We examined the thermal IR spectra of 35 rock samples and their fine-powder equivalents to better understand the relationship between spectral features, particle size, and composition. The Christiansen features of solid crystalline rocks differ from those of their powdered samples, and the magnitude of this difference changes systematically with rock type. Both the Christiansen feature (CF) and the transparency feature (TF) in spectra of particulate igneous rocks are useful in rock type determination. They correlate better with the SCFM chemical index of rock type than with other numerical rock type indices that we considered. Major rock type divisions (acidic, intermediate, basic, and ultrabasic) could be distinguished using both CF and TF, but some overlap occurred if one or the other spectral feature was used alone. Spectral contrast decreases as rocks become more basic, probably because of increasing opaque mineral content. Likewise, metallic iron, formed as a result of space weathering in an airless environment, appears to be the most likely cause of reduced spectral contrast in mature lunar soils. Thus reststrahlen features should be difficult to detect in remote-sensing measurements of such bodies as the Moon and Mercury. Moreover, the vacuum environment of an airless planetary surface appears to enhance the CF, but it may eliminate the TF, depending critically on particle size. The Martian pressure and radiative environment, on the other hand, appears to enhance the TF, which should be sought in spectra of finely particulate surface areas on Mars. Additional environmental simulation spectral experiments are needed to more accurately define the roles of pressure, particle size, opacity, and insolation angle on apparent emissivity.