Interpretation of infrared spectra of dioctahedral smectites in the region of OH-stretching vibrations

Abstract

Dioctahedral smectite samples of a wide range of compositions (beidellites, montmorillonites, nontronites, Fe-rich montmorillonites and Al-rich nontronites) were studied by infrared (IR) spectroscopy. A special sample-preparation technique was used to eliminate the contribution of molecular water. The OH-stretching regions of the spectra were decomposed and curve-fitted, and the individual OH-stretching bands were assigned to all the possible types of OH-bonded cation pairs that involve Al, Mg and Fe. The integrated optical densities of the OH bands were assumed to be proportional to the contents of the specific types of OH-linked cation pairs with the absorption coefficients being the same for all individual OH bands. Good agreement between the samples’ octahedral cation compositions calculated from the IR data and those given by crystal-chemical formulae was obtained for a representative collection of samples in terms of a unique set of individual OH-band positions that vary within narrow wavenumber intervals. This has allowed us to minimize the ambiguity in spectra decomposition caused by the poor resolution of smectite spectra and confirmed the validity of the resulting band identification.The bands associated with specific OH-bonded cation pairs in the spectra of smectites are, on the whole, shifted to greater wavenumbers with respect to the corresponding bands in micas. In addition to OH bands that refer to the smectite structure, AlOHAl and AlOHFe bands of the pyrophyllite structural fragments were identified. The band-position variation ranges overlap in a few cases (AlOHFe and MgOHMg; AlOHAl of smectite and AlOHFe of pyrophyllite-like component).Unambiguous interpretation of the OH-stretching vibrations was found to be possible only for smectite samples with known chemical compositions, so that IR data cannot be used for quantitative determination of octahedral cation composition of mixtures of dioctahedral 2:1 phyllosilicates. In the case of the studied monomineral smectites with known chemical compositions, IR data provided information on the short-range order/disorder in the distribution of octahedral cations along cation-OH-cation directions. This information can be employed, in conjunction with the data of other spectroscopic and diffraction techniques, in the analysis of short-range octahedral cation distribution.

Diagenetic and sedimentary controls on porosity in Lower Carboniferous fine-grained lithologies, Krechba field, Algeria: A petrological study of a caprock to a carbon capture site

“…6. The dominance of chlorite bands at 3560e3540 cm À1 strongly suggests that the chlorite is Fe-rich (as opposed to Mg-rich; Zviagina et al, 2004). Siderite is represented by the FTIR band at 1417 cm À1 and is clearly present in variable quantities in different samples, supporting the XRD data.…”

“…This mixture was then put in a Specac press at 10 ton to produce an FTIR pellet. The pellets were dried at 140 C for 14 hours in order to remove any adsorbed water (Zviagina et al, 2004). Adsorbed water can be a problem with FTIR because the important OH-stretching region (3000e4000 cm À1 ) can be overwhelmed by adsorbed water vibrations.…”

Computer simulations of cations order‐disorder in 2:1 dioctahedral phyllosilicates using cation‐exchange potentials and monte carlo methods

“…These TAOAT layers are stacked along the c crystallographic axis, and the space between two TAOAT layers is known as the interlayer space. This stacking is known 2 ] (x 5 0 2 1, y 51 2 0), where I is the interlayer cation species, if X or Y has a charge of less than three or four, respectively. Nonetheless, a tetrahedral charge exceeding 1 per formula unit can occur.…”

“…[1] In IR spectroscopy, the OH vibrational frequencies mainly depend on the first neighbor octahedral cations; these frequencies can yield information only about pairs of atoms that are linked to OH groups in the octahedral sheet, and so only short-range ordering information can be obtained. [2] Furthermore, the small crystal size of the clay minerals has always made it difficult to study them experimentally. For these reasons, in many cases, to obtain good results in cation ordering studies, a combination of experimental methods with molecular modeling, force field, first principles, and/or simulation methods is necessary.…”

Influence of redox processes on clay mineral transformation in Retisols in the Carpathian Foothills in Poland. Is a ferrolysis process present?

“…All weak absorption bands in the OH stretching region are related to vibrations of Al-OH-Al in the structure of kaolinite and/or dioctahedral mica (Table 6, Fig. 5) (Besson and Drits 1997;Madejová 2003;Zviagina et al 2004). Strong absorption bands present at~1165, 1085, 799, and 780 cm −1 in the Si-O and OH bending regions are related to quartz and silicates (Table 6, Fig.…”