Near-Infrared Spectroscopy Study of OH Stretching Modes in Pyrophyllite and Talc

Pyrophyllite and talc are both tetrahedra–octahedra–tetrahedra (TOT)-type phyllosilicates, but differences can be found in the stacking mode of the layers and the ion occupation. Fourier transform infrared spectroscopy was used to differentiate between pyrophyllite and talc. In the 400–600 cm−1 region, pyrophyllite exhibits six peaks, while talc only exhibits five peaks. In the 1000–1200 cm−1 region, pyrophyllite exhibits three clear peaks at approximately 1051, 1070, and 1121 cm–1; while talc only exhibits one strong peak near 1020 cm−1. The differences between pyrophyllite and talc in the near–infrared (NIR) region are clear in the 4000–4700 cm–1 region, and pyrophyllite exhibits an intense peak around 4615 cm–1, which is attributed to the combination of the OH and Si-O-Si stretching bands. Talc has a maximum peak located near 4324 cm–1, which is attributed to the OH stretching vibration. In addition, talc has a secondary peak near 4366 cm–1. Talc has two other weaker peaks around 4054 and 4180 cm–1. The 7000–7250 cm–1 region exhibits the first fundamental overtone of the OH group stretching vibrations. The common characteristic band of these two minerals is 7175–7183 cm−1. The first overtone of OH stretching vibrations can also be generated by adjacent peaks in the fundamental overtones. The peaks of these two minerals around 7094 cm−1 appear to be a combination of 3630 (±5) cm−1 and 3642 (±3) cm−1.The factor of the first fundamental overtone of the OH group stretching vibration is 1.95 (±0.003). Therefore, the characteristic peaks in the mid-infrared (MIR) and NIR regions can be used to distinguish between pyrophyllite and talc, providing a research basis for further exploration in related geological areas.