these metals up to 10 times higher than for gold (e.g., rhodium, Rh >520 /g). However, like gold, these metals are extremely rare in nature, with average tenors in terrestrial rocks of less than 1 mg per ton. How can they be concentrated in the Earth s crust to attain mineable (i.e., economically significant) levels that must be at least 1000 times greater?

The mechanisms controlling the distribution and concentration of PGE in nature are insufficiently understood, in particular the role of aqueous fluids that circulate in the Earth s crust and form hydrothermal ore deposits of common metals such as copper, zinc, silver or gold. Unlike

those metals, PGE have been known to be too poorly soluble in aqueous solutions in the presence of common salt (e.g., as Na+ and Cl ) and sulfur (as hydrogen sulfide, H2S and HS , or sulfate, SO42 ) compounds to allow significant PGE transport. However, this chemical knowledge contrasts with multiple instances of enhanced PGE remobilisation and concentration in different hydrothermal settings within the Earth s crust.

Researchers have attempted to solve this paradox by investigating the factors that may control the mobility of platinum, which is the most widely used element of the PGE group, in hydrothermal fluids. The most direct in- situ method available so far, synchrotron X-ray absorption spectroscopy (XAS), was used at beamline BM30 to measure both solubility and the molecular state of platinum in model hydrothermal fluids under controlled laboratory conditions similar to those in the shallow crust of the Earth (~300°C, ~2 km depth). These unique in-situ measurements, performed using a hydrothermal autoclave designed at the Néel Institute [1], were coupled with theoretical molecular dynamics simulations that made it possible to constrain the exact atomic-scale structure and composition of the species transporting platinum (Figure 3a,b). Such fluids dominantly contain chloride, sulfate and sulfide, and, in much smaller quantities, the trisulfur radical ion [S3

]−, discovered recently [2,3], but so far ignored in models of Pt solubility in aqueous solution.