engleski

On the origin of "iron-cross" twins of pyrite from Mt. Katarina, Slovenia

Iron‐cross twins of pyrite (FeS2) are regularly found in silts of upper‐Permian age at Mt. Katarina near Ljubljana (Slovenia). Pyrite mineralization started with a reduction of redcolored hematite‐rich primary sediment under the influence of sulphide‐rich fluids. A short period of magnetite crystallization preceded pyrite crystallization, indicating the onset of gradual reduction process following the sequence: hematite → magnetite → pyrite. Sulfur isotope analysis of pyrite crystals shows strong chemical fractionation of sulfur, suggesting its origin in the neighboring red‐ bed ore deposit. Other sulphides, such as chalcopyrite and galena, formed at the end of pyrite crystallization. Remnants of mineralizing fluids entrapped at the interfaces between the inclusions and hosting pyrite show trace amounts of Pb and Cu, indicating their presence in the solutions throughout the pyrite crystallization period. Two types of twin boundaries were identified in iron‐cross twins of pyrite: (i) Type‐A twin boundaries occupying {; ; ; ; ; ; ; 110}; ; ; ; ; ; ; planes and (ii) Type‐B boundaries occupying {; ; ; ; ; ; ; 100}; ; ; ; ; ; ; planes. While Type‐A twin boundaries are enriched with Cu, Type‐B boundaries are pure, indicating that Cu stabilizes the {; ; ; ; ; ; ; 110}; ; ; ; ; ; ; twin boundaries and is the necessary condition for the formation of iron‐ cross twins in pyrite. When the source of copper is disrupted, the two crystal domains continue to grow in predefined orientation along {; ; ; ; ; ; ; 100}; ; ; ; ; ; ; interfaces, which are energetically more favorable for pure pyrite.

red‐beds; hematite; magnetite; pyrite; FeS2; iron‐cross twins; stacking faults; twin boundaries; transmission electron microscopy; crystal growth

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