Thermal transformations of akaganeite and lepidocrocite to hematite: assessment of possible precursors to Martian crystalline hematite

We examine the possibility that crystalline hematite (alpha-Fe2O3) deposits on Mars were derived from the precursor iron oxyhydroxide minerals akaganeite (beta-FeOOH) or lepidocrocite (gamma-FeOOH) and compare them to an earlier study of goethite (alpha-FeOOH) and magnetite (Fe3O4) precursors. Both the mid-infrared and visible/near infrared spectra of hematite are dependent upon the hematite precursor mineral and the temperature of transformation. Laboratory spectra are compared to spectra from the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) and the Mars Exploration Rover (MER) Opportunity Mini-TES and Pancam experiments, allowing us to infer the formation environment of Martian crystalline hematite deposits. Akaganeite and lepidocrocite readily transform to hematite at temperatures of 300 and 500C, respectively. The visible/near-infrared and mid-infrared spectra of akaganeite-derived hematite are poor matches to data returned from TES, Mini-TES, and Pancam. The spectra of lepidocrocite-derived hematite are slightly better fits, but previously published spectra of goethite-derived hematite still represent the best match to MGS and MER spectral data. The experiments demonstrate that hematite precursor mineralogy, temperature of formation, and crystal shape exert a strong control on the hematite spectra.