Interlayer cations and moisture content greatly influence the molecular vibrations of H2O in montmorillonite as shown through reflectance spectroscopy in the infrared. The absorptions due to H2O have been studied in montmorillonites exchanged with H, Na, Ca, Mg and Fe3+ interlayer cations under variable moisture environments. Band assignments have been made for absorptions in the 3 µm region due to structural OH vibrations, symmetric and asymmetric H2O stretching vibrations and the H2O bending overtone. Changes in the energies of the absorptions due to H2O stretching vibrations were observed as the samples were dehydrated by reducing the atmospheric pressure. Absorptions near 3620 cm−1 and 3550 cm−1 have been assigned to water bound directly to cations (inner sphere) and surface-bonded H2O and absorptions near 3450 cm−1 and 3350 cm−1 have been assigned to additional adsorbed water molecules. Band assignments have been made for combination bands in the near-infrared as well. Absorptions near 1.41 μm and 1.91 μm are assigned to bound H2O combination bands, while the shoulders near 1.46μm and 1.97 μm are assigned to combinations of additional H2O molecules adsorbed in the interlayer regions and along grain surfaces.

Allophane and imogolite are common alteration products of volcanic materials. Natural and synthetic allophanes and imogolites were characterized in the present study in order to clarify the short-range order of these materials and to gain an understanding of their spectral properties. Spectral analyses included visible/near-infrared (VNIR), and infrared (IR) reflectance of particulate samples and thermal-infrared (TIR) emissivity spectra of particulate and pressed pellets. Spectral features were similar but not identical for allophane and imogolite. In the near-infrared (NIR) region, allophane spectra exhibited a doublet near 7265 and 7120 cm−1 (1.38 and 1.40 μm) due to OH2v, a broad band near 5220 cm−1 (1.92 μm) due to H2Ov+δ, and a band near 4560 cm−1 (2.19 μm) due to OHv+δ. Reflectance spectra of imogolite in this region included a doublet near 7295 and 7190 cm−1 (1.37 and 1.39 μm) due to OH2v, a broad band near 5200 cm−1 (1.92 μm) due to H2Ov+δ, and a band near 4565 cm−1 (2.19 μm) due to OHv+δ. A strong broad band was also observed near 3200–3700 cm−1 (~2.8–3.1 μm) which is a composite of OHv, H2Ov, and H2O2δ vibrations. Visible/near-infrared spectra were also collected under two relative humidity (RH) conditions. High-RH conditions resulted in increasing band strength for the H2O combination modes near 6900–6930 cm−1 (1.45 μm) and 5170–5180 cm−1 (1.93 μm) in the allophane and imogolite spectra due to increased abundances of adsorbed H2O molecules. Variation in adsorbed H2O content caused an apparent shift in the bands near 1.4 and 1.9 μm. A doublet H2Oδ vibration was observed at 1600–1670 cm−1 (~6.0–6.2 μm) and a band due to OH bending for O3SiOH was observed at ~1350–1485 cm−1 (~6.7–7.4 μm). The Si-O-Al stretching vibrations occurred near 1030 and 940 cm−1 (~9.7 and 10.6 μm) for allophane and near 1010 and 930 cm−1 (~9.9 and 10.7 μm) for imogolite. OH out-of-plane bending modes occurred near 610 cm−1 (16.4 μm) for allophane and at 595 cm−1 (16.8 μm) for imogolite. Features due to Si-O-Al bending vibrations were observed at 545, 420, and 335 cm−1 (~18, 24, and 30 μm) for allophane and at 495, 415, and 335 cm−1 (~20, 24, and 30 μm) for imogolite. The emissivity spectra were obtained from pressed pellets of the samples, which greatly enhanced the spectral contrast of the TIR absorptions. Predicted NIR bands were calculated from the mid-IR fundamental stretching and bending vibrations and compared with the measured NIR values. Controlled-RH X-ray diffraction (XRD) experiments were also performed in order to investigate changes in the mineral structure with changing RH conditions. Both allophane and imogolite exhibited decreasing low-angle XRD intensity with increasing RH, which was probably a result of interactions between H2O molecules and the curved allophane and imogolite structures.

