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

• Near-infrared spectra of anhydrous carbonates contain information on lattice modes.
• The bands originate from anharmonic coupling of internal mode(s) with lattice modes.
• The activated lattice modes are environmental cation specific.

Near-infrared (NIR) spectra of carbonates have proved important in many disciplines including planetary exploration. Classically bands in the 4000–6000 cm− 1 (2.5–1.67 μm) 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 (2.219 μm), 4900 cm− 1 (2.041 μm) and 5145 cm− 1 (1.944 μm)) 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.

Near-infrared (NIR) spectra of carbonates have proved important in planetary exploration. Classically bands in the 4000–6000 cm− 1 (2.5–1.67 μm) region are assigned to [CO32–] internal mode combinations and overtones. However band assignments remain equivocal. This study examines the mid-infrared and near-infrared spectra of powdered calcite and aragonite group minerals. As shown in the figure below when the arithmetic sum of mid-infrared derived internal mode combination band totals are plotted against complimentary NIR band frequencies in thirteen cases the difference in frequency (Δ cm− 1) closely coincides (< 16 cm− 1) with known lattice modes. This is interpreted to indicate that the NIR bands originate from anharmonic coupling of degenerate internal mode(s) with external (lattice) modes. In the case of two bands the greatest Δ cm− 1 values coincide with the smallest cation mass of the calcite and aragonite groups, i.e. the minerals magnesite and aragonite respectively, suggesting that the frequency of the lattice mode active in mixing with internal modes may increase with decreasing cation size. 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. MIR derived frequencies of combination bands plotted against NIR frequencies. Diamonds represent aragonite group minerals, squares calcite group minerals.Figure optionsDownload as PowerPoint slide