Pyrite For Sale from Milpillas Mine, Mexico

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UP-TO-DATE INFORMATION ABOUT COATED PYRITE CRYSTALS FROM MILPILLAS MINE, MEXICO

There has been a series of errors in identification of the gray-metallic-coated pyrite crystals from the Milpillas Mine, Mexico.
Now the mystery is solved: the coating is definitively CHALCOCITE.

Here is a chronology of the evolution of misidentifications:

1. The sulfide zone was encountered in 2015 at the Milpillas Mine and pyrite crystals with uncommon gray-metallic coating were found.

2. A wholesaler (to remain anonymous) sent specimens to Marcus Origlieri in March 2015 who tested them using SED-EDS analysis that detected copper (Cu), iron (Fe), and sulfur (S).

3. The wholesaler decided the coating was bornite and sold them as "Bornite coated Pyrite". I purchased a large lot, along with several other dealers.

4. I sold many at the time labeled per the wholesaler's identification, including to Prof. Allan Pring at the South Australian Museum.

5. Prof. Pring tested the coating, and failed to detect Fe, which is a significant component of bornite (= Cu5FeS4). Prof. Pring postulated (correctly) that the EDS analysis detected the Fe in the underlying pyrite.

6. Based on that analysis, I announced on this site the test results and stated that bornite was unlikely due to the lack of Fe in the tests, but that the coating identification was still unknown. At that time I suggested labeling them as "Pyrite with Chalcocite-Djurleite-Bornite coating" until definitive tests were performed.

7. I sent additional samples to the South Australia Museum where Ben Grguric mounted them on polished mounts and tested the coating further, described the coating as a 5-10 micron thick layer of chalcocite or djurleite (April 2016).

8. Jordi Fabre independently had specimens tested by GeoSpectra using alpha particle induced X-ray spectroscopy (August 2016), and by EDS on the longitudinally oriented thin section so the underlying pyrite was not exposed (December 2017) confirmed the identification of the coating as CHALCOCITE.

These events are an example of how one wholesaler, in a rush to sell a new find, failed to wait for proper testing, and propagated false information throughout the mineral community. A similar misidentification occurred with the "ajoite" from the Luputo Mine which was  actually chrysocolla.

I understand the rush to announce new finds, and I understand that EDS is fast and easy. But EDS is not a good test for all cases. When results are not definitive, it is wrong for the wholesaler to "decide" what they should be labeled. As dealers and collectors, it pays to ask questions regarding testing and identification.

Finally, it is noted that chalcocite coating on pyrite is known from several other localities, including the Burra Burra Copper Mine, South Australia, Australia; Chino Mine, Santa Rita District, Grant County, New Mexico; and Ducktown Mine, County, Tennessee in the USA. The latter were sold as "ducktownite" which were later identified as chalcocite-coated pyrite. The varietal name ducktownite still persists to describe pyrite crystals coated with gray-metallic chalcocite.

SPECIAL NOTE: The pyrite crystals at the Milpillas Mine were fractured after they crystallized and the crystal fragments were frozen in the white dickite-rich matrix, with thin fractures filled with matrix.  John White in the recent Rocks & Minerals magazine describes this as "disjoined" crystals. But the chalcocite coating is on all surfaces of the pyrite crystals, including the fractured surfaces. That means: 1.) pyrite crystallized; 2.) the pyrite crystals fractured and were frozen in matrix; 3.) the chalcocite coating formed on all surfaces of the pyrite at the end

 

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