Lepidocrocite to maghemite to hematite: A pathway to magnetic and hematitic Martian Soil

We examined decomposition products of lepidocrocite, which were produced by heating the phase in air at temperatures up to 525 degrees C for 3 and 300 h, by x-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic methods, and reflectance spectroscopy (visible and near-infrared (IR)). Single-crystal lepidocrocite particles dehydroxylated to polycrystalline particles of disordered maghemite that subsequently transformed to polycrystalline particles of hematite. Essentially pure maghemite was obtained at 265 and 223 degrees C for the 3 and 300 h heating experiments, respectively. Its saturation magnetization (J(S)) and mass specific susceptibility are similar to 50 Am-2/kg and similar to 400 x 10(-6) m(3)/kg, respectively. Because hematite is spectrally dominant, spectrally hematitic samples (i.e., a minimum near 860 nm and a maximum near 750 nm) also could be strongly magnetic (J(S) up to similar to 30 Am-2/kg) from the masked maghemite component. Analyses by TEM showed that individual particles are polycrystalline with respect to both maghemite and hematite. The spectrally hematitic and magnetic Mh + Hm particles can satisfy the spectral and magnetic constraints for Martian surface materials over a wide range of values of Mh/(Mh + Hm) either as pure oxide powders or (within limits) as components of multiphase particles. These experiments are consistent with lepidocrocite as the precursor of Mh + Hm assemblages on Mars, but other phases (e.g., magnetite) that decompose to Mh and Hm are also possible precursors. Simulations done with a copy of the Mars Pathfinder magnet array showed that spectrally hematitic Mh + Hm powders having J(S) equal to 20.6 Am-2/kg adhered to all five magnets.