Physico-chemical characterization of detrital sillimanite and garnet: Insights into REE elements, crystal structure and morphology

The Varkala–Kovalam coastal stretch of Southern Kerala, India, is enriched with economically significant heavy mineral deposits. In the present study, two such minerals like sillimanite, and garnet were recovered from the beach sands and subjected to detailed characterization studies related to their crystal structure, chemical composition, surface chemistry, and surface morphological changes. Advanced characterization studies involve Raman spectroscopy, X-ray photoelectron spectroscopy, ultraviolet–visible–near infrared spectroscopy, energy dispersive X-ray fluorescence, high resolution inductively coupled plasma mass spectrometry, and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). The Raman spectra strictly discriminate sillimanite from other A12SiO5 polymorphs and almandine from other silicate garnets. The average total rare earth elements present in sillimanite is 222 ppm and garnet is 1094 ppm with a maximum for Ce. The surface morphological changes induced by solution activity of chemicals and mechanical impacts during sediment movement and deposition were easily analyzed by SEM images. The study provides a vivid dataset of physico-chemical characteristics of these minerals, which could facilitate determining their grade and industrial applications.