Surface microstructures of lunar soil returned by Chang’e-5 mission reveal an intermediate stage in space weathering process

This study is conducted by a joint team from Chinese Academy of Sciences. They use aberration-corrected transmission electron microscopy (TEM), Electron-energy loss spectroscopy (EELS) and scanning transmission electron microscopy (STEM) to examine the microstructures and chemical compositions at nano/atomic scales of 25 soil grains (1-3 μm in size) from Sample CE5C0400YJFM00507 (1.5 g). The soil mainly includes minerals olivine, pyroxene, anorthite and glass bead. To avoid possible chemical contamination and ion-bombing-induced amorphization, we do not employ the focused ion beam (FIB) to cut the bulk samples except glass bead. Firstly, they unambiguously identify the wüstite FeO nanoparticles instead of npFe⁰ that are embedded in amorphous Si_xO_y rims outside the olivine grains. This unique rim structure has not been reported for any other lunar, terrestrial, Martian, or meteorite samples so far. Given that the nano-phase Fe is the final product of decomposing olivine Fe₂SiO₄, we suggest that wüstite FeO may serve as an intermediate state of the thermal decomposition process, and then the FeO may further transform into nano-phase Fe with the aid of in the presence of cosmic radiation or solar flare. Secondly, for pyroxene and anorthite, the chemical compositions of surface areas are identical to interior parts, and there is no Si_xO_y rim outside sample. Meanwhile, no foreign volatile elements deposition layer and solar flare tracks can be found on the surface or inside the olivine and other minerals. Such findings imply that the studied samples do not undergo severe space weathering, and the underlying mechanism deserves further investigation. It provides clues or constraints on the incipient formation mechanism of rim structure under space weathering.

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