Origin of pentlandite in Chang’e-5 lunar soils revealed by transmission electron microscopy

Sulfides are common minerals in lunar rocks and have great implications for lunar magma origin and subsequent evolution. Pentlandite as an important sulfide, usually coexisted with troilite, which could indicate the geological thermal history of lunar rocks. Previous researchers proposed three potential mechanisms to explain the origin of pentlandite in lunar soil: (i) the reaction between mobilized sulfur and metallic FeNi, ilmenite and an Fe-bearing silicate; (ii) it is formed by the reaction between migrating Ni and troilite; (iii) pentlandite may exsolve from the Ni-rich troilite during the cooling of rocks. Here, we used the scanning electron microscopy (SEM), X-Ray electron probe micro-analyzer (EPMA) and transmission electron microscopy (TEM) to decipher the formation mechanisms of pentlandite in Chang’e-5 (CE-5) lunar soils. Our results show that pentlandites occurred as lamella and veinlets in troilites from basalts and breccias, forming a troilite-pentlandite assemblage. Crystallographic data from TEM provide the first robust evidence that pentlandites from both basalts and breccias were exsolved from the host troilite during the magma cooling, rather than formed by the reaction between mobilized sulfur and metallic FeNi, or mobilized Ni with troilite. Furthermore, we found taenite was exsolved from pentlandite in lunar breccia, forming a troilite-pentlandite-taenite assemblage. Given exclusively exsolved taenite and higher Ni content in troilite in breccia than that in basalts, it suggests the origin of pentlandite in breccia may involve a geological process involving the addition of exotic meteorite materials. Finally, we established two atom shuffling models to describe the transformation mechanism from troilite to pentlandite, and pentlandite to taenite. This work provides new insights into the origin and geological evolution of lunar sulfides, and also provides new method for the study of mineral evolution in other extraterrestrial samples.