SPECTRAL CHARACTERIZATION OF A DEEPLY WEATHERED LATERITE PROFILE, KORAPUT, ORISSA, EASTERN INDIA

Abstract:

This contribution reports the spectral characterization of a 30-m long lateritic regolith profile over khondalite near Koraput, Eastern India. The area forms a part of the Archaean Eastern Ghats Mobile Belt, comprising high grade metamorphic rocks such as khondalites, charnockites, migmatites and granite gneisses. The area has been deeply weathered under tropical conditions leading to the development of thick laterite horizon. Samples were collected from the lateritic profile at intervals of 1-2 m. Fresh or slightly altered khondalite are exposed at the bottom and grade into laterite upwards. Reflectance spectra were collected using FieldSpec3 spectroradiometer in the VNIR-SWIR region. An Al-OH absorption feature at 2208 nm occurs throughout the profile. A blue shift of this feature related to the progressive leaching of Al and enrichment of Fe is observed in the top 5 m of the profile. An absorption feature at 2165 nm occurs throughout revealing a mixture of kaolinite, illite and smectite. Several Fe absorption features, possibly because of crystal field splitting, are observed at shorter wavelengths, particularly, 482-495 nm and 675-685 nm. Another Fe absorption feature due to intervalence charge transfer is observed around 416-428 nm. Absorption features at 2250 nm and 2263 nm suggest the presence of jarosite in the top 5 m of the profile. An absorption feature at 936 nm is observed in the top 12 m of the profile, which indicates progressive Fe enrichment. Fourier Transform Infra-Red (FTIR) spectroscopy in the TIR – MIR region support the above interpretations of the VNIR-SWIR spectra. Goethite shows distinct absorption features at 3000 nm and 6800-7000 nm. An absorption feature at 13000 nm reveals the presence of kaolinite and 15600 nm absorption feature confirms jarosite. Further, X-Ray diffraction (XRD) helped in identification and quantification of major mineral phases present in each sample. XRD results showed gradual decrease in kaolinitic content and successive increase in goethite content from bottom to the top of the laterite profile, an observation that is consistent with the spectral features. The implications of the mineralogical and geochemical variations along the profile for the process of regolith formation in the area are discussed.