Thermal Alteration of Nontronite and Montmorillonite: Implications for the Martian Surface

Introduction: OMEGA/Mars Express data was confirmed by CRISM/MRO data of the existence of clay minerals on the surface of Mars [1, 2]. These clays have been found in some of the oldest terrains of Mars and because of their Noachian age, they hold clues to the earliest history of the martian surface. Clays have been detected in outcrops surrounded by lava flows and in the ejecta of small impact craters [3, 4]. Previous studies have suggested that clays may form by impactinduced hydrothermal processes [5] while others imply that they formed much earlier and that thermally altered clays may be responsible for the properties of the red dust that covers the martian surface [6, 7]. In this study, we investigate the effects of thermal treatment on the spectral properties of clays, focusing on nontronite and montmorillonite, and their relation to the martian surface. Experimental and Analytical Methods: One-gram samples of nontronite (Fe, Mg) and montmorillonite (Al, Ca) were heated in a Lindberg tube furnace to temperatures ranging from 350C to 1150C for 4 to 24 hours. Samples were heated in air as well as under a steady flow of CO2 to more closely replicate early martian conditions. Each sample was allowed to cool overnight and weighed after heating. The samples’ extreme color change after heating was precisely characterized using Munsell soil color charts. Samples were analyzed using X-ray diffraction and near-infrared reflectance spectroscopy. Results and Discussion: Each heated sample of nontronite and montmorillonite showed significant color change. Nontronite samples changed from yellow-green to shades of reddish brown. Montmorillonite samples transitioned from gray to shades of orange. Weighing the heated samples showed there was an average of about 25% mass loss, most likely due to the loss of interlayer water, as it has been shown to be lost at lower temperatures than is structural water [6]. XRD spectra taken of nontronite samples heated to low temperatures (T < 750C) showed that the 001 peak had disappeared, consistent with the loss of interlayer water, but all other peaks were still intact. There was also very little difference in the spectra of samples heated in air and those heated in CO2, indicating the CO2 atmosphere had little, if any, effect on the transformation process. The XRD spectra of samples heated to intermediate temperatures (800C < T < 1000C) showed low intensity, broad peaks. These are evidence of a complex mixture of secondary nanocrystalline phases. The sharp, well-defined peaks in the XRD spectra of samples heated to high temperatures (T > 1100C) indicate the sample has melted and recrystallized into new phases (Figure 1). New phases such as hematite, cristoballite and sillimanite were identified.