To understand the aqueous history of Mars, it is critical to constrain the alteration mineralogy of the Martian surface. Previously published analyses of thermal infrared (λ = 6--25 ¿m) remote sensing data of Mars suggest that dark regions have ~15--20% clay minerals. However, near-infrared (λ = 1--3 ¿m) spectral results generally do not identify widespread clay minerals. Thermal infrared detections of clays on Mars are difficult to interpret owing in part to the relative paucity of published spectral analyses of clay minerals and clay-bearing materials using similar spectra (thermal infrared emission spectra). In this study, we present an analysis of the thermal emission spectral features (λ = ~6--25 ¿m or 400--1650 cm-1) of a suite of clay mineral reference materials and clay-bearing rocks, linking their spectral features to the crystal chemical properties of the clays. On the basis of this context provided by the emission spectral analysis of clay minerals and clay-bearing rocks, we reconsider the evidence for clay minerals on Mars from Thermal Emission Spectrometer (TES) results. We propose that global-scale clay abundances determined from TES probably represent a geologically significant surface component, though they may actually correspond to poorly crystalline aluminosilicates with similar Si/O ratios to clay minerals (0.3--0.4), rather than well-crystalline clays. If clay minerals or clay-like materials on Mars are poorly crystalline and/or dessicated, they may be detectable in the thermal infrared, but not easily detected with near-infrared data sets.