THE CHALLENGE OF DISTINGUISHING IRON (HYDR)OXIDES AND WHAT IT MEANS FOR MARS

In the nanophase regime, mineral structure becomes a function of grain size, due in part to surface curvature inhibiting normal termination of unit cells, leading to structurally distinct surface environments. The surface strain can propagate through the particle, forming unique and varied coordination environments from those seen in bulk materials. These structural differences are reflected in the mineral spectra to an extent that is not yet fully understood.

For the iron (hydr)oxides (FeOx), seven of the minerals are polymorphs of two chemical formulas. Therefore, in addition to the expected broadening, structural changes due to size can effectively blur the lines of spectral distinction between the phases, complicating spectral identification. X-ray diffraction (XRD) of nanophase FeOx can show substantial peak broadening but can be of limited usefulness becauseh feroxyhyte has the same major XRD peak as ferrihydrite, and the samples covering the solid solution from maghemite to magnetite have overlapping peak positions depending on crystallinity and cation substitution. Raman spectra are difficult to acquire without transforming FeOx to hematite, and spectra suffer from peak broadening and loss of some features. With care, most oxides are distinguishable using Raman, but in nanophase samples, the single, broad feature seen for ferrihydrite, magnetite, and maghemite is highly overlapped. Infrared (IR) absorptions are broadened due to a multitude of nonequivalent sites and visible absorptions shift due to cation substitution and crystallinity. IR spectra show sufficient variation from 1200-200 cm^-1 for distinguishing pure samples (with the exception of nanophase magnetite and maghemite) but the 400-200cm^-1 range is important. Mössbauer spectra, the industry standard for FeOx discrimination, display superparamagnetic doublets for sufficiently small oxides that can persist well below room temperature. Even at 4K, changes in grain size cause many Mössbauer spectra of FeOx to appear similar, and magnetite and maghemite are almost impossible to distinguish. Clearly, mixed samples analyzed by only one or two techniques will produce equivocal results, and only integrated spectral studies can positively identify iron (hydr)oxides (FeOx) on Mars or in the laboratory.