Discovery of Coesite on the Lunar Farside

Abstract

The Moon has been highly shocked as evidenced by numerous impact craters on its surface. High-pressure minerals are expected to form during these shock events and can be used to unravel the pressure and temperature conditions for the shock events. However, high-pressure minerals are rarely reported in the lunar returned samples, yielding a discrepancy with the prediction. The lunar soils returned by the Chang'e-6 (CE6) mission from the South Pole-Aitken (SPA) basin provide new opportunities to investigate the shock metamorphism of the lunar samples and the shock events on the Moon. Here, we reported the discovery of coesite in a shock-induced melt pocket from a CE6 mare basalt, which could have experienced a shock event with a peak pressure of ∼24 GPa. The coesite exhibits two types of occurrences, a polycrystalline aggregate in the center and a ring along the margin of a silica clast. The coesite could have been formed by solid-state transformation followed by partial conversion to silica glass during decompression. The coesite has a higher survival temperature and a slower back-transformation rate than most other high-pressure minerals, which are favorable for its preservation under high-temperature conditions of lunar soils induced by impacts. These findings provide new insights for the preservation of coesite in natural shock events and indicate that more thermal-resistant high-pressure minerals could have been formed and preserved in lunar samples than previously thought, providing new targets for studying the shock events on the Moon.