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

Abstract

Features attributed 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 Mossbauer 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. Samples were prepared by calcining (∼550°C) powders of high‐area silica gel that had been impregnated with ferric nitrate solutions. The samples are classified according to type of Mossbauer spectrum observed at 293 K. (1) Type S + D samples, which by Mossbauer granulometry contain hematite particles both larger and smaller than 10(2) nm, are characterized by a hematite sextet plus superparamagnetic doublet. (Uncertainties are given in parentheses and refer to the final digit(s).) (2) Type D samples, which contain hematite particles smaller than 10(2) nm, are characterized by only a superparamagnetic doublet and so contain only sp‐Hm. The presence of larger particles in type S + D samples is consistent with X ray diffraction data; the diffraction patterns of type S + D samples are characterized by a few, broad hematite lines, and type D samples have no lines because the particles are too small to coherently scatter X rays. Measurements of internal field strengths (Hint) at 22 K for both type S + D and type D samples show that Hint is not constant but decreases with decreasing particle diameter from 54.0 T for bulk‐Hm to 46.6 T for 5.4‐nm sp‐Hm. This dependence implies that phase identifications based solely on comparisons to bulk values of Hint are equivocal when superparamagnetic particles are present. Sp‐Hm (<10‐nm diameter) is much more magnetic than bulk‐Hm; the saturation magnetization at 293 K for type D samples is 7(2) A m2/kg as compared to 0–0.5 A m2/kg for bulk‐Hm. Optical properties of type S + D samples are similar to those of bulk‐Hm; in particular, a well‐defined band minimum is present near 860 nm. Optical properties of type D samples, with only sp‐Hm at 293 K, are significantly different in that a step‐shaped feature instead of a well‐defined band is centered near 860 nm. The transition from well‐defined band to step‐shaped feature occurs at a hematite particle diameter of ∼10 nm. The position of the UV‐visible absorption edge and the absorption strength at 860 nm depend on the number density of sp‐Hm particles, the Fe2O3 concentration, and the physiochemical properties of the support material. For 7(2)‐nm sp‐Hm particles dispersed within silica gel particles (35‐ to 74‐μm powder with 6‐nm pore diameter) the absorption edge shifts toward the visible, and the absorption strength at 860 nm increases with increasing number density of the sp‐Hm particles. Visually, the color change is from nearly white to tan to dark red. Optical properties of samples containing 7(2)‐nm sp‐Hm particles are essentially independent of temperature between 173 and 293 K.