Near-infrared spectroscopy: A powerful tool in studies of acid-treated clay minerals

The benefit of near-infrared (NIR) spectroscopy in studies of acid-treated clay minerals is demonstrated. The effects of mineral type, composition and content of non-swelling interlayers on the dissolution rate are investigated. Detailed analysis of the NIR region is performed by comparing the first overtone (2νOH) and combination (ν + δ)OH bands with the fundamental stretching (ν) and bending (ν) vibrations. Spectra of acid-treated samples show a gradual decrease in the intensities of the structural OH overtone (near 7100 cm−1) and combination (4600–4300 cm−1) bands reflecting a fewer number of octahedral atoms. The appearance of the 2νSiOH vibration for terminal (isolated) SiOH groups near 7315 cm−1 indicates the formation of a protonated silica phase. The band near 7130 cm−1 remaining in the spectra of acid-treated samples is assigned to 2νHOSiOH of geminal silanol groups. Thus the creation of geminal silanols, previously detected by 29Si MAS-NMR spectroscopy in acid-treated hectorite, is confirmed also by NIR spectroscopy. The assignment of the 4555 cm−1 band to the (ν + δ)SiOH combination enabled calculation of the wavenumber for the SiO–H bending vibration (∼810 cm−1) that is not observable in the mid-IR region due to overlapping with the Si–O band of amorphous silica (∼800 cm−1). The NIR spectra confirm that trioctahedral hectorite is much more susceptible to dissolution in HCl than dioctahedral nontronite. The dissolution rate of kaolinite present in the Badin clay as an admixture is lower than that of the main mineral nontronite. The accessibility of the interlayers for protons significantly influences the stability of clay minerals in HCl. Mixed-layered mineral illite/smectite with only 30% of swelling interlayers dissolves more slowly than smectite of similar chemical composition containing mainly swelling interlayers.