Spectral Properties of Anhydrous Carbonates and Nitrates

The spectral properties of anhydrous carbonates and nitrates are dominated by strong, sharp vibrational bands due to the CO₃²⁻ and NO₃⁻ anions observed as absorption bands in near-infrared spectra, as Reststrahlen features or absorption bands in mid-IR spectra, depending on particle size, and as peaks in Raman spectra. These spectral features provide a reliable means to identify the occurrence of carbonates and nitrates on planetary surfaces, which in turn contribute to our understanding of the environment and chemistry of planetary bodies. Four modes occur for carbonates and nitrates due to symmetric stretching (ν₁), out-of-plane bending (ν₂), asymmetric stretching (ν₃), and in-plane bending (ν₄). The vibrational absorptions of these spectral features vary with the mineral structure and the size of the cation, where the calcite-, dolomite-, aragonite-, and alkali-type structures result in different spectral features. Mid-IR bands for carbonates and nitrates occur from 1,040 to 1,105 cm⁻¹ for ν₁, from 810 to 906 cm⁻¹ for ν₂, from 1,275 to 1,590 cm⁻¹ for ν₃, and from 670 to 756 cm⁻¹ for ν₄. In Raman spectra the carbonate and nitrate absorptions are observed near 1,050–1,080 cm⁻¹ for ν₁, near 880 cm⁻¹ for ν₂, near 1,415–1,430 cm⁻¹ for ν₃, and near 680–700 cm⁻¹ for ν₄. NIR spectra include bands due to overtones and combinations at ∼1.75, 1.9, 2.0, 2.3, 2.5, 3.4, 4.0, and 4.6 μm for carbonates and ∼1.8, 2.0, 2.2, 2.4, 2.6, 3.5, 4.1, and 4.8 μm for nitrates. This study provides data for remote determination of carbonate and nitrate chemistry and will enable better characterization of these minerals on planetary bodies including Mars, Ceres, and Bennu.