Spectral signatures of rocks and mineral provide information on their chemical composition and crystal properties, grain size and roughness over a wide range of wavelengths from the visible to the thermal infrared.
In the Visible and Near-InfraRed (VNIR; 0.4÷1.0 µm) region, spectral features are dominated by electronic processes in transition metals, such as Fe, Mn, Cu, Ni, Cr, etc. Therefore, iron is the most important constituent having spectral properties in the VNIR, and the iron-rich minerals are characterized by low reflectance (high absorbance) below 0.7 µm.
Other minerals, which represent the major part of the Earth's surface rocks, such us Si, Al and some anion groups (e.g. silicates, carbonates, oxides) hydroxides, have less spectral features in the VNIR region, but exhibit much more evidences in the Short-Wave InfraRed (SWIR; 1÷3 µm) region. In fact, spectral features of hydroxyls and carbonates mark the SWIR region.
The hydroxyl ion is a widespread constituent occurring in rock forming minerals such as clays, micas, chlorite etc. It shows a vibrational fundamental absorption band at about 2.74÷2.77 µm and an overtone at 1.44 µm.
Carbonates, which are commonly in the Earth surface rocks in the form of calcite (CaC03), magnesite (MgC03), dolomite [(Ca-Mg) C03] and siderite (FeC03), shows a typical absorbance feature around 2.3 µm, instead the water content can be instead evaluated by the depth of absorption at 1,4µm and 1,9 µm.
Thermal InfraRed (TIR; 1÷20 µm) region, from a geological point of view, is a particularly important spectral region for remote sensing aiming at compositional investigations of terrestrial materials. In fact, the fundamental vibration features of many rock-forming mineral groups (e.g. silicates, carbonates, oxides, phosphates, sulphates, nitrates, nitrites, hydroxyls) occur in the TIR region. Briefly:
a) the silicates, which are most abundant group of minerals in the Earth's crust, shows vibrational spectral features due to the presence of Si04-tetrahedron around 8 µm to 12 µm; b) the carbonates show a weak feature around 11.3 µm that can be detected; c) the sulphates display bands near 9 µm and 16 µm; d) the phosphates also have fundamental features near 9.25 µm and 10.3 µm; e) the features in oxides usually occupy the same range as that of bands in Si-O, i.e. 8 µm to 12 µm; g) the nitrates have spectral features at 7.2 µm and the nitrites at 8 µm and 11.8 µm; h) the hydroxyl ions display fundamental vibration bands at 11 µm.