Time-Resolved Structural Transformations in Plagioclase Due to Impact Shock

Credit: SLAC National Accelerator Laboratory.

Plagioclase feldspars form the most abundant group of minerals in Earth’s crust and are ubiquitously present on extraterrestrial bodies such as Mars, the Moon, and asteroids. Due to its unique behavior in response to shock, plagioclase is commonly used to reconstruct impact histories and conditions for shocked rocks. For example, shock waves generated by impact events cause plagioclase to undergo structural transformations such as amorphization (losing its mineral structure and becoming glassy) and development of planar deformation features, which are used as geologic indicators of impact pressures in terrestrial impact rocks and heavily shocked chondritic meteorites. Although there have been extensive studies of feldspar transformations under shock, many questions regarding the details of these transformations and the exact pressures at which they occur have remained unresolved.

A research team led by Arianna Gleason of the Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory mimicked meteoritic impacts in the laboratory to explore how plagioclase transforms during extreme impacts. They used in situ X-ray diffraction from SLAC’s Linac Coherent Light Source to probe the phase transformation pathway on a sub-nanosecond timescale. The X-ray diffraction patterns obtained reveal how the atomic structure of plagioclase changes in response to increased pressures generated by impact events. The experiments showed that the amorphization of plagioclase begins at pressures of approximately 5 gigapascals (roughly 50,000 times Earth’s atmospheric pressure), which is much lower than previously suggested. The experiment also demonstrated a memory effect at approximately 32 gigapascals, in which the sample becomes amorphous but then partially returns to its original structure on a nanosecond timescale. This research could shed light on how minerals are affected by impacts at the atomic scale with sub-nanosecond resolution. In the future, similar studies conducted on other minerals could provide additional insights into impact conditions and histories on planetary bodies in the solar system. READ MORE