New Insight into the Relationships Between Structural and Ftir Spectroscopic Features of Kaolinites

Abstract

To resolve the existing ambiguities in the interpretation of the OH-stretching vibrations of kaolinites, relationships were, for the first time, established between the structural and Fourier-transform infrared (FTIR) spectroscopic features for a set of kaolinite samples which differed in terms of their relative amounts of coexisting high- and low-ordered phases. For this purpose, a representative collection of kaolinites differing in origin, particle size, and degree of disorder was studied by powder X-ray diffraction (XRD) and FTIR spectroscopy. Modeling of the experimental XRD patterns based on the orthogonal layer unit cell having a mirror plane showed each sample to be a mixture of nearly defect-free high-ordered (HOK) and low-ordered (LOK) kaolinite phases, with HOK varying from 86 to 4%. The wavenumbers, heights, areas, and full widths at half-maximum (FWHM) were determined for the OH-stretching bands at ~3697 (ν1), ~3670 (ν2), ~3652 (ν3), and 3620 cm–1 (ν4) by decomposition and fitting of the FTIR spectra. The FWHM(ν1)/FWHM(ν4) and FWHM(ν3)/FWHM(ν2) values were related linearly to the HOK content, which may be associated with the in-phase and out-of-phase character of the corresponding pairs of vibrations, respectively. A novel interpretation was suggested for the variations in the relative integrated intensities of the OH bands with the amount of the HOK phase. The intensity distribution of the ν2 and ν3 bands is controlled by the triclinic structure symmetry in the defect-free kaolinite and the mirror symmetry of the layers in low-ordered structures, in agreement with the observed evolution of the corresponding band intensities. The ν1 and ν2 band positions for the low-ordered samples are within the wavenumber range for the high-ordered samples. In contrast, the ν3 and ν4 band positions for the low-ordered samples are shifted toward higher wavenumbers, indicating that some of the low-ordered kaolinites should contain dickite-like structural fragments distributed among kaolinite layers.

The Kaolinite Crystallinity and Influence Factors of Coal-Measure Kaolinite Rock from Datong Coalfield, China

“…However, C5G crystallinity is more ordered than C5F2 and C8F due to its smallest offset. That said, it is unknown whether there were other clay minerals in the three samples that led them to show negative deviations [39]. Depending on the three methods, the crystallinity of C5G and C5F1 is better than that in C5F2 and C8F.…”

“…Kaolinite crystallinity was defined by analyzing the stretching vibration and bending vibration of the Si-O bond of kaolinite (Table 3). Generally, when the number of vibration waves of the Si-O bond shifted to the high-frequency region, this denoted better crystallinity and vice versa [39][40][41]. The infrared spectra of S4F (Figure 7a) and C2F (Figure 7b) illustrate that these kaolinites had absorption peaks in the four frequency bands of 3693 cm −1 (S4F) or 3695 cm −1 (C2F), 3670 cm −1 , 3656 cm −1 and 3620 cm −1 .…”

“…IR analysis can obtain the characteristic absorption peaks of several main minerals at once without the need for a single mineral separation. According to the position, shape and intensity of these characteristic absorption peaks, the mineral composition in the sample and their relative content can be determined [39][40][41]. In the infrared absorption spectrum, the absorption is generally recorded in the band range of 4000-200 cm −1 .…”

“…If the hydroxyl bonding force in the crystal layer is strong and the dehydroxy heat-absorbing valley in the differential thermal curve of kaolinite is strong and deep, the crystallinity is well ordered. On the contrary, if the dehydroxy heat-absorbing valley is wide and shallow and the temperature of the dehydroxy heat-absorbing valley is low [39,42], kaolinite is poorly crystallized. The heat absorption valley temperature range and the peak value of the differential heat curve fluctuated as a function of the structure and integrity of kaolinite itself and its mineral combinations [42].…”

Transformation of Ordered Albite into Kaolinite: Implication for the “Booklet” Morphology

“…For Ab, the absorption bands at 1156, 1098, and 997 cm –1 were attributed to the Si–O stretching vibrations, and the band at 1036 cm –1 was assigned to the Si­(Al)–O stretching vibration. , In the spectra of the hydrothermal products, three distinct absorption bands at 3696, 3656, and 3624 cm –1 were recorded. The two former bands were assigned to the stretching vibrations of inner surface hydroxyls in kaolinite, while the latter one was attributed to the stretching vibration of inner hydroxyls. ,, Regarding the reference sample Kaol-MM, two visible absorption bands assigned to the stretching vibrations of the inner surface hydroxyls appeared at approximately 3667 and 3656 cm –1 , respectively.…”