Experimental High-Pressure Mineralogy
Experimental High-Pressure Mineralogy
We are interested in properties of materials at high pressures and/or temperatures. The selection of material systems is usually motivated by a geological context but is not limited to this. The focus of the investigations is on the structural, physical and thermodynamic properties of crystals (minerals), melts, glasses and aqueous fluids and their interaction.
The deepest borehole in the world, at 12 km deep, only reaches a fraction of the Earth's radius of 6371 km. We therefore only have direct access to a very small part of the Earth. Most of the available information about the Earth's deep interior comes from indirect methods. Geophysicists, geologists and geochemists have produced many models of the Earth's structure and composition on this basis. However, to be able to test these models or understand contradictory results, we must understand the physical properties of geologically relevant materials under the pressure and temperature conditions of the Earth's interior. These extreme conditions of up to 364 GPa (3.64 million times atmospheric pressure) and around 6000 °C in the center of the Earth are very difficult to achieve experimentally, as only a few materials can withstand these conditions without breaking. The great hardness of diamond and other superhard materials alone makes it possible for very small sample quantities to be subjected to very high pressures. The sample is pressed together between two perfectly aligned diamonds in a so-called diamond anvil cell and, if necessary, heated simultaneously with lasers or an external electrical heater. Depending on the question, the sample can then be examined in-situ (during the high-pressure, high-temperature experiment) or ex-situ (after the experiment) using a variety of methods, such as X-ray diffraction, Raman spectroscopy or X-ray absorption spectroscopy and others.
Structure of silicate glasses at high pressure
The investigation of melt properties at very high pressures is very difficult, so silicate glasses (quickly cooled silicate melts) are often used as analogue materials to gain an initial understanding of the changes caused by pressure. The physical properties of melts and glasses are of great importance for understanding the development of the Earth and its current state. It is now assumed that large impacts during the formation of the Earth (e.g. the lunar-forming collision) led to the entire Earth being melted. To understand the processes at that time, we need an understanding of the properties of the pressure and temperature conditions of the Earth's interior. Silicate melts also play a central role in the processes of the recent Earth. This is visibly reflected on the surface in volcanism, but melts are also suspected in the deep Earth, e.g. at the core-mantle boundary (2891 km depth). In particular, we are investigating the structures and densities of silicate glasses using angle-dispersive X-ray diffraction and Raman spectroscopy, complemented by molecular dynamics simulations, in the MgO-CaO-FeO-Al2O3-SiO2 system up to several tenth of GPa. New technical developments have made it possible to obtain structural information on silicate glasses in the entire pressure range of the Earth's mantle (up to 135 GPa). This provides important information for understanding the Earth today and its development over time.
Hydrothermal diamond anvil cells
The hydrothermal diamond anvil cell (HDAC) is used for in-situ experiments on solids and liquids at high temperatures of up to 1000 K and high pressures up to 1 GPa. It is used in combination with various investigation methods. These include Raman, X-ray fluorescence and X-ray absorption spectroscopy as well as X-ray diffraction. With this approach, properties such as density, viscosity, solubility, complexation of cations in solutions and phase transitions are investigated. The cell we use (Bassett-type) consists of two parts. On both sides, there is a tungsten carbide seat to which a cut diamond is glued. The sample is measured through the diamonds, which are fortunately transparent to most types of radiation used in the above-mentioned investigation methods. The cell is heated during the experiment by heating wires that are wound around the tungsten carbide seat. The temperature is measured directly on the diamonds using a thermocouple. Heating increases the fluid pressure between the diamonds, which are connected by a metal ring (gasket), and thus also the pressure, which is measured using pressure sensors such as Cr-doped rubies or 13C diamonds. Current research using the HDAC is focused on metal speciation under hydrothermal conditions.
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