The study, led by Hongsheng Yuan of the Center for High Pressure Science and Technology Advanced Research in Shanghai, together with Lianjie Man and Leonid Dubrovinsky of the Bayerisches Geoinstitut at the University of Bayreuth and an international team of collaborators, is published in Nature Geoscience. Using laser-heated diamond anvil cells—devices that squeeze samples between two gem-quality diamond tips while laser beams raise temperatures to thousands of degrees—the researchers created conditions rivaling those near the core–mantle boundary, where pressures exceed one hundred gigapascals and temperatures climb above several thousand kelvin.

What emerged from these experiments were two hexagonal iron oxyhydroxides with the nominal compositions Fe5O12Hx and Fe7O12Hx, where x denotes a variable hydrogen content. These are not exotic compounds dreamed up in a computer simulation; they are crystalline phases whose structures were determined directly from in situ single-crystal X-ray diffraction measurements at synchrotron facilities, including the European Synchrotron Radiation Facility, PETRA III at DESY in Hamburg, and the Shanghai Synchrotron Radiation Facility. The crystallographic data underpinning the structure determinations have been deposited in the Cambridge Structural Database, and the broader experimental datasets are openly available through figshare, giving other researchers the means to scrutinize and extend the findings.

The significance of these phases lies in two properties highlighted by the team. First, they can form under water-undersaturated conditions, meaning they do not require an abundant free water reservoir to crystallize. This is crucial because the deep mantle is generally thought to be far drier than the shallow regions from which volcanic gases escape. Second, the new oxyhydroxides are remarkably dense. Density matters enormously in a planet whose interior is sorted by gravity: materials less dense than their surroundings tend to rise, while denser ones sink. The high densities of Fe5O12Hx and Fe7O12Hx mean that once formed, they would gravitationally settle and remain stably parked near the base of the mantle rather than escaping upward.

That combination of stability and density provides what the authors describe as a plausible mineralogical basis for the seismically observed ultralow-velocity zones. These zones, which can stretch for hundreds of kilometers horizontally but are only a few to tens of kilometers thick, reduce seismic shear and compressional wave speeds so severely that ordinary mantle minerals alone struggle to explain them. Hydrogen-bearing iron-rich phases, with their altered elastic properties and potentially partial molten character, have been increasingly invoked as candidates. Earlier work had identified a hydrogen-bearing iron peroxide and pyrite-type FeOOH as possible deep water hosts; the new hexagonal oxyhydroxides extend that family of compounds and demonstrate that iron, oxygen, and hydrogen can combine into dense crystalline architectures at the pressures and temperatures of the lowermost mantle.

The context for why water might be down there in the first place reaches back to the planet’s earliest history. Isotopic anomalies in helium and hydrogen recorded in deep-sourced volcanic rocks have long hinted that some primordial water—water inherited from the materials that accreted to form Earth—survives in isolated reservoirs deep in the mantle rather than having been lost entirely to space or mixed into the surface water cycle. A leading scenario involves a basal magma ocean: a dense, iron-rich melt that ponded at the base of the mantle during planetary differentiation and crystallized slowly over geological time. The question that has frustrated researchers is which minerals, if any, could have sequestered hydrogen during that crystallization. Most candidate hydrous silicates decompose at the relevant temperatures, and common mantle minerals such as bridgmanite hold very little water at depth. The new oxyhydroxides fill that gap, offering mineral hosts capable of locking hydrogen into the deep interior both from primordial sources and from water recycled downward by subducting slabs.

Subduction supplies the second pathway. Oceanic plates carry hydrated minerals down into the mantle, and laboratory studies in recent years have shown that some hydrous phases can survive transit to the lowermost mantle, potentially delivering water to the core–mantle boundary billions of years after Earth formed. If that arriving water reacts with iron-bearing mantle material—or with metallic iron from the core itself—the dense oxyhydroxides identified in this study could crystallize and accumulate. The result would be chemically distinct, hydrogen-rich lumps at the base of the mantle, exactly the kind of heterogeneity that seismic imaging keeps revealing.

The implications ripple outward from mineralogy into planetary evolution. The authors note that the stability of these phases provides a mechanism for transporting volatiles into Earth’s core, where hydrogen and oxygen could act as light elements dissolved in liquid iron alloy, or conversely for their episodic release back into the mantle. Mantle plumes—hot, buoyant upwellings that rise from the core–mantle boundary and feed surface hotspots such as Hawaii and Iceland—appear to be rooted in some of the same deep regions. If dense oxyhydroxide accumulations are destabilized or partially melted at the edges of plumes, the water they carry could be injected into the upwelling flow, eventually influencing the melting behavior, chemistry, and gas emissions of hotspot volcanoes at the surface. In this way, minerals formed nearly three thousand kilometers down could modulate the volatile budget of the planet across geological time.

