STOICHIOMETRIC ARSENOPYRITE, FeAsS, FROM LA ROCHE-BALUE QUARRY, LOIRE-ATLANTIQUE, FRANCE: CRYSTAL STRUCTURE AND MÖSSBAUER STUDY

Abstract

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, b 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 > 4s(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 d, and quadrupole splitting DEQ, as follows: SA 82.2%, dA 0.24(1) mm/s, DEQA 1.12(2) mm/s; SB 8.5%, dB 0.25(1) mm/s, DEQB 0.69(2) mm/s; SC 9.3%, dC 0.26(1) mm/s, DEQC 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 DEQ 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.