Investigation on Shock Metamorphism of Anatase by Supersonic Microprojectile Impact

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The phase relationships of TiO₂ polymorphs are of significance to the field of earth and planetary science, because these phases are crucial geochemical markers of natural shock occurrences and processes that take place in the crust and mantle of planets. In this study, we use a novel method called the laser-induced projectile impact testing (LIPIT) technique to investigate the shock metamorphism of TiO₂ polymorphs by controlled supersonic impacts of microparticles. The 3D digital microscope, synchrotron X-ray diffraction (XRD), focused ion beam/scanning electron microscopy (FIB/SEM), transmission electron microscopy (TEM), and density functional theory calculations are used to investigate and interpret the phase transformations of shocked anatase. The synchrotron XRD and TEM investigations of the impact region show the phase transformation of anatase to rutile, brookite, srilankite, and amorphous TiO₂ phase. According to the impact calculation, the shocked regions experienced a high pressure up to 2.1 GPa and high temperatures up to 986 °C. The shock waves created by impacts are attributed to shock-induced phase changes and lattice dynamic instability. The twinned rutile nanocrystals at the impact area have planar defects following {011} planes that formed under intense pressure or stress. The shearing on the rutile {011} planes can produce the epitaxial nucleation of srilankite at the rutile twin boundary. The methodology of the study, which combines LIPIT microprojectile experiments with simulations and characterization techniques, can help us better understand shock metamorphism in minerals and rocks. It will be helpful for expanding our understanding of the process by which shock metamorphism occurs on planetary bodies, including the Earth, Moon, Mars, and others.