Contact Pressure Effects on Vibrational Bands of Kaolinite during Infrared Spectroscopic Measurements in a Diamond Attenuated Total Reflection Cell

Abstract

Over the last decades infrared spectroscopy has become a frequently used method to investigate the structure and bonding properties of clay minerals. Along with classic transmission techniques, attenuated total reflection (ATR) spectroscopy has been applied as a very fast and easy method. In this study we compared transmission spectra of kaolinite with ATR spectra obtained by a single-reflection diamond ATR accessory (Golden Gate Mark II). The ATR spectra showed anomalous band positions that were obviously affected by the applied contact pressure of the sapphire anvil. Most of these vibrations can be assigned to basal Si–O bonds and all of them shifted their positions to lower wavenumbers. It is suggested that these peak shifts are due to changes in the Si–O–Si bond angle θ. They are caused by distortions and rotations of SiO₄ units within the tetrahedral sheet, due to shear forces perpendicular to the uniaxial pressure applied by the anvil. Furthermore, the intensity of a normally very weak transversal optical mode (TO mode) of the inner surface hydroxyls (3684 cm⁻¹) remarkably increased with increasing contact pressure, while the longitudinal optical mode (LO mode) at 3694 cm⁻¹ strongly decreased its intensity. This possibly is determined by a strong alignment of the platy kaolinite particles along their ab-planes due to the applied pressure.