Remote sensing of Mars at mid-infrared wavelengths could be used to identify different silicate rocks and minerals. Current knowledge suggests that the primary components of the surface of Mars are basaltlike rocks and a palagonitelike dust. To study the mid-infrared (7--20 μm) spectroscopy of this combination of materials, we examined the influence of Pahala ash palagonite on the biconical reflectance of quartzite and basalt. The reflectance of the palagonite is flat and low (1--2%), unlike the reflectance of quartzite and basalt. The reflectance of quartzite or basalt is significantly reduced by the addition of small amounts of the palagonite dust. A dusting of 1--20 μm palagonite particles and 20--250 μm particle clumps, covering 50% of the quartzite host, reduces the quartzite mid-infrared reflectance by 50%, as expected for an essentially black coating. Similar data were obtained for basalt. Even for extremely small amounts of palagonite (2--100 μm thickness, averaging 5 μm), the reflectance of the underlying rock is linearly related to exposed surface area. To confirm the opaque behavior of the Pahala ash palagonite coatings, we estimated the imaginary part (k) of the refractive index from transmission measurements. The strong absorption and low reflectance (high emissivity) of the palagonite at 8--11 μm and 17--25 μm explain its opaque behavior. Estimates of the extinction coefficient indicate that even a 5-μm thickness of Pahala ash palagonite is optically thick in these wavelength ranges where silicate rock identification is critical. Thus mid-infrared remote sensing of rocks on Mars will be practical only for dust-free areas. ¿ American Geophysical Union 1992