Terahertz time-domain spectroscopy: Probing pyrolysis behaviors and characteristic peaks of kaolinite, pyrophyllite, talc, and sepiolite

Clay minerals are key components of surficial deposits, with their structural features, particle size, and water content crucial for environmental evolution research and industrial applications. Traditional characterization methods have limitations, prompting the need for novel techniques to study their pyrolysis characteristics. Terahertz spectroscopy, a non-contact coherent technique with high signal-to-noise ratio, excellent stability, and high detection sensitivity, enables non-destructive acquisition of optical information (amplitude, phase) to differentiate crystal structures and chemical compositions. This study employed time-domain spectroscopy combined with thermogravimetric analysis and Fourier transform infrared spectroscopy to investigate the pyrolysis characteristics of kaolin, talc, pyrophyllite, and sepiolite. Results show that the terahertz optical parameters (absorption coefficient, refractive index) of kaolin vary significantly with concentration, particle size, and calcination temperature. Talc and pyrophyllite (2:1-type layered silicates) show distinctive terahertz spectral features due to varied octahedral cations, which can reflect their pyrolysis products and moisture content. Sepiolite is further investigated to clarify its phase transition temperatures and hydrous evolution behavior during thermal treatment. Contrary to traditional understanding, characteristic absorption peaks were identified in raw minerals: pyrophyllite at 1.1 THz and sepiolite at 1.53 THz. These findings confirm the feasibility of terahertz time-domain spectroscopy in clay mineralogy, offering a novel technical approach for pyrolysis research and promoting interdisciplinary integration between this terahertz technique and geoscience.