Crystal chemical correlations between the mid and near-infrared in carbonate minerals

Near-infrared (NIR) spectra of carbonates have proved important in many disciplines including planetary exploration. Classically bands in the 4000-6000cm-1 (2.5 – 1.67μm) region are assigned to [CO32-] internal mode combinations and overtones. However band assignments remain equivocal. This study examines three prominent bands ((ca 4505 cm-1 (2.219μm), 4900cm-1 (2.041μm) and 5145cm-1 (1.944μm)) from powdered calcite and aragonite group minerals. Results indicate that the bands originate from anharmonic coupling of degenerate internal mode(s) with external (lattice) modes. On this basis it is suggested that NIR data may provide an untapped source of detailed information on lattice mode frequencies and information on the type(s) of the environmental cation(s) in carbonate minerals.