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

陨石撞击是一个高度动态的过程，地质物质经历足够高的应力、温度、特别是应变速率条件，形成高压多晶（HPP）。对比冲击环境和准静态条件，石英HPP形成所需的压力和温度有较大差异。例如，与准静态环境相比，含硅辉石的冲击物表明冲击压力高于含硅辉石的冲击物。本提案的总体目标是更好地了解石英（MEMIN II活动的主要造岩矿物之一）的动态压缩及其在各种加载/卸载速率下的HPP。我们研究了HPP的形成是否取决于速率并受非流体静力偏应力的影响。研究了HPP的形成机理和中间非晶相的作用。相变将通过在极端条件波束线（ECB） P02.2上使用现场x射线衍射进行实时监测，该波束线位于汉堡DESY的最新且目前最亮的第三代同步加速器光源PETRA III上。我们的第一步是在MEMIN撞击实验的高冲击材料中寻找石英的HPP。虽然目前为止在冲击实验中还没有成功地检测到SiO2-HPP，但由于在我们的实验中，冲击持续的时间比以前的冲击恢复实验要长得多，因此有可能形成了纳米级的coesite或stishovite。采用两种不同类型的金刚石砧细胞对不同加载速率下的HPP进行了研究：膜驱动金刚石砧细胞（mDAC）将获得中等加载速率（3 GPa/s）。在高压缩率下，石英的进一步实验将使用改进的mDAC （300 GPa/s，应变率2 s-1）和压电驱动的动态金刚石砧细胞（dDAC） （500 GPa/s，应变率0.16 s-1）。该建议为研究实时条件下冲击波中的物质状态铺平了道路。目前正在建设中的欧洲自由电子激光器（XFEL）在未来的上述实验中发挥着至关重要的作用，它将减少100 fs完整衍射图样的采集时间。我们的实验随后进行了(i)处理获得的衍射图，（ii） Rietveld细化以获得细胞参数和HPP，以及（iii）彻底的EBSD和TEM研究。为了测试该提议的可行性，在PETRA III设备上使用mDAC对SiO2粉末进行了加载速率高达3 GPa/s的实验。关键衍射图的Rietveld分析表明，初始α-石英逐渐消失，形成了硅辉石和硅辉石HPP的组合。然而，我们的初步结果表明，α-石英-钴石-硅长石的后续转变顺序与静态高压实验中观察到的顺序不同。

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.