Non-equilibrium phase-transition features of Fe-bearing minerals in lunar mare basalt simulants during hydrogen reduction

The hydrogen reduction process can extract water resources to meet the needs of a lunar base, compensating for the engineering limitations of waterless lunar soil. However, the minerals present in lunar mare basalt simulants are not traditional iron ores but rather complex silicate and titanate crystals. The engineering and chemical properties, as well as the phase transition behavior of these minerals, determine the feasibility of in-situ extraction. This study investigates the mineral composition found in Chang’e-5 lunar mare basalt simulants, combining simulated hydrogen reduction experiments, thermodynamic calculations, and micro-area analysis. This study identifies a semi-amorphous layered intermediate in pyroxene and compares the mineral-dependent phase-transition features of pyroxene, olivine, and ilmenite during hydrogen reduction under the tested conditions. Pyroxene, due to its high vacancy density, forms a unique semi-amorphous layered structure. Olivine forms amorphous silica and more iron-deficient crystals. Ilmenite undergoes steady progress according to equilibrium reactions and phase transformations. These results suggest that phase transition may act as an important limiting factor in hydrogen reduction under the tested conditions. These findings indicate that, in the engineering implementation of water extraction, ilmenite has the highest priority, followed by olivine and pyroxene.