The features of the FeAsS arsenopyrite crystal structure disordering occurred in geological conditions were studied by elemental analysis, X-ray diffraction and Mossbauer techniques using natural samples. There were selected and studied three practically single-phase polycrystalline samples of Blagodatnoe and Olimpiada ore deposits (Krasnoyarsk region) with the chemical composition close to stoichiometry (I), with excess sulfur (II) and arsenic (III).

The Mossbauer (NGR) study detected the presence of the three forms of Fe(2+) in the substance. The first was associated with octahedral iron environment of [As₃S₃] type, the second with [S₆] and third with [As₆] forms conventionally associated with ordered arsenopyrite and FeS₂ FeAs₂ minerals. According to NGR data the iron atoms at 95% were in the main form in the sample (I). Other forms were present about in equal parts. There were detected 73% and 56% of iron in its basic form for samples (II) and (III) respectively with FeS₂, FeAs₂ dominated as by-forms.

X-ray powder crystal structure refinement was carried out in Space group P2₁/c. It was established that the iron atoms were in cationic positions of the arsenopyrite. Additional crystallographic positions of iron were not detected. The structure variation mainly occurred in the anion sublattice disordering the primary iron coordination from [FeAs₃S₃] to the condition described as [Fe(As_3-хS_x)(S_3-yAs_y)]. It was established that the mutual substitution of anions was accompanied by a small shift of the atomic positions due to the difference in the interatomic distances of Fe-S and Fe-As. Disordered condition admits the predominance of one of the anions. As a result, there are the set of the iron atom states with different composition and spatial configuration of the substituents in the coordination sphere in the real crystal. The disordering coordination of iron atoms obtained from X-ray diffracted data quantitatively corresponds to the data of NGR spectroscopy.