Diffusive hydrogen atoms in hydrous stishovite

A study from a team led by staff scientists from HPSTAR and Sun Yat-sen University discovered that stishovite and water prefer forming ordered superstructures with one-dimensional superionic conduit in Earth’s mantle. The research is published in the recent issue of the journal Science Advances.
Published in Chemistry
Diffusive hydrogen atoms in hydrous stishovite
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Stishovite has the same chemical formula as silica quartz (SiO2), er. While SiO2 is abundant on Earth’s crust, it is also a major component of basalt, a type of igneous rock that is rich in iron and magnesium. When basalt in oceanic crust subducts into Earth’s deep interiors, it is subjected to high pressures and temperatures, triggering basaltic melting and fluid release, i.e. liquid water. These fluids can then react with the surrounding mantle rock, changing its composition.

 “This might not be the case if stishovite can absorb a lot of water.” says Junwei Li, the leading author from the Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing China. “Stishovite can incorporate weight-percent level of water. Thus, Stishovite in Earth’s mantle can store such enormous amounts of water, that is equivalent to a few oceans of water on Earth’s surface.”

 The leading consensus of the geo-science community is that water incorporating into Stishovite is either realized by hydroxyl group (OH-) absorption into the crystal lattice or by forming defect structures via silicon replacement. Both mechanisms have been well studied in the past but it was unclear, as of now, which become dominant at the elevated pressure and temperature conditions of the Earth's mantle.

 “We employed a revolutionary structural searching algorithm called LASP that found the most favorable water coordination sites in dense stishovite at high temperatures,” said Dr. Shengcai Zhu, an associate professor from Sun Yet-Sen University, Guangzhou China. “This algorithm is based on a machine learning algorithm, accelerating the speed of the structural search by several orders of magnitudes.” Hydrous Stishovite contains more than 100 atoms per unit cell, and the combination of proper searching algorithms and machine learning make it possible to locate stable structures at reasonable computational costs.

 The results indicate hydrous Stishovite to be stabilized in stishovite-water superstructures exhibiting one-dimensional water channels. Hydrogen atoms will behave fluid-like inside the channels at temperatures above 1000 K, leading to an exotic 1D superionic state, similar to high pressure water-ices at high pressures and temperatures. Therefore, hydrous Stishovite could feature high ionic mobility, which could explain the previously observed electrical heterogeneity in Earth’s mantle. Given these encouraging results, the research team set out to synthesize samples in a laboratory environment. However, synthesis and characterization proved to be very challenging for those predicted stishovite-water superstructure

 

Caption: topmost panel, schematic plot for diffusive hydrogen (trajectories with various colors) in stishovite. Bottom panel, calculated mean squared displacement at selected P-T conditions.

“Although a series of hydrous stishovite samples have been synthesized in the past decade, either they contain a considerable amount of alumina or they are synthesized at relatively low pressure and temperature conditions.” said Dr. Yanhao Lin, a staff-scientist at HPSTAR. “For the first time, we synthesized Al-free Stishovite single crystals containing wt-% levels of H2O at P-T conditions approaching the lower-mantle geotherm,” he added.

 “Measuring hydrogen in nominally anhydrous minerals, especially for such minuscule samples as we were able to prepare, is technically challenging. Conventional structural characterization techniques such as X-ray diffraction are powerless due to the weak elastic scattering of hydrogen atoms.” Dr. Thomas Meier (HPSTAR) continued. “In order to solve this problem, we used a Nuclear Magnetic Resonance spectroscopy adopted for the detection of ultra-small samples. NMR is particularly sensitive to hydrogen nuclei.”

 The detected 1H-NMR signals, a fingerprint of the short-scale local environment of each nucleus of hydrogen, exhibited characteristics which were found to validate the super-structural predictions of the research team.

 “The unique stishovite-water superstructure opens exciting opportunities for exploring novel minerals. Hydrous stishovite is no longer the original stishovite by crystallographic definition. It is a new mineral.” added Dr. Ho-kwang Mao, the chief scientist at HPSTAR. Water is known to alter the structure of minerals but it is unexpected to form a superstructure with a water channel passing through. This might not be a unique structure for stishovite, but potentially exists in other minerals. The existence of superionic water channels will also largely modify the physical properties of stishovite. For example, since H-atoms diffuse along certain directions, its electrical properties will be highly anisotropic. The discovery of stishovite-water superstructures will refresh our conventional view of water reservoirs in Earth’s deep interiors.

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