HR: 0800h
AN: P21A-0210    [Abstracts]
TI: Thermal Transformations of Lepidocrocite and Akaganeite to Hematite: Examination of Possible Precursors to Martian Crystalline Hematite
AU: * Glotch, T D
EM: tglotch@notes.cc.sunysb.edu
AF: Stony Brook University, 255 Earth and Space Sciences, Stony Brook, NY 11794-2100, United States
AU: Kraft, M D
EM: mdkraft@asu.edu
AF: Arizona State University, School of Earth And Space Exploration Box 871404, Tempe, AZ 85287-1404, United States
AB: We examine the thermal transformations of lepidocrocite and akaganeite to hematite to determine if these minerals are viable precursors to Martian crystalline hematite. This is an extension of the work of Glotch et al. (2004) who demonstrated that precursor mineralogy and temperature of hematite formation have a strong influence on the infrared emissivity spectrum of coarsely crystalline hematite. Any indication that lepidocrocite or akaganeite were potential precursors to Martian crystalline hematite would yield information about the geochemical environment at Meridiani Planum at the time of their formation. In terrestrial settings, lepidocrocite is often thermodynamically unstable with respect to goethite and jarosite (if sulfate is present in the system). However, several factors can favor the formation of lepidocrocite. The presence of organics, a slow rate of Fe³⁺ hydrolysis, a low CO₂ fugacity, or the presence of excess Fe²⁺ in solution all favor the precipitation of lepidocrocite rather than goethite. Precipitation of akaganeite, on the other hand, is favored in hydrothermal environments with a high Cl content and elevated temperatures near 60°C. The synthetic precursor minerals were heated in air to 150, 300, 500, and 700°C. The resulting products were imaged by transmission electron microscopy (TEM) and analyzed by X-ray diffraction, mid-IR emission spectroscopy, visible/near-IR reflectance spectroscopy, mid-IR attenuated total reflectance spectroscopy. Results of these analyses indicate that both lepidocrocite and akaganeite are unlikely to be precursors to Martian crystalline hematite. Laboratory studies suggest that a goethite to hematite pathway is the most likely scenario for the formation of Martian crystalline hematite.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 5410 Composition (1060, 3672)
DE: 5464 Remote sensing
DE: 5494 Instruments and techniques
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting