Thermal stability of akaganeite and its desiccation process under conditions relevant to Mars

1,2Xiaohui Fu,1Liangchen Jia,3Alian Wang,1Haijun Cao,1,2 Zongcheng Ling,1Changqing Liu,1Erbin Shi,1Zhongchen Wu,1Bo Li,1Jiang Zhang
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113435]
1Shandong Provincial Key Laboratory of Optical Astronomy & Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, China
2CAS Center for Excellence in Comparative Planetology, Hefei, China
3Department of Earth and Planetary Sciences, McDonnell Center for Space Sciences, Washington University, St. Louis, MO, USA
Copyright Elsevier

Akaganeite has been found in Yellowknife Bay mudstones of the Gale crater by the Chemistry and Mineralogy X-ray diffraction instrument (CheMin) aboard the Curiosity rover. This phase has also been discovered in limited locations on Mars by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter. Akaganeite has also been proposed as a precursor candidate of hematite on Mars. To better constrain the stability and occurrence of akaganeite on Martian surfaces, structural and spectral modifications of akaganeite introduced by heating and desiccation were systematically investigated. We found that the phase transformation from akaganeite to hematite starts at 245 °C, which is accompanied by the removal of chloride in tunnels. We propose that geological activities (e.g., impact and volcanism on Mars) could heat the surrounding area and cause the transformation of akaganeite into hematite in Martian rocks and surface materials. Relative humidity (RH) variations result in water combination and overtone absorptions band strength changes. The CRISM spectrum of akaganeite detected in the Robert Sharp crater shows relatively weak 1.39 μm band compared to that of desiccated akaganeite under simulated Martian environments, indicating that akaganeite found on Mars could be highly desiccated. The water adsorption of akaganeite occurred when exposed to ambient laboratory conditions (RH ~65%). This suggests the water adsorption and desorption of akaganeite on Mars correspond to RH changes in a diurnal cycle.

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