Raman study of shock effects in lunar anorthite from the Apollo missions

Earth’s moon is a primary exploration target for space agencies around the world. The Moon records and preserves key information about fundamental processes that shape planetary crusts such as impact cratering. Understanding shock effects in lunar anorthite (Ca‐rich endmember of plagioclase feldspar), the principal component of anorthosite and the most common crustal mineral on the Moon, is key to the early evolution of the Moon and terrestrial planets in the solar system. However, there has not been a systematic study of shock effects in lunar anorthite using modern analytical techniques that could be used in future lunar surface exploration, such as Raman spectroscopy. This study examined 23 polished thin sections from all six Apollo missions using optical and Raman spectroscopy. We documented a variety of shock features recording low to moderate shock levels, including fractures, deformed twins, undulatory extinction, planar features, and partially isotropic plagioclase. A notable nonobservation was the absence of planar deformation features (PDFs) or completely isotropic (i.e., diaplectic glass) in this suite of samples. Raman spectroscopy results of the observed shock features show similar progressive changes in terrestrial samples: As shock level increases, band broadening, reduction of intensities, and peak loss were observed. Our Raman data are efficient in identifying shock levels and distinguishing planar features from PDFs and deformed twins, and differentiating amorphous areas from crystalline plagioclase, suggesting Raman spectroscopy as a useful tool for purposely selecting moderately to strongly shocked samples to return in future lunar missions. Our study can also help the interpretation of Raman data of impact materials from the past and future exploration missions and demonstrate the utility of Raman spectroscopy for documenting and selecting samples for future lunar missions.