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Thermal Transformations of Lepidocrocite and Akaganéite to Hematite: Examination of Possible Precursors to Martian Crystalline Hematite

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Thermal Transformations of Lepidocrocite and Akaganéite to Hematite: Examination of Possible Precursors to Martian Crystalline Hematite Seventh International Conference on Mars 3148.pdf THERMAL TRANSFORMATIONS OF LEPIDOCROCITE AND AKAGANÉITE TO HEMATITE: EXAMINATION OF POSSIBLE PRECURSORS TO MARTIAN CRYSTALLINE HEMATITE. T. D. Glotch1 and M. D. Kraft2, 1Jet Propulsion Laboratory, California Institute of Technology, [email protected], 2Arizona Sate University 2+ Introduction: Gray crystalline hematite (α-Fe2O3) the presence of excess Fe in solution [11,17-18] all has been discovered in several localities on Mars, in- favor the precipitation of lepidocrocite rather than goe- cluding Meridiani Planum, Aram Chaos, Iani Chaos, thite. Precipitation of akaganéite, on the other hand, is Aureum Chaos, and Valles Marineris, using data re- favored in hydrothermal environments with a high Cl turned by the Mars Global Surveyor (MGS) Thermal content and elevated temperatures near 60°C [15]. Emission Spectrometer (TES) instrument [1-5]. Data Samples and Methods: returned by the Athena science payload on the Mars Lepidocrocite-Precursor Series. The lepidocrocite Exploration Rover (MER) Opportunity showed that precursor (LPS2) used in this study is a synthetic yel- the gray hematite was concentrated in spherules— low-orange powder from Pfizer, Inc. described in pre- interpreted as concretions—which are eroding from a vious experimental work [19-20] It is composed of light-toned sulfate- and silica-rich outcrop [6-7]. acicular (lath-shaped) crystals with a mean size of Initial analyses of the TES hematite spectrum 0.03x0.9 μm. Samples were heated in air to 300°C and showed that it was dominated by [001] emission, and 500°C in an Omegalux LMF 3550 furnace for 24 this was confirmed by analyses of Mini-TES data [8- hours and then allowed to cool slowly to room tem- 9]. Previous work also investigated the role of precur- perature. sor mineralogy and morphology on the hematite spec- Akaganéite-Precursor Series. The akaganéite pre- tral character by thermally transforming goethite (α- cursor (AKG1) used in this study is a synthetic pre- cipitate prepared in the manner described by [15]. FeOOH) and magnetite (Fe3O4) to hematite at various temperatures and examining the subsequent IR spectra About 5 g of akaganéite were produced by dissolving in addition to VNIR spectra, TEM photomicrographs, 54.06 g FeCl3•6H2O in 4L distilled water and heating XRD patterns, and Mössbauer spectra [10]. That work to 40°C for 8 days. The precipitate was collected, showed that the IR emissivity spectrum of hematite washed, and dried. It is composed of somatoidal (spin- dle-shaped) crystals with a mean crystal size of ~150 derived by the high-temperature oxidation of magnet- nm in the long direction. Portions of the sample were ite is a poor fit to the Martian hematite spectrum heated in air to 150°C, 300°C, and 500°C in a furnace whereas the spectrum of hematite derived by the for 24 hours and then allowed to cool slowly to room lower-temperature thermal transformation of goethite termperature. was a good match. Thus, [10] concluded that a high- Sample Analysis. Mid-Infrared (200-2000 cm-1) temperature volcanic environment for the formation of thermal emission spectra were acquired on Arizona gray crystalline hematite on Mars was unlikely, and State University’s modified Nicolet Nexus 670 E.S.P. that a low-temperature aqueous environment condu- Fourier-Transform Infrared (FTIR) spectrometer. The cive to the initial formation of goethite was likely. spectrometer has a CsI beamsplitter and an uncooled In this work, we expand upon the initial analysis of deuterated triglycine sulfate (DTGS) detector. Follow- [10] by assessing the thermal transformations of lepi- ing the method of [10], samples were compressed at docrocite (γ-FeOOH) and akaganéite (β-FeOOH) to 10,000 psi in a Carver hydraulic press into compact hematite and discussing the implications of the results. pure pellets to increase the contrast of their IR spectra. Lepidocrocite and akaganéite in nature are often found Samples were heated overnight to 80°C in an oven and in association with other iron oxides and oxyhydrox- were actively heated at the same temperature as spectra ides [11-13], but specific environmental conditions can were being collected. Each spectrum presented in this favor or disfavor their formation. Therefore, if either work is an average 1000 scans collected on two differ- of these minerals are shown to be a possible precursor ent dates in 500 scan increments. to the Martian gray hematite, then specific inferences Transmission Electron Microscope (TEM) micro- can be made about the chemical environment on Mars graphs were acquired on ASU’s JOEL JEM-2010F at the time of their formation. TEM at ASU's Center for High Resolution Electron In terrestrial settings, lepidocrocite is often thermo- Microscopy at a 200 kV potential. Samples were pre- dynamically unstable with respect to goethite and pared by dispersing a