Arsenopyrite from La Roche-Balue quarry (Loire-Atlantique department, France), with the stoichiometric composition FeAsS, has been studied by X-ray single-crystal diffraction and 57Fe Mössbauer spectroscopy. Its unit cell is a 5.7612(8), b 5.6841(7), c 5.7674(8) Å, β 111.721(8)°, and V 175.46(4) Å3 (Z = 4). Taking into account very fine ubiquitous twinning on {101}, its crystal structure has been refined in the space group P21/c on the basis of 758 unique reflections [Fo > 4σ(Fo)] to R1 = 0.0298. Within uncertainty limits, it indicates three unmixed Fe, As and S positions. The 57Fe Mössbauer spectrum of this arsenopyrite shows two broad absorption peaks that were fitted using the superposition of three doublets denoted as A, B and C, with parameters relative to area S, isomer shift δ, and quadrupole splitting ΔEQ, as follows: SA 82.2%, δA 0.24(1) mm/s, ΔEQA 1.12(2) mm/s; SB 8.5%, δB 0.25(1) mm/s, ΔEQB 0.69(2) mm/s; SC 9.3%, δC 0.26(1) mm/s, ΔEQC 1.49(2) mm/s. This set of values agrees well with the most recently published results for arsenopyrite samples with a composition close to stoichiometry. The three distinct Fe positions indicated by Mössbauer spectroscopy are not visible in the X-ray study. Whereas the main doublet A would seem to correspond to the FeAs3S3 octahedron of the ideal structure, the two minor doublets B and C, with a ΔEQ shift towards marcasite and löllingite, respectively, may be due to local disorder (e.g., twin contact walls) changing the octahedral coordination of Fe to unequal As:S ratios.

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