Amorphization of feldspar under plate impact induced shock loading*

Feldspars are the most abundant mineral group of Earth’s crust and are also found on the lunar surface and in meteorites. As a result, shock metamorphism in feldspars is often used to identify past shock events in samples from natural impact sites. Hugoniot studies of feldspar have identified two phase changes at ~10 and ~30 GPa, interpreted as amorphization and a phase transition to the high-pressure hollandite phase, respectively. However, this interpretation was not confirmed with in situ measurements. To address this, we performed in situ X-ray diffraction (XRD) measurements for three natural feldspar samples (albite, microcline, and anorthosite) under plate impact shock loading using the two-stage light gas-gun at the Dynamic Compression Sector in Advanced Photon Source.

 

LiF impactors were accelerated to velocities of 3.6 and 5.6 km/s to achieve peak stress of ~30 and ~60 GPa. Four XRD images were collected during the loading and release process. At ~30 GPa, all samples showed two broad diffuse features, indicating amorphization. No clear signal of the high-pressure hollandite or other crystal phases was observed. In contrast to recent laser-shock experiments, recrystallization was not observed on release. At about ~60 GPa, two broad diffuse features were observed for microcline. On release, it changed to lower density amorphous (LDA) phase. For albite, the diffuse feature was not clear, although XRD peaks disappeared. On release, LDA phase appeared.