Phys Chem Minerals (2008) 35:569–581 DOI 10.1007/s00269-008-0249-z 123 ORIGINAL PAPER Thermal transformations of akaganéite and lepidocrocite to hematite: assessment of possible precursors to Martian crystalline hematite Timothy D. Glotch · Michael D. Kraft Received: 19 February 2008 / Accepted: 11 June 2008 / Published online: 4 July 2008  Springer-Verlag 2008 Abstract We examine the possibility that crystalline hematite (-Fe 2 O 3 ) deposits on Mars were derived from the precursor iron oxyhydroxide minerals akaganéite (-FeOOH) or lepidocrocite (-FeOOH) and compare them to an earlier study of goethite (-FeOOH) and magnetite (Fe 3 O 4 ) pre- cursors. Both the mid-infrared and visible/near infrared spectra of hematite are dependent upon the hematite precur- sor mineral and the temperature of transformation. Labora- tory 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. Aka- ganéite and lepidocrocite readily transform to hematite at temperatures of 300 and 500°C, respectively. The visible/ near-infrared and mid-infrared spectra of akaganéite- derived hematite are poor matches to data returned from TES, Mini-TES, and Pancam. The spectra of lepidocrocite- derived hematite are slightly better Wts, but previously pub- lished spectra of goethite-derived hematite still represent the best match to MGS and MER spectral data. The experi- ments demonstrate that hematite precursor mineralogy, temperature of formation, and crystal shape exert a strong control on the hematite spectra. Keywords Mars · Hematite · Lepidocrocite · Akaganéite · Infrared spectroscopy Introduction Crystalline hematite (-Fe 2 O 3 ) has been discovered in sev- eral localities on Mars, including Meridiani Planum, Aram Chaos, Iani Chaos, Aureum Chaos, and Valles Marineris, using data returned by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) instrument (Christen- sen et al. 2000, 2001a, b; Glotch and Christensen 2005; Glotch and Rogers 2007; Weitz et al. 2008). Data returned by the Athena science payload on the Mars Exploration Rover (MER) Opportunity at Meridiani Planum has shown that the crystalline hematite is concentrated in spherules that are eroding from a light-toned sulfate- and silica-rich outcrop (Squyres et al. 2004; McLennan et al. 2005; Glotch et al. 2006a). These spherules have been alternatively interpreted as concretions (Squyres et al. 2004; Chan et al. 2004), volca- nic lapilli (McCollom and Hynek 2005), or impact melt spherules (Knauth et al. 2005). In addition, hematite-rich spherules have been produced under hydrothermal conditions in the laboratory with jarosite in the system (Golden et al. in press) suggesting that jarosite may be the Martian hematite precursor. Remote sensing measurements of the other hema- tite-bearing terrains on Mars indicate that Mg- and Ca-bear- ing sulfates are also present in association with the hematite (Gendrin et al. 2005; Glotch and Rogers 2007; Noe Dobrea et al. 2008; Weitz et al. 2008). Initial analyses of the TES hematite spectrum showed that it is dominated by [001] emission (Lane et al. 2002), and this was conWrmed by analyses of Mini-TES data (Glotch et al. 2006b; Calvin et al. in press). Previous work also investigated the role of precursor mineralogy and mor- phology on the visible/near-IR (VNIR) and mid-infrared (MIR) spectral character of hematite by thermally trans- forming goethite (-FeOOH) and magnetite (Fe 3 O 4 ) to hema- tite at various temperatures and examining the subsequent T. D. Glotch (&) Department of Geosciences, Stony Brook University, Stony Brook, NY 11794-2100, USA e-mail: tglotch@notes.cc.sunysb.edu M. D. Kraft School of Earth and Space Exploration, Arizona State University, Temp, AZ 85287-1404, USA