Experimental simulation of silica melting and amorphization under extreme shock conditions

拟建的研究项目侧重于在行星体超高速撞击过程中记录的极端高压条件下形成的二氧化硅玻璃结构的表征。温度、压力和冲击时间可以影响硅玻璃的形成机制——硅熔体的快速淬火或石英的固态转变——以及它们的短期和中期顺序，即硅的配位数和多面体环的大小。因此，限制石英玻璃的形成机制和结构将有可能阐明撞击过程。为了达到这一目标，将在石英上进行三种主要在冲击持续时间上不同的高压实验：激光照射（几纳秒）、高爆（大约一微秒）和金刚石砧细胞（长达几分钟）。新形成的玻璃的中短程阶序的表征将通过几种分析方法的组合使用：拉曼、电子能量损失光谱、x射线吸收光谱、电子衍射和x射线衍射。为了解释衍射数据，将计算对分布函数，并与硅玻璃的理论模型进行比较。在第一次实验和获得第一批结构数据后，将通过相同的分析方法和程序，从不同大小（即不同冲击持续时间）的陨石坑中选择天然二氧化硅冲击玻璃进行研究。与自然样品的比较将是将实验方法与自然联系起来的关键，从而更好地理解由地外撞击引起的极端高压条件下的熔化和非晶化过程。由于结合了不同的实验和分析技术以及对自然样品的平行研究，该项目将是第一个完全致力于理解二氧化硅冲击玻璃的形成机制和结构的综合工作。

The proposed research project focuses on the characterization of the structures of silica glasses formed under extreme high-pressure conditions recorded during hypervelocity impacts of planetary bodies. Temperature, pressure, and shock duration can affect the mechanisms of formation of silica glasses – rapid quenching of a silica melt or solid-state transformation of quartz – and their short- and intermediate-range order, namely, the coordination number of silicon and the size of the polyhedral rings, respectively. Thus, constraining both the mechanisms of formation and the structures of silica glasses will be possible to shed light on the impact cratering process. In order to reach this goal, three types of high-pressure experiments, which differ mainly in terms of shock duration, will be performed on quartz: laser irradiation (a few nanoseconds), high explosive (around one microsecond), and diamond anvil cell (up to a few minutes). The characterization of the short- and intermediate-range order of the newly formed glasses will be pursued by the combined use of several analytical methods: Raman, electron energy loss spectroscopy, X-ray absorption spectroscopy, electron diffraction, and X-ray diffraction. For the interpretation of the diffraction data, pair distribution functions will be calculated and compared with theoretical models of silica glasses. After the first experiments and the acquisition of the first structural data, selected natural silica impact glasses from craters of different sizes (i.e., different shock duration) will be investigated through the same analytical methods and procedures. The comparison with natural samples will be the key to relate the experimental approaches to nature, and then, to gain a better understanding of the melting and amorphization processes under extreme high-pressure conditions induced by extraterrestrial impacts. Thanks to the combination of different experimental and analytical techniques and the parallel study of natural samples, this project will be the first comprehensive work entirely devoted to understanding the mechanisms of formation and the structures of silica impact glasses.