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The effect of the particle size on the fundamental vibrations of the [CO3(2-)] anion in calcite.

This study examines the effects of particle sizes between 3 and 121 μm on the fundamental vibrations of the [CO3(2-)] anion in calcite [CaCO3] as analyzed by total attenuated reflectance infrared spectroscopy (ATR-IR) and Raman spectroscopy (RS). The ATR-IR absorbance intensity ratios of the [ν4/ν3] [712 cm(-1)/1393 cm(-1)], [ν4/ν2] [712 cm(-1)/871 cm(-1)], and [ν2/ν3] [871 cm(-1)/1393 cm(-1)] share the same profile for grain size fractions 121 μm through to 42 μm mode. Between 42 and 3 μm mode the three ratios sharply decline in a systematic manner, consistent with a nonuniform decrease in spectral contrast of the [CO3(2-)] internal modes. Raman intensity increased with decreasing particle size from 121 μm until 19 μm mode particle size fraction thereafter decreasing sharply. The [ν4/ν3], [ν1/ν3], and [ν4/ν1] intensity ratios normalized against the corresponding intensity ratio of the 121 μm particle size fraction indicate that the [ν4/ν3] ratio changes by 22%. Both ATR-IR and Raman indicate two critical points in internal mode behavior of the Raman and infrared active ν4 and ν3 internal modes, the first between 42 and 59 μm size range and the second between 19 and 5 μm. Results are interpreted in terms of specular to volume (diffuse) coherent transitions of internal modes and with further grain refinement internal mode specific optically thick to thin transitions.