The role of volume scattering in reducing spectral contrast of reststrahlen bands in spectra of powdered minerals

As particle size decreases, the spectral contrast of reststrahlen bands also decreases. It has been generally agreed that this loss of spectral contrast is due to the increased porosity associated with fine particle size, resulting in formation of photon traps. That is, that the pores acted like small black bodies. However, we show here that the reststrahlen bands change in shape as well as intensity, and this change in shape can only be explained by the occurence of substantial volume scattering at fine particle size, rather than by photon trapping. It appears that the role of porosity is to physically separate 1- 5-μm-diameter particles that are optically thin, even in the reststrahlen bands. When such particles are separated by more than a wavelength, they scatter independently as optically thin, volume-scattering particles. When packed closely together, however, they scatter coherently as if they were large, optically thick, surface-scattering particles. Thus, the loss of spectral contrast of reststrahlen bands for fine particle size materials appears to be due directly to particle size and only indirectly (but critically) to porosity. Coarse particle size materials may also display greatly reduced spectral contrast and altered band shape in their spectra if the larger particles are coated with clinging fines. The practical implication of this finding for remote sensing of the Earth, Moon, Mercury, Mars, and the asteroids is that the spectral features displayed by some particulate materials may be substantially changed from those seen in spectra of solids, requiring the use of a separate spectral search library to identify component minerals.