The permittivities of pulverized samples of coal and limestone were measured over a range of bulk densities at 11.7 GHz and 20/spl deg/C. The measured values were used, along with particle densities, determined by pycnometer measurements, and the Landau & Lifshitz, Looyenga (LLL) dielectric mixture equation to determine the solid material permittivities. Subsequently, similar measurements were taken on a 35%-65% coal-limestone mixture, and the LLL equation and the complex refractive index (CRI) dielectric mixture equation were used to calculate the permittivities of powdered coal, limestone, and the 35%-65% mixture at various bulk densities. Results compared to the measured permittivities showed that the LLL equation provided much better estimates than the CRI equation, with errors of 0.15% in predicting the dielectric constants of powdered coal, 1.4% in predicting the dielectric constants of powdered limestone, and 0.5% in predicting the dielectric constants of the coal-limestone mixture.