Earth's Stony Legacy – A Solid Foundation
Minerals and rocks have ever since been central to human progress as tools, as construction and raw materials, and as media for art and documentation. Solid rocks constitute the largest volume fraction of planet Earth, including Earth's crust and mantle, providing a solid foundation for life on Earth's surface and for human civilization. The rocks of Earth's crust and mantle formed as the products of ongoing, dynamic processes and hold records of Earth's past, from the planet's formation until today. Reading this record and projecting Earth's dynamics into the future requires to understand the properties of rocks and minerals as the building blocks of planets.
Rocks are composed of minerals. The properties of the minerals that form a rock will therefore determine the properties of the rock. Many technologically important materials are also composed of minerals or mineral-like materials such as concrete and ceramics. As with natural rocks, the properties of technological materials, such as mechanical, chemical, and optical properties, will again be inherited from their mineral components.
My research focuses on the physical properties of minerals. Most minerals form crystals. Crystals are characterized by their periodic arrangement of atoms, ions, and molecular groups in space. It is this three-dimensional atomic order that imparts particular physical properties to crystals. For example, the physical properties of crystals typically depend on the direction of an acting force or perturbation, i.e., crystals show anisotropy (from ancient greek άν-ϊσος-τρόπος for un-equal-direction).
To investigate their physical properties, we irradiate minerals and crystals with electromagnetic waves, such as X-rays, lasers, or infrared radiation, and observe the interaction between a mineral sample and the electromagnetic waves. Depending on their wavelengths, electromagnetic waves interact with minerals by being scattered, diffracted, absorbed, or re-emitted. All these different types of interactions contain information about different mineral-physical properties.
The minerals that form the rocks in Earth's deep interior experience high pressures and high temperatures that change their atomic structure and physical properties. To simulate the extreme conditions deep within the Earth and other planets, we use diamond anvil cells and intense lasers to compress and heat mineral samples. The diamond anvils serve as windows for electromagnetic waves allowing us to measure the physical properties of a mineral while it is being compressed and heated. In this way, we can determine the physical properties of different minerals and rocks and compare them with geophysical observations to understand the structure and composition of Earth's deep and otherwise inaccessible interior.
The Structure of Earth's Interior
Earth's interior can be subdivided into shell-like layers, which are defined by prominent changes in geophysical properties at their boundaries. Changes in density and seismic wave speeds can be detected from the Earth's surface by measurements of the gravitational field, the moment of inertia, tidal deformation, and the characteristics of seismic waves emitted by strong earthquakes. For example, compressional waves (P waves) can propagate through solid and liquid materials while shear waves (S waves) cannot travel through liquids. As a result, the liquid outer core creates a shadow zone for S waves. In general, density and seismic wave speeds are connected to the elastic properties of a material through the compressibility and rigidity of the material. The elastic properties of minerals and rocks can be determined in experiments and compared to geophysical observations. In this way, we seek to understand which materials compose the deep interior of the Earth and how the structure and composition of Earth's interior arose as products of fundamental, persistent processes that are shaping and changing the Earth and other planets since their formation until today.
Buchen, J., Marquardt, H., Speziale, S., Kawazoe, T., Boffa Ballaran, T. & Kurnosov, A. (2018) High-pressure single-crystal elasticity of wadsleyite and the seismic signature of water in the shallow transition zone. Earth Planet. Sci. Lett., 498, 77–87.
Dziewonski, A. M. & Anderson, D. L. (1981) Preliminary reference Earth model. Phys. Earth Planet. Inter., 25, 297–356.
Finger, L. W., Hazen, R. M., Zhang, J., Ko, J. & Navrotsky, A. (1993) The effect of Fe on the crystal structure of wadsleyite β-(Mg1–xFex)2SiO4, 0.00 ≤ x ≤ 0.40. Phys. Chem. Minerals, 19, 361–368.
Momma, K. & Izumi, F. (2011) VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Cryst., 44, 1272–1276.
Shatskiy, A., Litasov, K. D., Matsuzaki, T., Shinoda, K., Yamazaki, D., Yoneda, A., Ito, E. & Katsura, T. (2009) Single crystal growth of wadsleyite. Am. Mineral., 94, 1130–1136.
Smyth, J. R. & Hazen, R. M. (1973) The crystal structure of forsterite and hortonolite at several temperatures up to 900°C. Am. Mineral., 58, 588–593.
© Johannes Buchen 2022