Water inside Earth’s interior is crucial to assist mantle convection. Without it, mantle convection would be inefficient, volcanic activity would cease, as would plate tectonics on the surface.
Hydrous minerals help transport water deep into Earth’s mantle, and form part of a cycle that regulates the sustained presence of water, both in the interior and on the surface, on geological time scales. To understand the deep-water cycle, it is crucial to study the properties of hydrous minerals under the conditions present in Earth’s mantle. Brucite, chemical formula Mg(OH)2, is one of the simplest hydrous minerals and stores significant amounts of water in the form of hydroxyl groups. It is assumed to decompose in the mantle transition zone, but recent work by Dr Andreas Hermann, with collaborators from Florida State University, shows that the mineral can exist in a more compact high-pressure phase instead, which pushes the stability region of brucite several hundreds of kilometers deeper and into the lower mantle. Brucite might be present at much greater depths, and in much larger quantities, than previously thought. This changes the current thinking about the water storage capacity of the deep Earth, and the role hydrous minerals play in transporting and holding water inside our planet.
The work involved quantum-mechanical calculations that screened thousands of potential crystal structures of brucite under extreme pressure conditions. The new phase, which has a network structure akin to anatase, emerged eventually from those calculations. Besides being more stable than the known brucite structure at high pressures and temperatures, Dr Hermann and his collaborator showed that the new phase has a unique elastic response, which might enable its direct detection in seismic signatures, as well as specific vibrational properties that should make its detection in laboratory experiments straightforward.
The research is published in the Proceedings of the National Academy of Sciences.