*Article* **Near-Infrared Spectroscopic Study of OH Stretching Modes in Kaolinite and Dickite**

**Shaokun Wu 1, Mingyue He 1,\*, Mei Yang <sup>2</sup> and Bijie Peng <sup>1</sup>**


**\*** Correspondence: hemy@cugb.edu.cn

**Abstract:** Kaolinite and dickite are differently ordered polytypes of kaolinite-group minerals, whose differences are in the stacking mode of layers and ion occupation. Fourier transform infrared spectroscopy was used to collect information about the differences between the two minerals. The common characteristics of kaolinite and dickite are bands near 4530 and 7068 cm−1, which are attributed to the combination of the inner Al-OH stretching vibration and outer Al-OH bending vibration and the overtone of the inner Al-OH stretching vibration, respectively. The difference is that kaolinite has secondary peaks at 4610 and 7177 cm<sup>−</sup>1, and the secondary peak of dickite is near 4588 cm<sup>−</sup>1. The OH stretching vibration has the first fundamental overtone of the stretching vibration in the range of 7000–7250 cm−1. In addition to the overtones generated by single OH stretching vibrations, overtones combining different OH stretching vibrations are also found, which are formed by adjacent peaks of OH stretching vibrations. The average factor of the first fundamental overtone with an OH-group stretching vibration is approximately 1.95. The near-infrared spectrum (NIR) of phyllosilicates is closely related to their structure and isomorphism. Therefore, the near-infrared region can distinguish between kaolinite and dickite and provide a basis for deposit research and geological remote sensing.

**Keywords:** near-infrared spectroscopy; kaolinite; dickite; OH group
