Spectral characteristics of clay minerals in the 2.5–14 μm wavelength region

Identification and quantification of clay minerals, particularly those that are responsible for susceptibility of soils to expansion and shrinkage, is a constant focus of research in geotechnical engineering. The visible, near infrared and short wave infrared wavelength regions are well explored. However, little is understood about the spectral characteristics of such clay minerals in the wavelength longer than 2.5 μm. The objective in this study was to explore the potential of laboratory spectroscopy in the 2.5–14 μm wavelength region for characterizing clay minerals. Montmorillonite, illite and kaolinite were investigated, for these clay minerals are key indicators of soil expansion and shrinkage potential. Characteristic absorption bands and their changes for mixtures of clay minerals were determined. Partial least squares (PLS) regressions in combination with continuum removal analyses were used to determine wavelength regions that best discriminate differences in mineralogical contents. Spectral contrast was high in the 3–5 μm wavelength region but overall low in the 8–14 μm. The clay minerals were characterized by strong, diagnostic absorption bands. Much of the variation in compositions of the mixtures was explained by the PLS models (coefficients of correlations of > 0.90). Thus, spectroscopy in the 2.5–14 μm wavelength region is a useful technique for characterizing clay minerals.

Research highlights
► Spectral characteristics of clay minerals and mixtures in the 2.5-14 µm.
► Absorption band differences for clay mineralogical identification and characterization.
► PLS regression analysis for quantitative clay mineral estimation from spectra.
► PLS regression models explained much of the variation in clay compositions.