Bulk synthesis of stoichiometric/meteoritic troilite (FeS) by high-temperature pyrite decomposition and pyrrhotite melting

1Juulia-Gabrielle Moreau,1Argo Jõeleht,1Jaan Aruväli,2Mikko J. Heikkilä,3Aleksandra N. Stojic,2Thomas Thomberg,1Jüri Plado,4Satu Hietala
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13782]
1Department of Geology, Institute of Ecology and Earth Science, University of Tartu, Ravila 14A, Tartu, 50411 Estonia
2Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), FI-00014 Finland
3Institut für Planetologie, Westfälische Wilhelms Universität Münster, Wilhelm-Klemm-Str. 10, Münster, 48149 Germany
4Geological Survey of Finland, Neulamäentie 5, Kuopio, FI-70211 Finland
Published by arrangement with John Wiley & Sons

Stoichiometric troilite (FeS) is a common phase in differentiated and undifferentiated meteorites. It is the endmember of the iron sulfide system. Troilite is important for investigating shock metamorphism in meteorites and studying spectral properties and space weathering of planetary bodies. Thus, obtaining coarse-grained meteoritic troilite in quantities is beneficial for these fields. The previous synthesis of troilite was achieved by pyrite or pyrrhotite heating treatments or chemical syntheses. However, most of these works lacked a visual characterization of the step by step process and the final product, the production of large quantities, and they were not readily advertised to planetary scientists or the meteoritical research community. Here, we illustrate a two-step heat treatment of pyrite to synthesize troilite. Pyrite powder was decomposed to pyrrhotite at 1023–1073 K for 4–6 h in Ar; the run product was then retrieved and reheated for 1 h at 1498–1598 K in N2 (gas). The minerals were analyzed with a scanning electron microscope, X-ray diffraction (XRD) at room temperature, and in situ high-temperature XRD. The primary observation of synthesis from pyrrhotite to troilite is the shift of a major diffraction peak from ~43.2°2θ to ~43.8°2θ. Troilite spectra matched an XRD analysis of natural meteoritic troilite. Slight contamination of Fe was observed during cooling to troilite, and alumina crucibles locally reacted with troilite. The habitus and size of troilite crystals allowed us to store it as large grains rather than powder; 27 g of pyrite yielded 17 g of stochiometric troilite.

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