Spectroscopic Methods in the Study of Kaolin Minerals and Their Modifications by Jacob (Theo) Kloprogge
Author:Jacob (Theo) Kloprogge
Language: eng
Format: epub, pdf
ISBN: 9783030023737
Publisher: Springer International Publishing
In soils, kaolinite is formed at pressure-temperature conditions of the Earth’s surface (i.e., 25–30 °C and atmospheric pressure). However, kaolinite also persists until late diagenesis at temperatures lower than 200 °C, and lithostatic pressure lower than 100 MPa or hydrostatic pressure of 30–40 MPa; furthermore, it can result from hydrothermal alteration , at 100–200 °C and 0.1–10 MPa. Friedrich and Weidler (2010) reported on the effects of contact pressure using diamond and germanium ATR infrared spectroscopy on kaolinite vibrational bands. The bands in the OH-stretching region of kaolinite are displayed in Fig. 5.40a. The strong vibration ν5 of the inner OH group occurred in all spectra at its expected position (3619 cm−1). Also, the weak out-of-phase vibrations ν2 and ν3 of the inner-surface hydroxyl s were observed at nearly the same positions in all spectra, ν3 at 3652 cm−1 (transmission, TR), 3650 cm−1 (ATR from suspension, no pressure, SUS), and 3651 cm−1 (ATR with pressure, ATR) and ν2 at 3668 cm−1 (TR), 3665 cm−1 (SUS), and 3669 cm−1 (ATR), but their intensities decreased with increasing pressure. Furthermore, in the SUS and ATR spectra an additional weak shoulder around 3640 cm−1 was observed. The strongest differences were visible in the region around the strong in-phase symmetric stretch ν1. The band component analyses revealed one band at the expected position (3694 cm−1) in all spectra, but in the SUS and ATR spectra an additional band occurred at 3684 cm−1 (ν4), whose intensity increased with increasing pressure. All bands that were affected by the anvil pressure exhibited a shift in their positions to lower wavenumbers with increasing pressure. The Si-O in-plane vibration shifted from 1028 cm−1 at 8.7 MPa to 1022 cm−1 at 104.9 MPa, while the shift of the second Si-O in-plane vibration was even stronger (1004–994 cm−1). However, the position of the inner-surface OH-deformation band at 939 cm−1 did not shift. Yet, the deformation band of the inner OH-group shifted from 910 cm−1, a position close to that measured with the other methods, to 904 cm−1. Similarly, the two Si-O deformation bands shifted from 530 and 461 cm−1 to 523 and 453 cm−1, respectively. The spectral differences between ATR and TR spectra were restricted mainly to basal Si-O vibrations and to some vibrations of the inner-surface OH group s (Fig. 5.40b). The vibrations related to the octahedral sheet exhibited hardly any effects. This is somehow surprising, because a number of publications have reported complex deformation processes, especially in the octahedral sheet and phase transitions in kaolin group minerals, when exposed to hydrostatic pressure s (Holtz et al. 1993; Johnston et al. 2002; Dera et al. 2003; Butler and Frost 2006). However, these spectral changes appeared at much higher pressures (1–4 GPa) than those applied in the ATR cell (Johnston et al. 2002; Dera et al. 2003; Butler and Frost 2006). Moreover, most of the bands in the region between 1240 and 440 cm−1 shifted to higher wavenumbers, reflecting the multiaxial compression of the clay mineral structure (Butler and Frost 2006). These
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