Spectroscopic study of the dehydration and dehydroxylation of phyllosilicate and zeolite minerals: Applications to laboratory study and Martian exploration

Abstract

Phyllosilicates on Mars mapped by infrared spectroscopic techniques may have been affected by dehydration and/or dehydroxylation associated with volcanism or shock heating induced by meteor impact. The effects of heat-induced dehydration and/or dehydroxylation on the infrared spectra of 14 phyllosilicates from four structural groups (kaolinite, smectite, sepiolite-palygorskite, and chlorite) and 2 natural zeolites are reported in the 1st and 2nd parts of this dissertation. Pressed powders of size-separated phyllosilicate and natural zeolite samples were heated incrementally from 100 to 900 ??C, cooled to room temperature, and measured using multiple spectroscopic techniques: mid-infrared (mid-IR) (400-4000 cm-1) attenuated total reflectance (ATR), mid-to-far-IR reflectance (50-1400 cm-1), mid-to-far-IR emission (100-1400 cm-1), and near-infrared (NIR) reflectance (1.2-2.5 ??m) spectroscopies. Correlated thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) data were also acquired in order to clarify the thermal transformation of each sample. The results indicate that (1) all phyllosilicates exhibit characteristic degradations in both NIR and mid-to-far-IR spectral properties between 400-800 ??C, mainly attributable to the dehydroxylation and recrystallization processes as temperature increases. Spectral features of natural zeolites persist to higher temperatures compared to features of phyllosilicates during heating treatments. (2) The thermal behaviors of phyllosilicate infrared (IR) properties are greatly influenced by the compositions of the octahedral cations: changes in both the NIR and mid-to-far-IR spectra of phyllosilicates tend to occur at lower temperatures (300 to 400 ??C) in the Fe3+-rich samples as compared to the Al3+-rich types (400-600 ??C); Mg2+-trioctahedral phyllosilicates hectorite, saponite, and sepiolite all display major mid-to-far-IR spectral changes at 700 ??C, corresponding to the formation of enstatite; phyllosilicates that have minor replacement of Mg2+ for Al3+ in octahedral positions (e.g. cheto-type montmorillonite and palygorskite) show an absorption band at ~920 cm-1 that becomes strong at 900 ??C. (3) Spectral results have provided suggestive evidence for the scenario that some phyllosilicates could lose all original spectral features in mid-to-far-IR region while maintaining their characteristic hydration bands in NIR region in the same temperature range. Using the mid-IR emissivity and NIR spectra collected in the 2nd part, a detailed analysis of phyllosilicates in the ancient Nili Fossae region of Mars was conducted in the 3rd part of this dissertation. Analysis of remote sensing data including both thermal and near IR dataset suggests that thermally altered nontronite (400 ??C) is likely present in the Nili Fossae region of Mars mixed with unaltered nontronite. The mixing of altered and unaltered phyllosilicates is the likely cause for the apparent disconnect between previous VNIR and TIR observations of nontronite-bearing surfaces in Nili Fossae.