Spectroscopic study of the dehydration and/or dehydroxylation of phyllosilicate and zeolite minerals

摘要

Phyllosilicates on Mars mapped by infrared spectroscopic techniques could have been affected by dehydration and/or dehydroxylation associated with chemical weathering in hyperarid conditions, volcanism or shock heating associated with 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 two natural zeolites are reported here. Pressed powders of size‐separated phyllosilicate and natural zeolite samples were heated incrementally from 100°C to 900°C, cooled to room temperature, and measured using multiple spectroscopic techniques: midinfrared (400–4000 cm~(-1)) attenuated total reflectance, midinfrared reflectance (400–1400 cm~(-1)), and far‐infrared reflectance (50–600 cm~(-1)) spectroscopies. Correlated thermogravimetric analysis and X‐ray diffraction data were also acquired in order to clarify the thermal transformation of each sample. For phyllosilicate samples, the OH stretching (～3600 cm~(-1)), OH bending (～590–950 cm~(-1)), and/or H_2O bending (～1630 cm~(-1)) bands all become very weak or completely disappear upon heating to temperatures > 500°C. The spectral changes associated with SiO_4 vibrations (～1000 cm~(-1) and ～500 cm~(-1)) show large variations depending on the compositions and structures of phyllosilicates. The thermal behavior of phyllosilicate IR spectra is also affected by the type of octahedral cations. For example, spectral features of Al~(3+)‐rich smectites are more stable than those of Fe~(3+)‐rich smectites. The high‐temperature (>800°C) spectral changes of trioctahedral Mg~(2+)‐rich phyllosilicates such as hectorite, saponite, and sepiolite result primarily from crystallization of enstatite. Phyllosilicates withmoderateMg~(2+) concentration (e.g., palygorskite, clinochlore) and dioctahedral montmorillonites (e.g., SAz‐1 and SCa‐3) with partial Mg~(2+)‐for‐Al~(3+) substitution all have new spectral feature developed at ～900 cm~(-1) upon heating to 800°C. Compared with phyllosilicates, spectral features of two natural zeolites, clinoptilolite and mordenite, are less affected by thermal treatments. Even after heating to 900°C, the IR spectral features attributed to Si (Al)‐O stretching and bending vibration modes do not show significant differences from those of unheated zeolites.