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，地质材料经历了足够高的应力、温度，特别是应变率的条件，形成了高压晶型(HPP)。当比较冲击环境和准静态条件时，形成石英HPP所需的压力和温度是相当不同的。例如，与准静态环境相比，含柯石英的冲击岩表明冲击压力比含辉钛矿的冲击压力高。这项建议的总体目标是更好地了解石英的动态压缩--第二代运动的主要造岩矿物之一--以及它们在不同加载/卸载速率下的HPP。我们研究HPP的形成是否依赖于速率并且受到非静力偏应力的影响。将研究HPP的形成机制和中间非晶相的作用。在汉堡DESY的新的、目前最亮的第三代同步加速器光源Petra III上，通过使用极端条件光束线(ECB)P02.2的原位X射线衍射，将实时监测相变。我们的第一步是在梅明陨石坑实验的高冲击材料中寻找石英的HPP。虽然到目前为止，在冲击实验中还没有成功地检测到SiO_2-HPP，但在我们的实验中，由于冲击持续的时间比以前的冲击恢复实验要长得多，所以确实有可能形成了纳米级柯石英或辉钛矿。用两种不同类型的金刚石压腔对不同加载速率下的超高压进行了研究：膜驱动的金刚石压腔将获得中等的加载速率(3 GPA/S)。在高压缩比下对石英的进一步实验将使用改进的MDAC(300GPA/S，应变率2 S-1)和压电式动态金刚石压腔(DDAC)(500 GPA/S，应变率0.16 S−1)。这一建议为实时条件下研究冲击波中的物质状态奠定了基础。未来上述实验的一个关键作用是目前正在建设的欧洲自由电子激光器(XFEL)，它将减少获得100fs全衍射图案的时间。我们的实验之后是(I)对获得的衍射图进行处理，(Ii)Rietveld精化以获得晶胞参数和HPP，以及(Iii)全面的EBSD和TEM研究。为了验证这一建议的可行性，在Petra III装置上使用MDAC对二氧化硅粉末进行了加载速度高达3 Gpa/S的实验。对关键衍射图进行的Rietveld分析表明，最初的α石英消失为柯石英和矽石高压共生。然而，我们的初步结果表明，随后从α石英到柯石英再到辉钛矿的转变并不是按照静态高压实验中观察到的顺序进行的。

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.