The case of the Milpillas chalcocite-covered pyrite: a case of analysis misinterpretation.

Some time ago, beautiful specimens of pyrite crystals from Milpillas mine (Mexico) covered by a thin layer of chalcocite (Cu2S), were put on the mineralogy market as Bornite (Cu5FeS4). The crystal shape and color of the crystals didn’t match well with Bornite. Moreover, the geochemistry of the deposit make more likely that the specimens are composed by chalcocite-covered pyrite crystals (a relatively common phenomenon in the supergene enrichment zone of some copper deposits). Despite these observations, the identification were considered legit after a misinterpreted analysis, performed apparently (according to some picture captions) by Marcus Origlieri, analyses for which we didn’t found any reference and we were unable to check it out, so we cannot comment it.

Chalcocite-covered pyrite, sold as Bornite after a bad analysis.

Another specimen analysed.

In order to solve the misidentification, we performed a series of analysis on two specimens, using particle-induced X-ray spectroscopy, SEM-EDS, XRD and optical microscopy. Here we present the results found:

Our recommendation to buyers and collectors is that RELABEL these specimens in their collections, and warn to the dealers that are incorrectly labelled of they find it on sale.

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