Spectral and other physicochemical properties were determined for a suite of submicron powders of hematite (alpha-Fe2O3), maghemite (gamma-Fe2O3), magnetite (Fe3O4), goethite (alpha-FeOOH), and lepidocrocite (gamma-FeOOH). The spectral reflectivity measurements were made between 0.35 and 2.20 micrograms over the temperature interval between about -110 degrees and 20 degrees C. Other physicochemical properties determined were mean particle diameter, particle shape, chemical composition, crystallographic phase, magnetic properties, and Mossbauer properties. Only the magnetite powders have significant departures from the stoichiometric phase; they are actually cation-deficient magnetites having down to about 18.0 wt % FeO as compared with 31.0 wt % FeO for stoichiometric magnetite. A structured absorption edge due to crystal field transitions and extending from weak absorption in the near-IR to intense absorption in the near-UV is characteristic of the ferric oxides and oxyhydroxides and is responsible for their intense color. Particularly for hematite, the number and position of the spectral features are consistent with significant splitting of the degenerate cubic levels by noncubic components of the crystal field. The position of the crystal-field band at lowest energy, assigned to the envelope of the components of the split cubic 4T1 level, is near 0.86, 0.91, 0.92, and 0.98 microgram at room temperature for hematite, goethite, maghemite, and lepidocrocite, respectively. Comparison with Mossbauer data suggests covalent character increases sequentially through the aforementioned series. The positions of the spectra features are relatively independent of temperature down to about -110 degrees C. The maximum shifts observed were on the order of about 0.02 microgram shortward for the ferric oxyhydroxides. Variations in the magnitude of the reflectivity of the hematite powders as a function of mean particle diameter are consistent with scattering theory. The absorption strength of the crystal-field bands increases with increasing mean particle diameter over the range 0.1-0.8 micrometer; visually this corresponds to a change in color from orange to deep purple. The position of the split cubic 4T1 band shifts longward by about 0.02 micrometer with decreasing mean particle diameter over the same range; this trend is consistent with wavelength-dependent scattering. The cation-deficient magnetite powders are very strong absorbers throughout the near-UV, visible and near-IR; their spectral properties are independent of temperature between about -110 and 20 degrees C.

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 (o•-Fe203) 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 (---550øC) 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 Hin t 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 Hin t 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 Fe20 3 concentration, and the physio...

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