Effect of precursor mineralogy on the thermal infrared emission spectra of hematite: Application to Martian hematite mineralization

Observations from the Thermal Emission Spectrometer (TES) instrument aboard the Mars Global Surveyor (MGS) spacecraft led to the discovery of two isolated deposits of gray, crystalline hematite located in Meridiani Planum and Aram Chaos and several smaller deposits in Valles Marineris. Several pathways for formation of these hematite deposits have been proposed, involving both aqueous and nonaqueous processes. This work uses the precise shape and position of spectral features in the Martian hematite thermal emission spectrum to constrain hematite formation pathways. Thermal infrared emission spectra, X-ray powder diffraction patterns, Mössbauer spectra, and transmission electron microscope (TEM) photomicrographs were obtained for synthetic and natural hematite samples derived by (1) dehydroxylation of fine-grained goethite and (2) oxidation of magnetite. Collectively, the instrumental analyses show that the mineralogical composition and crystal morphology of precursor samples and the time and temperature conditions under which decomposition to hematite occur determine the crystallinity and crystal morphology of the hematite product. Comparison of laboratory and MGS-TES spectra shows that the Martian hematite spectra correspond closely with a synthetic hematite sample derived by pseudomorphic and topotactic dehyroxylation of goethite at 300°C. Spectra of goethite-precursor samples dehydroxylated at higher temperatures provide increasingly poor fits. Spectra of hematite samples derived by high-temperature thermal oxidation of magnitite are also poorer fits to the Martian hematite spectrum. Thermal emission spectra of goethites heated at lower temperatures ate characterized by the spectral signatures of both hematite and goethite and are not consistent with the Martian spectra. The characteristics that distinguishes the synthetic hematite sample with the Mars-like spectral signature from the other synthetic hematite samples is the high proportion of crystal surfaces that are crystallographic {001} faces (c faces) for the former but not the latter. The high proportion of {001} face area results because the largest surface of the lath-shaped hematite particles is the (001) face, as determined by TEM. Thus a possible formation pathway for hematite in Meridiani Planum, Aram Chaos, and Valles Marineris is precipitation of goethite from aqueous solutions as lath-shaped crystals, possibly as a stain, cement, and/or massive deposit, followed by pseudomorphic and topotactic dehydroxylation to hematite at temperatures below ∼300°C.