Quantitative Mineralogy of Planetary Silicate Ternary Mixtures Using Raman Spectroscopy

A 532-nm-excited lunar Raman spectrometer (LRS) has been selected as a scientific payload of the Chang'e-7 mission, exploring mineralogy assemblages in the lunar south polar region. However, the quantification of dark-colored silicate minerals via Raman spectroscopy is an urgent requirement for upcoming Raman applications in future lunar and planetary explorations. Therefore, we conducted detailed laboratory studies on the Raman quantification of lunar silicate minerals using ternary mixtures of feldspar, olivine, and augite. Quantitative models were established employing the observed linear relationship between Raman integrated intensities and mineral proportions. The significant correlation coefficients (>0.94) and small RMSE (<= 4.20 wt.%) confirmed the performance of these models. A series of methods (multipoint sampling, multispectral averaging, peak area extraction, and spectral parameter ratios) were jointly used to ensure that the models were not significantly affected by crystal orientation, chemical inhomogeneity, and instruments. Factors (s2/s1) describing the relative Raman scattering cross sections were introduced to calibrate the Raman counts. Our results indicated that the relative Raman scattering efficiency of feldspar, olivine, and augite is 1.4:2.4:1, which can be used to improve the quantitative accuracy of the point-counting method if polymineralic mixing spectra are dominant. The models were validated across different samples using laboratory mixtures and lunar soil (CE5C0600). The lithology of the Chang'e-5 soils is basaltic/gabbroic according to the quantitative mineralogy returned from our models that is consistent with the results from traditional methods. This research will be of particular significance for accurately determining the mineral abundances for Chang'e-7 and other planetary missions. As such, crucial information can be inferred to understand the geological evolution of the exploration regions. Plain Language Summary Several Raman payloads are currently or planning to detect minerals on lunar and planetary surfaces. The proportion of minerals could provide clues about the processes that formed them and hence help decipher the planet's geological history. However, previous studies on planetary materials using Raman spectroscopy mainly focused on mineral identification. In this study, we introduce new Raman quantitative models based on spectral measurements of silicate ternary mixtures in the laboratory. The linear relationships between Raman integrated intensities and mineral proportions are observed. We demonstrate that our quantitative models based on this linear relationship are more accurate than previous attempts and are independent of instrumental setup. Using these models, we obtain the relative mineral proportions of lunar soil (CE5C0600) and confirm that augite is the dominant mineral at the Chang'e-5 landing site. Besides, our results indicated that the intrinsic relative Raman scattering cross section of feldspar, olivine, and augite is 1.4:2.4:1. This coefficient is helpful for more accurate mineral quantification.