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China University of Geosciences has developed a new method for simultaneous determination of grain size and mineral composition of Chang’e 5 lunar soil

The grain size and mineral composition of lunar soil are of great significance for interpreting orbital remote sensing spectral data and understanding lunar magmatism and space weathering processes. Since the 1970s, scientists began to use various means to study lunar soil samples, but the methods used by predecessors usually need to consume more samples, and it is difficult to obtain information on mineral composition, particle size, morphology and so on at the same time.

Recently, the Chinese and English versions of science of China:geoscience published online the research results on the grain size and mineral composition of Chang’e 5 lunar soil by the team of professors she Zhenbing and Wang zaichong of China University of Geosciences (Wuhan). The first author is doctoral student caokenan.Based on the Raman micro particle analysis technology, the research team developed a new method to simultaneously determine the particle size and mineral composition of particle samples with very low sample consumption, and successfully applied it to the study of Chang’e 5 lunar soil samples (Fig. 1).

Fig. 1 flow chart of simultaneous determination of lunar soil particle size and mineral composition by Raman spectrum automatic microparticle analysis technology

The researchers will be about 30 μ The Chang’e 5 sample of G was dispersed on the aluminized glass slide (Fig. 1a), and then the large-area image mosaic and depth of field synthesis of lunar soil particles were carried out with a 50x objective lens in the dark field reflected light mode. The particles were automatically identified and the particle distribution map was reconstructed according to the brightness of different positions in the image (Fig. 1b). After obtaining the granularity information, select 1~45 μ M of lunar soil particles were analyzed by automatic Raman spectroscopy to obtain the spectrum with high signal-to-noise ratio (Fig. 1c), and the particles were automatically identified through the lunar soil mineral spectrum database built by the team to obtain the particle size, volume and other information of each mineral phase (Fig. 1D, Fig. 2), and calculate the mineral model abundance.

Fig. 2 for 6mm × Reconstruction results of mineral composition and distribution of 7307 montmorillonite particles within 3mm

Different colors represent different minerals

The analysis results of 24881 particles showed that the average particle size of Chang’e 5 lunar soil was 3.5 μ m. And it has a unimodal distribution (Fig. 3a), indicating that it has a high maturity. Although most particles have small particle sizes (< 6 μ m) , but greater than 8 μ M particles account for more than 90%of the total volume (Fig. 3b).

Fig. 3 grain size distribution characteristics of Chang’e 5 lunar soil

After studying the mineral pattern abundance of Chang’e 5 lunar soil, the researchers found that μ The mineral composition within the particle size range of M is pyroxene (39.4%), plagioclase (37.5%), olivine (9.8%), iron titanium oxide (1.9%), glass (8.3%) (FIG. 4A). This result is basically consistent with the previous results obtained by X-ray powder diffraction analysis. In addition, it was also found that the content of olivine and pyroxene in lunar soil decreased gradually with the decrease of particle size, while the content of plagioclase increased:the particle size was 20~45 μ The pyroxene content in lunar soil samples between M is the highest (49%), followed by plagioclase (32%), olivine (11%) and glass (8%), while iron titanium oxide, phosphate and siliceous minerals do not appear; With the decrease of grain size, the abundance of plagioclase gradually increases, while the abundance of pyroxene and olivine decreases significantly (Fig. 4b-4c). This trend is also common in Apollo samples (Fig. 4D), which may be caused by the fact that plagioclase is more easily broken than mafic minerals during space weathering (such as impact of micro meteorite).

Fig. 4 mineral composition ((a) ~ (c)) of Chang’e 5 lunar soil and its comparison with Apollo lunar soil (d)

The study also identified some trace mineral phases in the lunar soil, such as apatite, quartz, cristobalite and orthopyroxene. The discovery of orthopyroxene was reported for the first time, indicating that the lunar soil of Chang’e 5 may contain a very small amount of materials from the lunar plateau.

The above results provide a ground truth reference for interpreting the spectral remote sensing data of the northern part of the storm ocean where Chang’e 5 landed, and a new perspective for understanding the deep and surface evolution history of this region of the moon.

The advantages of this method are:(1) it only takes about 30 minutes each time μ G sample, while obtaining multi-dimensional information, the sample loss is minimized, and the sample preparation process is simple, which greatly reduces the possible sample pollution caused by this link; (2) A diversified information database of mineral grain size and composition can be quickly established in a short time, which is helpful to discover rare mineral phases; (3) Further development will provide technical support for rapid analysis of micro particle samples returned from Mars, asteroids and other celestial bodies in the future.

thankThe samples used in this study were provided by the National Astronomical Observatory of the Chinese Academy of Sciences. The analysis and testing were completed in the State Key Laboratory of biogeography and environmental geology of China University of Geosciences (Wuhan). The instrument used was WiTEC α 300r confocal Raman spectroscopy and particlescout (v5.3.14.106) automatic microparticle analysis system. The research was supported by the civil aerospace technology pre research project of the National Space Administration (d020205), the National Natural Science Foundation of China (42172337) and the State Key Laboratory of biogeography and environmental geology (gbl12101).