VNIR spectral signatures of chromium-substituted goethite and hematite in Sukinda ultramafic laterites, India: From mineral spectra to spaceborne hyperspectral mapping

This study demonstrates, to our knowledge, the first spaceborne hyperspectral attribution of minor-element substitution in natural iron (oxyhydr)oxides, whereby incorporation of Cr into goethite and hematite produces diagnostic spectral features in the visible to near-infrared (VNIR) wavelength region. Hyperspectral satellite imagery of ferruginous laterite over chromitite-bearing ultramafic rocks at Sukinda (Odisha, India) was integrated with field sampling, petrographic and mineralogical characterization, bulk and mineral-scale chemical analyses, and laboratory VNIR spectroscopy supported by Cr-doped synthetic reference materials. Across the weathering profile—from chromitite–serpentinite bedrock upward through saprock–saprolite to ferruginous laterite and duricrust/ferricrete—Cr is strongly partitioned into goethite and hematite, locally reaching concentrations of up to 20 wt% within goethite. Laboratory reflectance spectra of natural and synthetic goethite and hematite materials reveal Cr-induced modifications to a key crystal-field absorption feature (CFA 1 ) in the ~650–700 nm range, with the band minimum shifting to longer wavelengths as Cr concentration increases. This behaviour supports a two-component model comprising CFA 1 -Fe (~655 nm) and CFA 1 -Cr (~700 nm), attributed to ligand-field transitions of octahedrally coordinated Fe 3+ and Cr 3+ , respectively. Equivalent CFA 1 shifts in PRISMA hyperspectral imagery delineate Cr-rich alteration domains, with continuum-subtracted CFA 1 minima ≥685 nm enabling robust discrimination of Cr substitution in iron (oxyhydr)oxides at low, single-digit concentrations (~2 wt% Cr, approximated from synthetic reference materials). These results demonstrate that spaceborne hyperspectral data can resolve Cr enrichment in ferruginous laterites, enabling new applications in mineral exploration, environmental monitoring, and potentially planetary surface investigations.