Dynamic loading and unloading of SiO2 aggregates. Real-time phase transformation monitored by means of synchrotron beam diffraction

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Meteorite impact is a highly dynamic process where geological materials experience conditions of stress, temperature and in particular strain rate that are sufficiently high to form high pressure polymorphs (HPP). Pressure and temperature required for the formation of HPP of quartz are considerably different when comparing impact environments with quasi-static conditions. For instance, coesite-bearing impactites indicate higher shock pressures than those containing stishovite, in contrast to quasi-static environments. The overall goal of this proposal is to gain a better understanding of the dynamic compression of quartz – one of the main rock-forming minerals of the MEMIN II campaign - and their HPP for a variety of loading/unloading rates. We investigate if the formation of HPP is rate dependent and influenced by non-hydrostatic deviatoric stresses. The mechanism of HPP formation and the role of intermediate amorphous phases will be studied. Phase transformations will be monitored in real-time by using in situ x-ray diffraction at the Extreme Conditions Beamline (ECB) P02.2, at the new and currently brightest 3rd generation of synchrotron light sources PETRA III at DESY, Hamburg. Our first step is the search for HPP of quartz in highly shocked material of the MEMIN cratering experiments. While SiO2-HPP have not been successfully detected in shock experiments so far, there is a chance that nm-sized coesite or stishovite were indeed formed as the shock prevailed in our experiments for a much longer period than in previous shock recovery experiments. The study of HPP at various loading rate is carried out with two different types of diamond anvil cells: Moderate loading rates (3 GPa/s) will be obtained with a membrane-driven diamond anvil cell (mDAC). Further experiments on quartz at high compression rates will use an improved mDAC (300 GPa/s, strain rate 2 s-1) and a piezo-electric-driven dynamic diamond anvil cell (dDAC) (500 GPa/s, strain rate 0.16 s−1 for a metal ) This proposal paves the way to study the state of matter within shock waves under real-time conditions. A crucial role for the above experiments in the future represents the European Free electron laser (XFEL) that is currently under construction and will reduce acquisition time for a full diffraction pattern 100 fs. Our experiments are followed by (i) processing of the obtained diffraction pattern, (ii) Rietveld refinement to derive cell parameters and HPP, and (iii) a thorough EBSD and TEM study. To test the feasibility of the proposal, experiments on SiO2 powders using a mDAC at the PETRA III facility were carried out at loading rates up to 3 GPa/s. Rietveld analysis performed on key diffractograms show the disappearance of initial α-quartz towards an association of coesite and stishovite HPP. However, our preliminary results indicate that the subsequent transformation from α-quartz to coesite to stishovite is not in the order it was observed in static high-pressure experiments.