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

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 Fe-57 Mossbauer spectroscopy. Its unit cell is a 5.7612(8), b 5.6841(7), c 5.7674(8) angstrom, beta 111.721(8)degrees, and V 175.46(4) angstrom(3) (Z = 4). Taking into account very fine ubiquitous twinning on {101}, its crystal structure has been refined in the space group P2(1)/c on the basis of 758 unique reflections [F-o > 4 sigma(F-o)] to R-1 = 0.0298. Within uncertainty limits, it indicates three unmixed Fe, As and S positions. The Fe-57 Mossbauer 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 delta, and quadrupole splitting Delta E-Q, as follows: S-A 82.2%, delta(A) 0.24(1) mm/s, Delta E-QA 1.12(2) mm/s; S-B 8.5%, delta(B) 0.25(1) mm/s, Delta E-QB 0.69(2) mm/s; S-C 9.3%, delta(C) 0.26(1) mm/s, Delta E-QC 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 Mossbauer 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 Delta E-Q shift towards marcasite and lollingite, respectively, may be due to local disorder (e.g., twin contact walls) changing the octahedral coordination of Fe to unequal As:S ratios.