Iron-cross twins of pyrite are well known among mineralogists, however it is quite surprising that the conditions of their formation remain unexplored. To address this question we studied pyrite twins from the Upper Permian silts of Mt. Katarina near Ljubljana (Slovenia), which represent one of the most typical geological environments for twinned pyrite. Mineralization of pyrite starts with a reduction of the primary red-coloured hematite-rich sediment by sulfide-rich fluids that penetrated the strata. A short period of magnetite crystallization is observed prior to pyrite crystallization, which indicates a gradual reduction process. Sulfur isotope analysis of pyrite shows an enrichment in δ34S, suggesting its origin from the neighbouring red-bed deposit. Other sulfides, such as chalcopyrite and galena, formed at the end of pyrite crystallization. Remnants of mineralizing fluids trapped at the interfaces between the inclusions and host pyrite show trace amounts of Pb and Cu, indicating their presence in the solutions throughout the period of pyrite crystallization. An electron microscopy and spectroscopy study of twin boundaries showed that interpenetration twinning is accomplished through a complex 3D intergrowth of primary {110} Cu-rich twin boundaries, and secondary {100} boundaries that are pure. We show that approximately one monolayer of Cu atoms is necessary to stabilize the {110} twin structure. When the source of Cu is interrupted, the two crystal domains continue to form {100} interfaces, that are more favourable for pure pyrite.

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