Unearthing water's deep structure: Raman revelations

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  • Published: Apr 1, 2013
  • Channels: Raman
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Supercritical depths

Microscopic structure of water at elevated pressures and temperatures c/o Christoph Sahle

Raman X-ray scattering has been used by a German-Finnish-French collaboration to investigate the behaviour of water subjected to the kinds of high pressures and temperatures it experiences deep within the Earth.

The Earth, as far as we know, is the only planet that retains vast quantities of water present in all three states of matter under natural conditions - ice at the poles and in glaciers, above boiling point in erupting geysers and elsewhere, and of course as the liquid in great oceans, lakes and rivers. But water has some otherworldly properties particularly when it is subjected to high pressures and high temperatures. Writing in the Proceedings of the National Academy of Science (PNAS), a team from Finland, France and Germany describes its experiments to reveal what happens when water is exposed to the kind of conditions found deep within the Earth - above 22 megapascals and hotter than 374 Celsius. This is beyond the critical point of water and in this range our familiar friend becomes a highly aggressive solvent, this behaviour being vital to the physical chemistry of the Earth's crust and mantle.

Tectonic water

"Without water in Earth's interior there would be no material cycles and no tectonics, explains Max Wilke from the GFZ German Research Centre for Geosciences. How the water under these conditions affects processes in the upper mantle and crust is still the subject of intense research and a question that Wilke and colleague Christian Schmidt of GFZ together with Christoph Sahle, Christian Sternemann, Alexander Nyrowa, Kolja Mendea and Metin Tolan of the TU Dortmund, Susi Lehtola, Simo Huotari, Mikko Hakalab, Tuomas Pylkkaenen and Keijo Haemaelaeinen of the University of Helsinki hope to answer. The team has now carried out X-ray Raman scattering experiments on water held in the GFZ's diamond anvil cell. The X-ray source was from a powerful beamline at the European Synchrotron Radiation Facility ESRF in Grenoble under Laura Simonelli.

The X-ray studies allowed the researchers to first extract the microscopic structure of water as a function of pressure and temperature. Data analysis was based on ab initio molecular dynamics simulations and density functional theory (DFT). Their molecular dynamics simulations by Sandro Jahn a member of the GFZ team reveal much about water's anomalous behaviour. "A spatial statistical analysis using Ripley's K-function shows that this model is homogeneous on the nanometre length scale," the team explains. "According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime." In particular, the team adds, the average number of hydrogen bonds per water molecule falls to about 0.6 at 600 Celsius and a pressure of 134 MPa.

Aqueous order

"The study shows that the structure of water continuously develops from an ordered, polymerized structure to a disordered, marginally polymerized structure at supercritical conditions," explains Wilke. "The knowledge of these structural properties of water in the deep earth is an important basis for the understanding of chemical distribution processes during metamorphic and magmatic processes."

The team concludes that this is just the start of investigations with much wider implications for our understanding of water beneath the Earth's surface: "The presented experimental spectra provide a benchmark for further theoretical investigations using other means to simulate the atomic structure and other approaches to calculate XRS spectra from the models, such as BSE-based methods," they say.

Related Links

Proc Natl Acad Sci 2013, online: "Microscopic structure of water at elevated pressures and temperatures"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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