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

The near-infrared (NIR) spectra of powdered carbonate minerals is of significant multidisciplinary interest. However, there is a paucity of accounts of the behaviour of NIR bands as a function of particle size in well-constrained experimental settings, although such studies may hold promise in revealing band-specific traits and identifying spectral features amenable to quantified measurement of particle size fractions. To these ends, this study examines the NIR spectral response in the 2.5 to 1.66μm wavelength (4000-6000cm-1 wave number) region from powdered calcite in size fractions ranging from 3 to 121μm mode particle sizes. Results show that the behavioural response of the NIR spectra is varied. Those assigned to 3v3: 2.34μm (4270cm-1) and 2.3μm (4350cm-1) wavelengths show a linear relationship (r2 = 0.9992), with decreasing particle size and absorption intensity. Bands at 2μm (5007cm-1), 1.96μm (5109cm-1) and 1.87μm (5336cm-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 3μm mode particle size powder relative to the 5μm mode powder. The differing behaviours of 3v3 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.