Incomplete liquid immiscibility drives Fe-S-Si heterogeneity in OAOpq revealed by Chang’e-5 impact glass beads

The space weathering on the lunar surface leads to the extensive accumulation of optically active opaque particles (OAOpq) with different sizes, leading to a darkening or reddening of the reflectance spectrum. However, the formation mechanisms of OAOpq in lunar soil are complex and controversial, especially for submicrometer impure S-bearing particles, which account for the largest mass proportion. In this study, we characterized the OAOpq with various sizes and focused on analyzing the structural heterogeneity and formation mechanisms of S-bearing OAOpq in the Chang’e-5 impact glass beads. The OAOpq are divided into three types based on size, including small (<20 nm), medium (20–1000 nm), and large OAOpq (>1000 nm). Small OAOpq are identified as pure metallic iron, while medium to large OAOpq are usually dominated by impure S-bearing metallic iron. The Type-A texture in large OAOpq is characterized by interwoven texture consisting of metallic iron matrix and internal troilite particles accompanied with some iron silicide grains, while Type-B texture in medium OAOpq has typical metallic iron core and troilite shell with local Si-rich layer. Thermodynamic analysis indicates that the structural heterogeneity originates from differential initial compositions in the complex Fe-S-Si ternary system, which can be attributed to the liquid immiscibility during the impact process. These results indicate that the liquid immiscibility by impacts is incomplete and multistage, resulting in forming multicomponent systems and various particles with different textures. These OAOpq with various textures possibly have potential differential optical effects, which may have varying degrees of influences on the surface reflectance spectrum.