The Effect of the Particle Size on the Fundamental Vibrations of the [CO32-] Anion in Calcite

This study examines the effects of particle sizes between 3-121µm on the fundamental vibrations of the [CO32-] 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] [712cm-1/1393cm-1], [ν4/ν2] [712cm-1/871cm-1] and [ν2/ν3] [871cm-1/1393cm-1] share the same profile for grain size fractions 121µm through to 42µm mode. Between 42 and 3µm mode the three ratios sharply decline in a systematic manner, consistent with a non-uniform decrease in spectral contrast of the [CO32-] internal modes. Raman intensity increased with decreasing particle size from 121μm until 19μm mode particle size fraction thereafter decreasing sharply. The [v4/v3], [v1/v3] and [v4/v1] intensity ratios normalised against the corresponding intensity ratio of the 121μm particle size fraction, indicate that the [ν4/ν3] ratio changes by 22%. Both ATR-IR and Raman indicate two critical points in internal mode behaviour of the Raman and infrared active ν4 and ν3 internal modes, the first, at between 42-59µm size range and the second between 19 and 5µm. 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.