MATRIX EFFECTS FOR REFLECTIVITY SPECTRA OF DISPERSED NANOPHASE (SUPERPARAMAGNETIC) HEMATITE WITH APPLICATION TO MARTIAN SPECTRAL DATA

The effect of the matrix on reflectivity spectra of nanophase (superparamagnetic) hematite (np-Hm) dispersed within it was studied over the 350-2200-nm wavelength range. Np-Hm is characterized by particle diameters less than ~10 nm and has properties distinctly different from larger diameter hematite particles (bulk-Hm). Data were obtained for four series of powder samples which have different matrix properties and variable concentrations of np-Hm dispersed within discrete powder particles. It is possible to match the Fe2O3 concentration, magnetic properties, and spectral data for Martian surface material with a laboratory mixture whose only ferric-bearing phase is hematite. What made this possible is a matrix material with suitable optical scattering properties and differences in optical and magnetic properties of np-Hm and bulk-Hm. Most of the hematite is present as strongly magnetic np-Hm, but some bulk-Hm is required to give a shallow ~860-nm band. Hematite is thus very likely present on Mars and may even be the dominant ferric mineralogy. -from Authors