The natural mineral PhS samples were studied. The physical reasons of the Plasmon reflectance edge st low-frequency are discussed. According to the relations between the slope of plasmon reflectance edge and the contents of natural impurities,the influence of characters of different chemical composition and crystal strictures in different samples on the plasmon reflectance edge is explained. And according to the positive and negative correlations among the contents of different impurities in samples with resped to the cause and effect relations of natural formation, the feasibility of obtaining information from far infrared reflectance spectral structures which are usable to indicate the mineral formation conditions is discussed.