Magnetic phases in lunar fines: metallic Fe or ferric oxides?

The ferromagnetic resonance (g = 2.08 ± 0.03) observed for the Apollo 11 and 12 lunar fines is characterized by an asymmetric lineshape with a narrower appearance on the high field side. This asymmetry together with an anisotropy energy which varies from + 640 to + 500 G over the temperature range of 80 to 298 K indicates that the ferromagnetic resonance arises from metallic Pe having the body-centered cubic structure and not from hematite, magnetite or other Fe³⁺ ions in magnetite-like phases. The g-value, the lineshape asymmetry, and the temperature dependence of the linewidth for the Apollo 14 and 15 fines as reported by other workers are found to be essentially similar to those observed for the Apollo 11 and 12 fines, except a narrower linewidth is observed in the case of the Apollo 14 fines (600–650 G vs 700–800 G, at X-band and room temperature). This difference in linewidth is found to be correlated with the Ni and/or Co content in these samples. The ESR signal for the so-called ‘unknown ferric oxide’ phase of Griscom and Marquardt is also shown to have frequency and temperature dependences significantly different from those for the lunar fines.