Spectral reflectance properties of magnetites: Implications for remote sensing

Abstract

Magnetite (Fe3+(Fe2+Fe3+)2O4) is ubiquitous in Earth and planetary materials, forming in igneous, metamorphic, and sedimentary settings, sometimes influenced by microbiology. Magnetite can be used to study many and varied planetary processes, such as the oxidation state of magmas, paleomagnetism, water–rock interactions such as serpentinization, alteration and metamorphism occurring on meteorite parent bodies, and for astrobiology. The spectral reflectance signature of magnetite in the ultraviolet, visible, and near-infrared is somewhat unusual compared to common planetary materials, suggesting that remote detection and characterization of magnetite should be possible. Here we present a systematic investigation of the reflectance spectral properties of magnetite using natural and synthetic samples. We investigate the effects of chemical substitutions, grain size variations, and mixtures with other phases in order to better constrain remote spectral searches for, and interpretation of, magnetite-bearing lithologies. Magnetite is characterized by high extinction over the entire wavelength range considered here, and therefore surface scattering dominates over volume scattering. Magnetite reflectance spectra are strongly influenced by the presence of delocalized electrons above the Verwey transition temperature (∼120 K), leading to metal-like scattering behavior, that is, high extinction, surface scattering dominant, and a general increase in reflectance with increasing wavelength, i.e. “red-sloped and featureless”. Superimposed upon the metal-like reflectance are local reflectance maxima which we ascribe to Fresnel reflectance peaks corresponding to Fe–O oxygen–metal charge transfer processes (∼0.27 and ∼0.39 µm) and Fe-related field–d orbital transitions (∼0.65 µm). We also find a systematic shift in the wavelength position of the 0.65 µm Fresnel peak with increasing chemical impurity in magnetite. Magnetite reflectance spectra are most similar to those of titanomagnetite and wüstite, and unlike those of other Fe–(Ti) oxides, such as ilmenite, hæmatite, ulvospinel, maghemite, pseudobrookite, and armalcolite.