Beidellites may exist on Mars and represent intermediate alteration products; their presence would indicate different alteration environments than previously identified because montmorillonite is a low-grade alteration mineral whereas beidellite is a higher-temperature alteration mineral, and often represents a step toward illite formation. The reflectance spectra of beidellites are under study to support their orbital detection on Mars, where spectral signatures of other Al-rich phyllosilicates have been observed. Reflectance spectra of ten Al-rich smectites are presented here which include pure beidellites and Al smectites having compositions between those of beidellite and montmorillonite, and emphasis is placed here on the OH combination bands near 4545 cm−1 (2.2 μm) as these vibrational features are commonly used in the identification of phyllosilicates on Mars. Shifts were observed in the Al2OH band centers, which occur near 4590 cm−1 (2.18 μm) in reflectance spectra of beidellite and near 4525 cm−1 (2.21 μm) in reflectance spectra of montmorillonite. These are compared with the Al2OH bending vibrations observed near 941–948 cm−1 (10.5–10.6 μm) for beidellite and near 918–926 cm−1 (10.8–10.9 μm) for montmorillonite. Although the octahedral site cation composition provides the greatest influence on the vibrational energies of the M2OH groups, the tetrahedral site cation composition also influences these vibrations. Shifts were observed in the Si-O-Al bending vibrations from 552 and 480 cm−1 (18.1 and 20.8 μm) in beidellite spectra to 544 and 475 cm−1 (18.4 and 21.0 μm) in montmorillonite spectra. Gaussian modeling of the 4545 cm−1 (2.2 μm) bands led to the discrimination of four overlapping bands in each of the ten Al smectite spectra examined in this study. Shifts in the band center and area of the primary spectral band are coordinated with substitution of Al for Si in the tetrahedral sheet. This is consistent with beidellites having a greater tetrahedral layer charge than montmorillonites. The observed spectral differences were sufficiently large that these Al-rich smectites can be differentiated in orbital data of Mars. A pure beidellite-type spectrum is observed in an isolated Al phyllosilicate-bearing outcrop in Libya Montes, a region where Fe-rich smectite is common but Al-rich smectite is rare. Beidellite-type reflectance spectra were also observed in one area of the Nili Fossae region. In contrast, a variety of Al phyllosilicates were found in the ancient rocks at Mawrth Vallis, including some smaller clay-bearing regions exhibiting spectral signatures more consistent with beidellite-like than montmorillonite-like chemistry.

Altered basaltic tephra from Haleakala, Maui, are characterized using multiple techniques in order to identify the minerals formed under a variety of conditions and to understand the soil formation processes here. We collected samples that are representative of typical bulk weathered material in the crater, as well as solfataric alteration in a hydrothermal environment. For this study X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy are coupled with spectroscopic techniques including Mössbauer, visible-infrared reflectance, transmission infrared, and thermal infrared emission spectroscopies to analyze these samples. The unaltered tephra are composed of feldspar, glass, pyroxene and olivine. Observed alteration products include Fe oxides, phyllosilicates and sulfates, as well as SAED amorphous Al-Si-bearing material. These samples are potential analogs for altered volcanic material on Mars as the pedogenic influences and contact with plants and animals are minimal. Results from this study may help to determine spectral signatures of these samples that could be used for identification on Mars of the minerals observed here.

The Al-clay-rich rock units at Mawrth Vallis, Mars, have been identified as mixtures of multiple components based on their spectral reflectance properties and the known spectral character of pure clay minerals. In particular, the spectral characteristics associated with the ~2.2 μm feature in Martian reflectance spectra indicate that mixtures of AlOH- and SiOH-bearing minerals are present. The present study investigated the spectral reflectance properties of the following binary mixtures to aid in the interpretation of remotely acquired reflectance spectra of rocks at Mawrth Vallis: kaolinite-opal-A, kaolinite-montmorillonite, montmorillonite-obsidian, montmorillonite-hydrated silica (opal), and glass-illite-smectite (where glass was hydrothermally altered to mixed-layer illite-smectite). The best spectral matches with Martian data from the present study’s laboratory experiments are mixtures of montmorillonite and obsidian having ~50% montmorillonite or mixtures of kaolinite and montmorillonite with ~30% kaolinite. For both of these mixtures the maximum inflection point on the long wavelength side of the 2.21 μm absorption feature is shifted to longer wavelengths, and in the case of the kaolinite-montmorillonite mixtures the 2.17 μm absorption found in kaolinite is of similar relative magnitude to that feature as observed in CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data. The reflectance spectra of clay mixed with opal and of hydrothermally altered glass-illite-smectite did not represent the Martian spectra observed in this region as well. A spectral comparison of linear vs. intimate mixtures of kaolinite and montmorillonite indicated that for these sieved samples, the intimate mixtures are very similar to the linear mixtures with the exception of the altered glass-illite-smectite samples. However, the 2.17 μm kaolinite absorption is stronger in the intimate mixtures than in the equivalent linear mixture. Modified Gaussian Modeling of absorption features observed in reflectance spectra of the kaolinite-montmorillonite mixtures indicated a strong correlation between percent kaolinite in the mixture and the ratio of the area of the 2.16 μm band found in kaolinite to the area of the 2.20 μm band found in montmorillonite.