摘要：
Chang'e-5 for the first time, and found that the chromite nanocrystals on the lunar surface can grow outward from the surface of olivine like mushrooms after rain. Recently, the relevant results were published in the American Mineralogist.
Crystallization is one of the most important physical and chemical processes in human production and life. The traditional crystal growth process is observed on Earth, but it is still unknown whether crystal growth in other celestial environments is different from that on Earth. For example, the moon, which is closest to Earth, is constantly bombarded by meteorites and various high-energy particles on its surface due to the lack of atmospheric protection; Under the extreme high temperature and pressure generated by bombardment, the particle size, morphology, growth mode, and mineral composition of lunar surface crystals are significantly different from those of surface crystals.
▲ Schematic diagram of mushroom growth structure
To investigate the mechanism of these peculiar crystal growth phenomena, the research team used in situ focused ion beam sampling method to extract samples from lunar soil breccia debris of Chang'e-5 mission. High resolution transmission electron microscopy loaded with electron energy loss spectroscopy detection system was used to conduct micro nano mineralogical studies on the samples. They observed a rare dendritic structure composed of chromite pyrite nanospheres and zero valent iron nanospheres in the sample, growing vertically from the surface of olivine like mushrooms.
The structure and composition analysis under electron microscopy revealed the process of the formation of the "mushroom" structure: firstly, the debris was impacted by micro meteorites, and the Fe2+contained in it underwent disproportionation reaction, producing nano zero valent iron balls and Fe3+in the molten glass, while producing nano sulfide iron balls or their immiscible complexes with zero valent iron balls; Secondly, iron containing nanospheres undergo directional adhesion with olivine at high temperatures; Thirdly, directional attachment generates defect sites with high potential energy, where Cr3+in olivine and Fe2+in glass aggregate and crystallize into chromite at the defect sites; Fourthly, under non-equilibrium conditions, chromite continues to grow in dendritic form into the glass, pushing the nanospheres outward until the energy of the system is exhausted to the point where it cannot cross the energy barrier of crystallization. This process is supported by lattice mismatch calculation and thermodynamic simulation calculation.
