A unique pyrrhotite, having a new 5С type superstructure, has been found in a specimen from an alpine hydrothermal vein. Precession photographs show that it differs from the previously-described 5С pyrrhotites in that the superstructure reflections are offset from Bragg positions. The composition is Fe9S10, and this phase is thus a polymorph of so-called hexagonal pyrrhotite of that composition. Fluid inclusion work indicates that it formed at a temperature of at least 160–170°C and that it is temperature.
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