Thermal infrared directional emissivity of quartz was measured for several particle size ranges and packing fractions. The samples measured were (1) a polished quartz slab; (2) a 75--250 μm powdered quartz sample, cleansed of clinging fines; (3) a 0--75 μm powdered quartz sample, sifted into a fairy castle structure; and (4) the same sample compressed to minimize porosity. The spectra of the particulate samples showed a strong dependence on exitance angle, particle size and packing fraction. In addition, thermal gradient effects significantly affected the measured emissivity of the fine, sifted sample. The measured directional emissivity was modeled by first using Mie theory to calculate single-particle scattering properties of a quartz spheres of appropriate size at a single wavelength and radiative transfer theory to calculate the flux reflected from an optically thick, plane-parallel ''atmosphere'' composed of particles with these scattering properties. For the 75--250 μm sample, close packing of individual particles was accounted for by subtracting the diffraction contribution to the scattering cross section. These calculations are repeated wavelength-by-wavelength to determine the spectral directional hemispherical reflectance of the quartz sample. Kirchhoff's law was then used to obtain spectral directional emissivity. This model, which uses optical constants derived from widely used oscillator parameters for quartz, reproduces the directional emissivity spectrum of the large powdered quartz to better than 10%. However, model calculations for 0--75 μm particle size quartz were less successful. This could be due to the several approximations used in the model, or to a possible error in the oscillator parameters. More successful calculations based on different optical constants suggest that the widely used oscillator parameters may well be wrong. ¿ American Geophysical Union 1995