A Validated SEM-EDS Workflow for Screening Rare High-Mg Minerals in Fine-Grained Chang’e-6 Lunar Soil

Accurate localization of rare high-Mg minerals in lunar soil is hindered by their small grain size and occurrence in compositionally heterogeneous matrices. High-Mg minerals are operationally defined as olivine with Fo > 80 (Fo = 100 × Mg/(Mg + FeT) molar basis), orthopyroxene with Mg# > 80 (Mg# = 100 × Mg/(Mg + FeT) molar basis), and clinopyroxene with Mg# > 70. A particle-resolved scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) screening method was developed for Chang’e-6 lunar soil. The key advance is that Mg-bearing particles are not only identified by phase mapping but are further classified according to Mg# for pyroxene and Fo for olivine, enabling high-Mg candidates to be directly ranked and spatially located across large mapped areas. Analytical conditions were optimized using Monte Carlo simulations, counting-statistics requirements, particle-size constraints, and phase-segmentation performance. The final conditions were 15 kV, 10 nA, a dwell time of 2000 μs, a resolution of 1024 pixels, frame processing of 4, a boundary tolerance of 10, and a grouping level of 0. Quantitative SEM-EDS results agreed well with electron-probe microanalysis (EPMA) analyses of the same grains. Application of the method showed that Mg-bearing phases accounted for approximately 1.8% of the mapped compositional area and enabled rapid localization of high-Mg olivine and pyroxene grains for subsequent high-precision analyses. The workflow provides a practical basis for selecting high-Mg candidates for subsequent studies of lunar mantle provenance and Lunar Magma Ocean differentiation.