Spectral and other physicochemical properties of submicron powders of hematite (α‐Fe₂O₃), maghemite (γ‐Fe₂O₃), magnetite (Fe₃O₄), goethite (α‐FeOOH), and lepidocrocite (γ‐FeOOH)

Abstract

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.

A non-destructive spectroscopic study of the decoration of archaeological pottery: from matt-painted bichrome ceramic sherds (southern Italy, VIII-VII B.C.) to an intact Etruscan cinerary urn

“…Therefore, for the red decorations of all sherds the reflection spectral pattern ( Figure 5 shows the spectrum of sample 9) resembles the one reported for -Fe2O3, with an edge around 545 nm, a shoulder at about 640 nm and a minimum at about 860 nm [33]. Nevertheless, variations in the relative intensities and, above all, in the exact wavelength values take place from one sample to another.…”

Chemical, multispectral, and textural constraints on the composition and origin of rocks at the Mars Pathfinder landing site

“…Reflectance at a blue (440 nm) wavelength is in general much less variable between Martian rocks and soils [Soderblom, 1992] general correlation between the red/blue ratio and iron oxidation state in natural composite particles (oxidatively altered volcanic tephra). The peak reflectance of typical Martian spectra [Mustard and Bell, 1994], as well as the spectra of' most ferric minerals [Sherman et al, 1982;Morris et al, 1985;Bishop, 1995], occurs near 750 ran. However, in a few minerals (most notably ferrihydrite and maghemite), the peak occurs instead at or near 800 nm (Figure 7).…”

Preparation and Properties of Hematite with Structural Phosphorus

“…5 % were bright red. Purple hues in hematite are associated with large particle sizes (Morris et al, 1985). So, it can be hypothesized that particles sintered at 1073 K, the degree of sintering decreased with increasing P content.…”