Evidence for pigmentary hematite on Mars based on optical, magnetic, and Mössbauer studies of superparamagnetic (nanocrystalline) hematite.

Features atrributed to ferric iron in remotely sensed spectral data of Mars and the magnetic nature of Martian soil at the Viking landing sites are consistent with the occurrence of hematite (α-Fe₂O₃) as both superparamagnetic (nanocrystalline) hematite (sp-Hm) and larger-diameter hematite (bulk-Hm) particles. These hematite particles most likely occur in pigmentary form, that is, as particles dispersed throughout the volume of a relatively spectrally neutral (silicate?) material. Likely physical forms of this pigmented volume include rocks, dust and soil particles, and coatings (weathering rinds) thereon. Accommodation of Martian data by hematite is a result of differences in optical and magnetic properties of sp-Hm and bulk-Hm particles. Optical, magnetic, and Mössbauer properties of sp-Hm particles dispersed within particles of high-area silica gel are reported in this study and compared to the corresponding properties of bulk-Hm powders.