The experimental achievement behind these conclusions should not be understated. Diamond anvil cell experiments at lowermost mantle conditions are notoriously difficult: samples are measured in volumes smaller than the width of a human hair, laser heating must be controlled with precision to avoid catastrophically melting the diamond anvils, and extracting single-crystal diffraction data from polycrystalline samples squeezed to megabar pressures demands sophisticated analysis software and high-brightness synchrotron beams. The team combined these techniques with electron microscopy and chemical analysis of recovered samples to characterize the new phases from multiple angles, including constraints on their hydrogen contents and densities at high pressure. Extended data on unit-cell parameters and refined atomic positions support the structural assignments, and the authors report that chemical analysis of recovered material found no evidence of contamination from the rhenium gasket or other experimental artifacts that might otherwise mimic the observed compositions.

Much remains to be tested. Whether the new oxyhydroxides can account quantitatively for the seismic properties of ultralow-velocity zones, how much water the deep mantle could realistically store in them, and how they interact with surrounding mantle minerals at the atomic scale are questions for future experiments and modeling. But the study reframes a long-standing problem in Earth science: the search for the planet’s missing deep water no longer needs to rely on sparse hydrous silicates that barely survive at depth. Iron, the most abundant element in Earth’s core and a major component of the deep mantle, combined with oxygen and hydrogen, may instead form its own family of dense reservoir minerals. If those reservoirs exist at the core–mantle boundary today, they could be simultaneously a vault preserving water from the planet’s birth, a sink for water recycled from its surface, and a chemical trigger for the plumes that reshape its surface—tying together, in a single crystal structure, processes that span the whole depth and history of our planet.

Subject of Research: Dense iron oxyhydroxide phases as potential water reservoirs in the deep lower mantle

Article Title: Dense iron oxyhydroxides as possible reservoirs of water under deep mantle conditions

Article References: Yuan, H., Man, L., Kupenko, I., Yin, Y., Aprilis, G., Pantousas, A., Pakhomova, A., Zhou, W., Pierru, R., Yang, K., Glazyrin, K., Rodrigues, J. E., Garbarino, G., Ball, J. A. D., Bykova, E., Hu, Q., Sossi, P. A., Frost, D. J., & Dubrovinsky, L. (2026). Dense iron oxyhydroxides as possible reservoirs of water under deep mantle conditions. Nature Geoscience. https://doi.org/10.1038/s41561-026-02067-1

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02067-1

Keywords: iron oxyhydroxides, deep mantle water, core–mantle boundary, ultralow-velocity zones, diamond anvil cell, high-pressure experiments, basal magma ocean, mantle plumes, subduction, hydrogen storage, volatile cycles, Nature Geoscience

The significance of the result lies in what it says about the intermediate regime between known phases. For decades, high-pressure physicists have mapped the phase diagram of water with increasing precision, identifying nearly twenty distinct crystalline forms of ice, from the hexagonal ice Ih that falls as snow to exotic proton-ordered varieties stabilized only at low temperature and high pressure. Ice X occupies a special place in that catalog. Above roughly sixty gigapascals, the oxygen atoms lock into a body-centered arrangement and the hydrogen atoms settle at the midpoints between them, so that each proton is shared equally by two neighboring oxygens. The material ceases to be describable as discrete water molecules bound by hydrogen bonds; it becomes a network of oxygen ions threaded by protons, a so-called symmetrized hydrogen-bonded crystal.

What happens beyond that regime has been the subject of intense theoretical debate. Computational studies have predicted that as pressure climbs further, the oxygen sublattice itself should reorganize, exploring structures such as the cubic Pn-3m framework and more densely packed arrangements in which the coordination of oxygen atoms increases beyond the eightfold geometry of ice X. Some predictions place such transitions at pressures approaching the terapascal range, conditions comparable to those found in the interiors of giant icy planets such as Uranus and Neptune, where water is believed to constitute a substantial fraction of the mantle. Confirming any of these predictions experimentally has been extraordinarily difficult, because the pressures involved push the capabilities of diamond anvil cell technology to their limits.

The new work observes that before any wholesale transformation occurs, the ice X lattice begins to distort. Rather than maintaining the ideal symmetric geometry assumed in textbook treatments, the crystal develops a structural deviation under extreme compression at room temperature. This distortion is not a trivial elastic response; it signals that the symmetrized hydrogen-bond network, long treated as a stable endpoint of compression, is itself unstable at sufficiently high pressure. The observation provides direct experimental evidence that the pathway from ice X to denser structures is mediated by a symmetry-lowering intermediate state, a detail that theoretical models will now need to reproduce.

