Chromites of Nuggihalli Schist Belt, Western Dharwar Craton, Southern India: a comprehensive spectroscopic investigation

Chromite is a significant mineral within the spinel group, functioning as a petrogenetic indicator and an important resource for numerous industrial uses. This study investigates the chromite occurrence of the Nuggihalli Schist Belt (NSB), which belongs to the Archean supracrustal greenstone belts of Western Dharwar Craton (WDC), southern India, through a comprehensive spectroscopic analysis. Chromites of the study area occur as diverse textures within ultramafic rocks, reflecting the complex evolutionary processes involving multiple stages of crystallization, and post-magmatic alterations. Based on the modal abundance, the chromite samples have been classified into three distinct groups, that is, Group I (80–90%), Group II (60–80%), and Group III (<60%). The chromites of this study were characterized using multiple spectroscopic techniques such as X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, Visible-Near-InfraRed (VNIR) spectroscopy, and laser Raman analysis. XRD spectra of chromite exhibit their characteristic 2θ peaks at 12°,18°,24°, and 35°. The FTIR data reveal transmittance in and around 960–976 and 1001–1007 cm⁻¹, corresponding to the vibrations of oxygen ions perpendicular to the bond axis and the oscillations of Cr-ions within the isotropic field of their tetrahedral and octahedral coordination, respectively. Laser Raman spectra show intensity peaks at around ∼610, ∼520, ∼450, and 684 cm⁻¹, which are attributed to E_g and F_2g and A_1g vibrational modes of Cr³⁺ ions. The shortward shift observed in the 684 cm⁻¹ peak is due to the compositional changes and Fe³⁺ substitution (chromite to ferritchromite) in the crystal structure. VNIR spectroscopy of chromite reveals a prominent absorption band at around 2000 nm. The observed shift in the 2000 nm band center is distinct to groups I, II, and III, indicating an abundance-dependent band shift. Through this study, we have correlated the characteristic spectral features and their shift across the various groups of chromite and the corresponding textures, which in turn give implications regarding the stages of crystallization and subsequent alteration. This study also contributes significantly to the spectral database of ore minerals, potentially aiding effective and precise future mineral exploration.