The Moon no longer generates a global magnetic field, but traces of its ancient magnetism remain locked in rocks and soil. Researchers analyzing Chang'e-6 lunar samples have now identified γ-Fe, a face-centered cubic form of metallic iron, for the first time in natural lunar material. The mineral was found inside tiny impact-glass particles, where it exists as nanoscale grains.
Under ordinary conditions, γ-Fe is stable only at high temperatures and typically converts to α-Fe as it cools. The team proposes that carbon impurities, rapid cooling during impacts, and protection from the surrounding glassy matrix allowed this rare phase to survive on the lunar surface. In the two impact-glass samples studied, γ-Fe was actually the dominant form of iron present.
Using off-axis electron holography, the researchers observed that larger γ-Fe particles can form a stable single-vortex magnetic state and respond consistently to external magnetic fields. That stability suggests γ-Fe could act as a previously unrecognized recorder of magnetic information, preserving clues about conditions during the Moon's magnetic past.
Because γ-Fe and α-Fe form under different conditions and behave differently magnetically, they may capture separate stages of lunar impact events. The study, led by Prof. Haifeng Du of the Chinese Academy of Sciences, was published in PNAS. Further research is needed to determine how much these minerals can reveal about the evolution of the Moon's magnetic field.