Coesite Formation at Low Pressure during Supersonic Microprojectile Impact of Opal

The shock metamorphism of opal-A induced by supersonic microprojectile impacts in an advanced laser-induced projectile impact test (LIPIT) was investigated using synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM). The three-dimensional topography of the impact area is similar to the meteorite crater that has a circular outline with an uplifted rim. The post-impact synchrotron XRD and TEM analyses show the phase transformation of opal-A to coesite together with opal-CT, tridymite, and cristobalite. The impact simulation estimates that the shocked areas experienced pressures up to similar to 0.6-0.7 GPa. The results suggest that the transition pressure from opal-A to coesite is lower than the transition pressure of quartz to coesite (similar to 2 GPa). The impact-induced phase transformation of opal-A to coesite at low pressure could be related to the local precursor structure (coesite domain) and water content (similar to 6.7 wt %) in opal-A. The investigation of minerals via the LIPIT technique provides a new way of simple and cost-effective shock-metamorphic experiments. The combined micron-scale ballistic test, impact simulation, and high-resolution characterization techniques will be useful to study the shock metamorphism of various minerals from macro-, micro-, to nanoscale, which will help understand the impact phenomena on Earth, Mars, and many other types of meteorites and asteroids.