powdered sample in acetone and jarosite (if sulfate is present in the system). However, covering a carbon film-covered grid with the dilute several factors can favor the formation of lepido- suspension. Powder X-ray diffraction spectra were crocite. The presence of organics [14], a slow rate of acquired on a Rigaku D/Max-IIB XRD instrument 3+ Fe hydrolysis [15], a low CO2 fugacity [13, 16], or using Cu- Kα radiation at 50 kV and 30 mA. Several Seventh International Conference on Mars 3148.pdf authors have investigated the visible/near-IR (VNIR) Thermal emission spectra of the lepidocrocite ther- spectral properties of lepidocrocite and akaganéite and mal transformation series are shown in Figure 4b. Af- their thermal transformation products [21-22], so these ter heating to 300°C for 24 hours, sample LPSH2-300 techniques are not employed in this work. displays an infrared spectrum that is a combination of Results: hematite and maghemite. This is consistent with previ- X-Ray Diffraction. Powder X-ray diffraction spec- ous studies that show that maghemite is an intermedi- tra that were acquired of the akaganéite and lepido- ate product in the thermal transformation of lepido- crocite samples and their thermal transformation prod- crocite to hematite [12]. After heating to 500°C for 24 ucts are shown in Figure 1. The relatively narrow lines hours, the emissivity spectrum of sample LPSH2-300 of the akaganéite sample AKG1 (Figure 1a) indicate is that of pure crystalline hematite. There is only a that it is well crystalline. The XRD pattern of the sam- small kink near 390 cm-1 indicating that most of the ple derived by heating the akaganéite to 500°C emissivity is due to emission from the (001) crystal (AKGH1-500) indicates that the sample has com- face. pletely transformed to hematite, and the narrow lines Discussion and Conclusions: Figure 5 shows Mar- indicate it is well crystalline. An additional XRD pat- tian crystalline hematite spectra derived from the TES tern of akaganéite heated to 300°C (AKGH1-300) is [3] and Mini-TES [23] datasets along with hematite identical to the AKGH1-500 pattern, indicating that samples derived from lepidocrocite and akaganéite as the transformation to hematite was complete at 300°C. well as the best-fit goethite-derived hematite from The XRD pattern of the lepidocrocite sample LPS2 [10]. It is evident that the akaganéite-derived hematite (Figure 1b) contains shallow, broad lines, indicating AKGH1-300 is a poor fit to the Martian crystalline that it is poorly crystalline. Upon heating to 500°C, hematite in terms of band shape and the relative mini- however, the sample is completely converted to well mum emissivities between the bands. The lepido- crystalline hematite. crocite-derived hematite LPSH2-500 spectrum is a Transmission Electron Microscopy. TEM micro- better fit to the Martian hematite spectra, in terms of graphs of samples LPS2 and LPSH2-500 and AKG1 band position and the relative minimum emissivities and AKGH1-300 are shown in figures 2 and 3. Micro- between the bands. However, the band centered at 540 graphs of samples LPS2 and LPSH2-500 (Figure 2) cm-1 is broader than is seen for the Martian hematite both show acicular crystals, indicating that the hema- spectrum. This is consistent with other synthetic hema- tite (LPSH2-500) is pseudomorphic after the precursor tite spectra created at temperatures of 500°C or higher lepidocrocite (LPS2). The acicular hematite crystals [10]. The best overall fit to the Martian hematite spec- appear similar to those produced by the thermal trans- tra is still the low-temperature goethite-derived hema- formation of acicular goethite to hematite [10]. tite spectrum GTSH2-300, discussed by [10]. Hematite sample AKGH1-300 and its akaganéite Lepidocrocite and akaganéite are iron oxyhydrox- precursor AKG1 (Figure 3) display very different mor- ides that form under specific Eh-pH conditions, and phologies. The AKG1 crystals are somotoidal and will precipitate from solution under varying circum- elongated in the [001] direction. Upon heating to 300 stances. Like goethite, these minerals can be thermally °C, the bcc anion packing of akaganéite breaks down transformed to hematite, which is observed at Merid- and rearranges into the hcp packing structure of hema- iani Planum on Mars. During the lepidocrocite-to- tite [12]. The resultant sample is composed of hexago- hematite transformation, maghemite forms as an in- nal and pseudo-hexagonal plates. termediate product. High temperatures (>500°C) are Thermal Emission Spectra. Emissivity spectra for needed to transform lepidocrocite completely to hema- the akaganéite thermal transformation series are shown tite. As discussed by [10], spectra of hematite formed in Figure 4a. After heating to 150°C for 24 hours, the at high temperatures are inconsistent with the Martian AKGH1-150 spectrum is very similar to the original hematite spectra. AKG1 spectrum. Previous work had shown that the The akaganéite-to-hematite transformation takes thermal transformation process from akaganéite to place at much lower temperatures (<300°C) than the hematite starts at ~150°C [12], so some change was lepidocrocite-to-hematite transformation.
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