Multifactor-controlled mid-infrared spectral and emission characteristic of carbonate minerals (MCO3, M = Mg, Ca, Mn, Fe)

Carbonate minerals have been playing an important role in determining the history of the earth's atmosphere, geology, and hydrology and have received extensive attention. In this study, infrared spectral characteristics and infrared radiation properties of four carbonate minerals (calcite, rhodochrosite, siderite, magnesite) were highlighted and investigated by using X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), energy dispersive X-ray spectrometer (EDS), differential scanning calorimeter (DSC) and infrared spectroscopy (IR) (absorption and emission spectroscopy). Infrared absorption and thermal emission spectra systematically illustrated the effect of cations (Ca2+, Mg2+, Fe2+, Mn2+) on shifting the positions and infrared performance of the carbonate absorption bands. Three specific modes of the carbonate anion (CO32−) in mid-infrared range, derived from the out-of-plane bending, asymmetric stretching, and in-plane bending vibration (ν2, ν3, and ν4 modes, respectively) were found to be blue-shifted with the decrease of cationic radius, bond length and lattice volume. The average emissivity of calcite, rhodochrosite, siderite, and magnesite were calculated as 0.954, 0.934, 0.907, and 0.883, respectively, in the temperature range of 50–155 °C and the mid-infrared range of 400–2000 cm−1. Emissivity and thermal radiation performance of carbonate minerals can be influenced by several interplaying factors. In particular, higher emissivity was proportional to longer bond length (cation-oxygen and carbon–oxygen with linear correlation coefficients (R2) of 0.751 and 0.721, respectively) and larger cationic radius (R2 = 0.872), which gave rise to lower vibrational frequency, narrower vibration range, and reduced absorbed energy. Furthermore, the heat capacity of four minerals presented a positive correlation with their infrared radiant energy within the temperature. It thus can be concluded that complex and multifactor interactions occur within the crystal structure affecting the fundamental vibrational frequencies of CO32− group, and infrared performance was consequently influenced. This paper can provide theoretical reference for demonstrating the effect of crystal structure on infrared radiation performance and spectral characteristics of various minerals, which is conducive to distinguish minerals based on their features in infrared spectroscopy.