The thermal emission properties of two Mars soil analogs, palagonite and iron-substituted montmorillonite clay, and their physical mixtures are presented in the 5 to 25 μm (2000 to 400 cm-1) region. The clay spectrum has both a greater number of spectral features and higher emissivity contrast when compared to the palagonite spectrum. Emissivity peaks located near 8.9, 11.1, and 19.9 μm (1120, 905, and 505 cm-1) and troughs (restsrahlen features) located near 8.8, 9.7, 18.9, and 22.2 μm in the spectrum of the clay provide the ability to identify the presence of clay when it is physically mixed with the palagonite. In the mixtures studied, when 15 wt% of clay is mixed with 85 wt% palagonite, the 19.9 μm emissivity peak and the 9.7 μm reststrahlen trough are readily identifiable, but the 8.9 and 11.1 μm peaks and the 8.8, 9.3, and 18.9 μm troughs are only marginally identifiable. This indicates that identification of clays on Mars, using midinfrared data, should rely upon the sensitivity of the 19.9 and 9.7 μm features to the clays' presence. ¿ American Geophysical Union 1996