Thermal alteration of nontronite and montmorillonite: Implications for the martian surface

To test the effects of meteorite impacts on martian phyllosilicate deposits, we heated two smectites (non-tronite and montmorillonite) to temperatures ranging from 350 degrees C to 1150 degrees C for durations of 4-24 h in two different atmospheres, under air and a steady flow of CO2. Samples were analyzed using X-ray diffraction (XRD) and near-infrared (NIR) and mid-infrared (MIR) reflectance spectroscopy. Interlayer water was lost after heating to temperatures of similar to 400 degrees C. Between 400 degrees C and 700 degrees C, nontronite converted to an intermediary phase which conserved the XRD pattern of untreated nontronite with the exception of the 0 0 1 peak. Nanocrystalline high-temperature phases formed for both smectites at temperatures between 700 degrees C and 1000 degrees C. Finally, after being heated to temperatures above similar to 1100 degrees C, the samples melted and recrystallized into secondary phases. Secondary high-temperature phases included sillimanite and cristobalite for both smectites plus hematite for nontronite. NIR and MIR reflectance spectra significantly evolved with increasing temperature. NIR spectra of smectites showed that 1.4 and 1.9 mu m bands decrease in intensity and disappear above 700 degrees C. Similarly, the 2.2-2.3 mu m metal-OH band showed a decrease in intensity as well as an overall shift towards lower wavelengths (for nontronite) due to the thermal resistance of the Al-OH bond compared to the Fe-OH bond. NIR spectra of montmorillonite showed a gradual increase in band depth up to temperatures between 500 degrees C and 600 degrees C, then decreased with increasing temperature. In the MIR spectra of samples heated to temperatures above similar to 1100 degrees C, newly formed bands confirmed the secondary phases identified by XRD. Similarities between the NIR spectra of our heated samples and the phyllosilicates in some martian craters imply that these phyllosilicates were altered by the impact-generated heat and thus were not formed post-impact. In addition, NIR reflectance spectra provide a proxy for shock temperatures of smectites up to 700 degrees C while MIR is optimum for identification of high-temperature phases of smectites above 700 degrees C. (C) 2010 Elsevier Inc. All rights reserved.