The Raman spectra of freshly cleaved pyrrhotites: The effect of atmospheric weathering and laser‐induced oxidation

Abstract Pyrrhotite is a common iron sulfide mineral described as a non‐stoichiometric formula Fe 1‐ x S (0 < x < 0.125), permitting a lot of superstructures based on the hexagonal NiAs crystal subcell. Due to its abundance as well as its characteristic magnetic properties, many aspects of the research have been carried out both in geochemistry and material sciences. However, surprisingly, Raman spectra reported as “pyrrhotite” are remarkably different literature‐by‐literature. The reason for the differences is probably due to its reactive surface allowing the accelerated surface oxidation by laser‐induced heating in air and atmospheric weathering by long‐time exposure. However, to consider its crystal and/or surface structure, standard Raman spectra from fresh surfaces, that is, free from oxidation, are inevitable for the assignments of geological samples and for studying the chemical reaction that occurred on the surfaces. Here, we measured the Raman spectra of freshly cleaved surfaces of pyrrhotites sampled from different mines with changing the measurement conditions. As a result, only a very weak and broadened peak at around 130 cm −1 was observed from the fresh surfaces. It is suggested that the Raman measurements in air rapidly proceed the surface oxidation. In addition, the atmospheric weathering is also found to give new intense Raman bands, which have been sometimes assigned to the original peaks of pyrrhotite crystal. The Raman spectra of pyrrhotites both in lower‐wavenumber and in anti‐Stokes region were also measured for the first time, confirming the spectroscopic features of pyrrhotite above.