The near-infrared (NIR) spectra of powdered calcite in the 3-121 μm mode particle size range

Results show that the behavioural response of the NIR spectra is varied. Those assigned to 3 v3: 2.34 μm (4270 cm−1) and 2.3 μm (4350 cm−1) wavelengths show a linear relationship (r2 = 0.9992), with decreasing particle size and absorption intensity. Bands at 2 μm (5007 cm−1), 1.96 μm (5109 cm−1) and 1.87 μm (5336 cm−1) assigned to combinations between v3 overtones and v1 and its overtone show an initial decrease in absorption intensity, but an increase in absorption intensity in the 3 μm mode particle size powder relative to the 5 μm mode powder. The differing behaviours of 3 v3 overtone related bands with respect to combinations containing v1 or its overtone occur at particle size ranges in which the optical thick to thin transition occurs at mid-infrared (MIR) frequencies. The separate NIR behavioural responses of specific bands are qualitatively reminiscent of MIR types 1 and 2 behaviour of [CO32−] anion internal modes at the optical thick to thin transition.