LEPIDOCROCITE TO MAGHEMITE TO HEMATITE - A PATHWAY TO MAGNETIC AND HEMATITIC MARTIAN SOIL

We examined decomposition products of lepidocrocite, which were produc ed 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 micros copy (TEM), magnetic methods, and reflectance spectroscopy (visible an d near-infrared (IR)). Single-crystal lepidocrocite particles dehydrox ylated to polycrystalline particles of disordered maghemite that subse quently transformed to polycrystalline particles of hematite. Essentia lly 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 sp ectrally 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. An alyses by TEM showed that individual particles are polycrystalline wit h respect to both maghemite and hematite. The spectrally hematitic and magnetic Mh + Hm particles can satisfy the spectral and magnetic cons traints for Martian surface materials over a wide range of values of M h/(Mh + Hm) either as pure oxide powders or (within limits) as compone nts of multiphase particles. These experiments are consistent with lep idocrocite 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 magne t array showed that spectrally hematitic Mh + Hm powders having J(S) e qual to 20.6 Am-2/kg adhered to all five magnets.