Based on comprehensive analysis of the crystal structure, chemical and phase composition of natural arsenopyrite of the “Panimba” deposit (Krasnoyarsk region), an analytical expression for calculating the impurity density in structure of minerals type of marcasite are received. Based on comprehensive analysis of the crystal structure, chemical and phase composition of natural arsenopyrite of the “Panimba” deposit (Krasnoyarsk region), an analytical expression for calculating the impurity density in structure of minerals type of marcasite are received.