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Mineralogical ClassificationOn the origin of »iron-cross« twins of pyrite from Mt. Katarina, Slovenia

5th Mar 2016 18:47 UTCUwe Kolitsch Manager

Rečnik, A., Zavašnik, J., Lei, J., Čobić, A., Daneu, N. (2016): On the origin of »iron-cross« twins of pyrite from Mt. Katarina, Slovenia. Mineralogical Magazine, 80, (in press).

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 Upper-Permian silts of Mt. Katarina near Ljubljana (Slovenia), that represent one of the most typical geological environments for twinned pyrite. Mineralization of pyrite starts with a reduction of the primary red-colored hematite-rich sediment by sulphide rich fluids that penetrated the strata. A short period of magnetite crystallization is observed prior to pyrite crystallization, which indicates a gradual reduction process. Sulphur isotope analysis of pyrite shows an enrichment in δ34S, suggesting its origin from the neighboring red bed deposit. Other sulphides, 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. Electron microscopy and spectroscopy study of twin boundaries showed that interpenetration twinning is accomplished through complex 3D intergrowth of primary {110} Cu-rich boundaries, and secondary {100} boundaries that are pure. We show that ~ 1 monolayer of Cu atoms is necessary to stabilize the {110} twin structure. When the source of copper is interrupted, the two crystal domains continue to grow in predefined orientation along {100} interfaces, favorable for pure pyrite. The presence of Cu thus appears to be necessary condition for the formation of iron-cross twins in pyrite.

Confirms the general picture (see other publications of A. Rečnik) that monolayers of impurity atoms are responsible for twinning.

6th Mar 2016 23:55 UTCDan Costian

Uwe,

Very interesting research explaining the genesis of such rarities.

I uploaded recently an "iron cross" pyrite specimen, which landed in the less-visited user gallery area

http://www.mindat.org/photo-734913.html

Dan

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