Design and Validation of a Rapid and Accurate Identification Scheme for Clay Minerals in Soils by Combining Different Optical Analysis Methods

The identification of clay minerals is an essential step in soil research. However, many traditional methods have the limitations of tedious steps, slow speed, complicated preprocessing, and poor discrimination. A novel scheme for rapid and accurate identification of clay minerals in soils was proposed based on the chemical composition and structure information of kaolinite, talc, illite, vermiculite, montmorillonite, chlorite, and sepiolite obtained by combining different optical analysis methods. In this scheme, the chemical index of alteration (CIA) values was first calculated from the X-ray fluorescence spectroscopy (XRF) data for the overall classification, and then the main clay minerals were identified by the characteristic peaks of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), or Raman spectroscopy (Raman). Furthermore, the 15 certified reference materials (CRMs) distributed in various regions of China and an actual chernozem sample (E125(degrees)6.744 ', N44(degrees)25.018 ') were analyzed to validate the accuracy and applicability of the scheme. The results showed that there were significant latitudinal differences in the main clay minerals of soils with a gradual transformation of montmorillonite and vermiculite to kaolinite from north to south in China, and the major clay minerals of chernozem sample located in northeast China were montmorillonite and vermiculite, which were consistent with previous reports and actual situations. Therefore, the novel scheme proposed here greatly improves the accuracy and convenience of identifying clay minerals in soil by combining four optical instruments, which can provide strong technical support for the study of the formation and transformation mechanism of clay minerals.