Following this distortion, the material transforms into ice XXII, the newly identified phase. According to the study, ice XXII is the most dense H2O polymorph that has been experimentally observed to date. Density is the central currency of high-pressure physics: when a crystal adopts a denser packing, it lowers its Gibbs free energy under compression, and the sequence of phases a substance passes through as pressure increases is essentially a sequence of increasingly efficient ways of stacking its atoms. That water, a molecule of just three atoms, continues to find new packing solutions at pressures far above those at which most substances have long since surrendered their molecular identity, is a striking illustration of the richness hidden in its phase diagram.

Experiments of this kind rely on diamond anvil cells, instruments that compress a microscopic sample between two gem-quality diamond anvils. The sample chamber in such an experiment is measured in micrometers, and the pressures achieved can exceed those at the center of the Earth. Structural information is typically extracted by X-ray diffraction, in which a synchrotron beam is focused through the diamonds onto the tiny compressed sample, and the resulting diffraction pattern is used to reconstruct the arrangement of atoms. At the highest pressures, the diffraction signals become faint and the analysis demanding, since the diamond itself contributes background and the sample may be non-hydrostatically strained. Detecting a subtle distortion of the ice X lattice, and then the emergence of an entirely new diffraction signature corresponding to ice XXII, therefore represents a considerable experimental achievement.

The room-temperature character of the transition is also noteworthy. Many of water’s high-pressure phases were originally discovered or characterized at low temperatures, where kinetics slow down enough for metastable structures to be trapped and proton ordering to occur. A transition observed at ambient temperature demonstrates that the transformation is driven by thermodynamic stability under compression rather than by temperature-assisted rearrangement, and it means the new phase is accessible under conditions closer to those relevant to planetary interiors, where water-bearing layers are hot as well as compressed. This strengthens the connection between laboratory measurements and geophysical models of ice giant planets, where the behavior of water at megabar and higher pressures influences magnetic field generation, heat transport, and interior structure.

For theorists, the observation of a distorted ice X precursor poses a concrete benchmark. First-principles calculations based on density functional theory have long predicted a landscape of candidate high-pressure structures for water, but the calculated transition pressures and the ordering of phases have been sensitive to computational details, including the treatment of proton quantum effects such as zero-point motion and tunneling. Protons are light enough that their quantum behavior significantly modifies the energy landscape, and nuclear quantum effects are believed to influence the precise pressure at which hydrogen bonds symmetrize. Any successful model of the ice X to ice XXII transition must now account for both the symmetry-lowering distortion and the density of the final structure, constraints that should help discriminate among competing computational approaches.

The discovery also extends a numbering scheme that has become a running chronicle of water’s complexity. Ice phases were historically labeled in order of discovery, from ice II and ice III in the early twentieth century through the recent additions of ice XIX, ice XX, and ice XXI in the past few years, several of which were identified through careful calorimetry and dielectric measurements of low-temperature, high-pressure samples. Ice XXII now continues that sequence, and its identification as the densest experimentally observed polymorph underscores that the inventory of water’s solid forms is still incomplete. Each new phase refines the phase diagram that planetary scientists, chemists, and physicists share, and each raises the question of where the sequence ends, or whether it ends at all before water’s molecules give way to an ionic or superionic state at the highest compressions.

Beyond its fundamental interest, the result speaks to a broader theme in modern materials physics: that even substances as thoroughly studied as water conceal structural surprises when probed under extreme conditions. The techniques refined in this work, combining ultrahigh static compression with high-resolution structural probes, are applicable to other hydrogen-dominated systems, including ammonia and methane, which together with water shape the interiors of icy planets. As synchrotron and free-electron laser sources deliver ever brighter beams and anvil geometries improve, the frontier of static compression continues to advance, bringing laboratory conditions closer to the interiors of the solar system’s most enigmatic worlds. With ice XXII now on the books, the high-pressure chapter of water’s story has gained a new and unexpectedly dense installment, and the search for the next polymorph is already underway in laboratories around the world.

Subject of Research: Ultrahigh-pressure structural distortion of ice X and its transition to the dense ice XXII polymorph of water

Article Title: Ultrahigh pressure phase transition in ice

Article References: Osmond, I., Kuzovnikov, M. A., Lewis, D. A., Shuttleworth, H. A., Marqueño, T., Dixon, M. J., Robertson, C. E. A., Cherepnalkoski, V., Wang, B., Hermann, A., Peña-Alvarez, M., & Howie, R. T. (2026). Ultrahigh pressure phase transition in ice. Nature Materials. https://doi.org/10.1038/s41563-026-02732-1

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02732-1

Keywords: ice X, ice XXII, high-pressure physics, water polymorphs, phase transition, diamond anvil cell, hydrogen bonds, crystal structure, ice giant planets, X-ray diffraction, condensed matter physics, dense ice