Spectral characteristics of the iron oxides with application to the Martian bright region mineralogy

Abstract

In an effort to determine the nature and significance of the iron oxide mineralogy on Mars, reflectance spectra of eight polymorphs of FeOOH and Fe 2 O 3 are presented together with additional data on these materials. The Kebulka‐Munk theory is used to qualitatively constrain the effect of other components that might interfere with iron oxide absorption features. Although the wavelength range 0.4–1.0 μm is potentially the most diagnostic spectral region for delineating the iron oxide phases, the effect of temperature complicates the identification of a given Fe 3+ phase based on the position of the 6 A 1 → 4 T 1 absorption feature. The Fe 3+ crystal field transitions are spin forbidden, but most of the iron oxide polymorphs exhibit anomalously intense crystal field absorption features resulting from magnetic coupling between adjacent FeO 6 octahedra. The resulting deviations from observed remote sensed reflectance spectra of Mars may provide a basis for the exclusion of many iron oxide phases as significant components of the Martian Fe 3+ mineralogy. Comparison with the visible region spectra of Martian bright regions suggest that the predominant Fe 3+ ‐bearing phase may be a magnetically disordered material. Possible candidates include amorphous gels, some ferric sulphates, and other minerals in which Fe 3+ ions in the crystal structure are not magnetically coupled.