Figure 3. Plot of laboratory spectra of mont-morillonite, kaolinite, gypsum, and muscovite from Salisbury et al. (1991), converted to qualitative emissivity using Kirchoff's Law. These minerals commonly occur in rocks and soils and have spectral features in the 8–14 µm atmospheric window measured by ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) that can substantially affect TIR (thermal infrared) spectral response. Where these and other minerals occur in mixtures with quartz, the presence of quartz can be spectrally obscured, as discussed in text.