The influence of structural Fe, Al and Mg on the infrared OH bands in spectra of dioctahedral smectites

Abstract

Visible to near-infrared (NIR) reflectance spectra and mid-IR transmittance spectra are presented here for a collection of dioctahedral smectites. Analysis of the structural OH vibrations is performed by comparing the NIR combination and overtone bands with fundamental stretching and bending absorption features in the mid-IR region. Second derivatives are used to determine the actual band centres, which are often shifted slightly by a spectral continuum in the reflectance or transmittance spectra. New bands have been identified near 4170 and 4000 cm–1 in the NIR spectra of nontronite with tetrahedral substitution. A related band is observed near 4100 cm–1 for montmorillonites with substantial tetrahedral and/or octahedral substitution. These bands are correlated with the mid-IR bands near 680 cm–1 for nontronite and near 630 cm–1 for montmorillonite. Comparison of the OH overtone and combination bands with the fundamental stretching and bending vibrations gives consistent results.

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument

“…1b). This combination of absorptions is consistent with high-Fe chlorites [9][10][11] . Some spectra from Mawrth Vallis show strong 1.9-mm and 2.2-mm features and a weak 1.4-mm band (spectrum 1 in Fig.…”

“…If water is present in a mineral (for example interlayer water in smectite clays), then an absorption centred near 1.9 mm is observed as a result of the combination of the H-O-H bending and stretching vibrations 29 . Absorptions near 2.2 mm are indicative of Al-OH vibrations, whereas those between 2.28 and 2.35 mm are commonly indicative of Fe/Mg-OH vibrations [8][9][10] . Quantitative abundances can be estimated from computationally intensive, nonlinear spectral deconvolution 31 .…”

The stratigraphy and history of Mars' northern lowlands through mineralogy of impact craters: A comprehensive survey

“…Fe/Mg Phyllosilicates Fe/Mg phyllosilicates-we use the term to refer to smectites and mixed-layer clay minerals but not chloriteare identified using 1.39-1.41, 1.91, and 2.29-2.31 μm absorptions due to H 2 O overtones and combination modes as well as a Fe/Mg-OH stretching mode (Figure 9). For Fe/Mg smectites, the centers of absorption features shift due to variation in Fe or Mg as cations in the phyllosilicates: 1.41 and 2.31 μm for Mg-OH and 1.39 and 2.28-2.29 μm for Fe-OH absorptions [Clark, 1999;Bishop et al, 2002], sometimes accompanied by weaker 2.4 and 2.5 μm absorption features. In Martian spectra, some 1.41 μm absorption features are weak or absent, which could be a result of an alteration rind or coating.…”

Kolsuz-Ulukisla-Nigde Clays, Central Anatolian Region - Turkey and Petroleum Exploration

“…However, for illite at 916 and 833 cm -1 (Al-Mg-OH) deformation and at 525 and 470 cm -1 (O-Si-O) bending frequency bands were observed after the thermal treatment. This bands are resulted due to re-arrangement of Al and Mg atoms within the crystal structure during the thermal treatment (Bishop et al, 2002). Only (Si-O) strecthing at 988 cm -1 among the vibrational frequencies of chlorite at 3660, 988, 819, 543, and 441 cm -1 was not observed after the thermal treatment.…”

“…Mg, Si and Ca ions, products of such conversion, could form minerals like calcite and dolomite facilitated by increasing temperature and these new minerals deposit within shale. This in turn results in a decrease in porosity and permeability and an increase in density (Bishop et al, 2002;Dunoyer de Segonzag, 1970).…”