Carbonates under shock compression: stability, phase transitions, and chemical reactions with silicates

At the time of application for the first funding period in 2015, there was no experimentally verified method of shock wave synthesis that allowed the pressures required for a [CO3]–[CO4] transition to be reached. Regardless of the fact that, for example, calcium carbonate is stable under pressure-temperature conditions corresponding to those of the deep mantle, this compound decays after shock wave treatments during pressure relief due to the high temperatures, often followed by subsequent recombination into the starting compound upon further cooling.

For this reason, the focus of the work within the first funding period 2015–2017 consisted of the development of a method which now makes it possible for the first time to expose carbonates to a dynamic pressure of up to 200 GPa without exhibiting a degassing behavior. This required the following steps and the design of a completely reworked sample-holder set-up: The elimination of Mach effects to prevent "upstream jetting", followed by the impedance theory on the relaxation curve and the control of the adiabatic decompression and the final use of W/Cu heavy metals to ensure "quasiisentropic compression".

100% of the tested samples were recovered under all selected experimental conditions and the experiments are comparable and reproducible. The new self-sealing sample holder (Fig. 1) ensures an air-tight sealing of the specimens even after the shock wave tests and makes them much easier to handle.

Due to the use of halides (for example, NaCl), no acid addition is required to remove the samples from the impedance powder.

Now, with these four new developments, it is possible for the first time to use the shock wave synthesis as a complementing tool to the diamond anvil cell for the search for novel carbonate phases. Initial experiments have already been made.

Fig. 1 Sawed sample container after a successful shock wave experiment (sample pressure approx. 180 GPa). The sample container consists of W/Cu, the cylindrical sample piston is made from carbon steel, the flyer plate made of Cr-Ni steel and the specimen holder with the sample made of high-purity titanium. (Photo taken by Dr. Thomas Schlothauer.)