Reflectance spectroscopy of soils post-heating—Assessing thermal alterations in soil minerals

Abstract We examined the potential of reflectance spectroscopy across the VIS-NIR–SWIR spectral region to identify, ex post facto, the thermal processes that occur in heated soils and minerals, and thus to monitor mineral changes in the soil. We systematically investigated five Israeli soils and six standard soil minerals using a controlled furnace from 150 °C to 1000 °C (at 50 °C intervals). Samples which had been heated and cooled (soils and minerals) were measured for reflectance across the 350–2500 nm spectrum using an ASD Field-Pro spectrometer. To confirm the findings extracted from the spectral information, traditional analytical methods (XRD, DTA-DTG, PSD, SEM–EDS) were used. The spectral information was found to be a good indicator for thermal processes occurring in the soil based on mineralogical changes. The spectral changes across different spectral regions provided information about dehydroxylation of goethite to form hematite (VIS-NIR); clay deformation (lattice collapse) due to dehydroxylation processes (VIS-NIR–SWIR); the condition of calcite (SWIR), and indirect processes, such as re adsorption of water and rehydration of the new minerals (e.g. pyrolysis of the calcite product) (SWIR). We propose the use reflectance spectroscopy for post-thermal analysis (RS-PTA), as a valid post-heating diagnostic method for thermal processes occurring in minerals and soils. The suggested method is simple and easy to perform, and can be used in all domains (laboratory and field, point and image) to monitor heat-induced changes in soil minerals.