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:
-
The sulfide zone was encountered in 2015 at the Milpillas Mine and pyrite
crystals with uncommon gray-metallic coating were found.
-
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).
-
The wholesaler decided the coating was bornite and sold them as
"Bornite coated Pyrite". I purchased a large lot, along with
several other dealers.
-
I sold many at the time labeled per the wholesaler's identification, including
to Prof. Allan Pring at the South Australian Museum.
-
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.
-
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.
-
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).
-
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
PYRITE WITH CHALCOCITE COATING FROM MILPILLAS MINE,
MEXICO

#75761, Pyrite with Chalcocite coating,
Milpillas Mine, Mexico (m) $55 |

#75760, Chalcocite over Pyrite, Milpillas
Mine, Mexico (m) $12 |

#81556, Pyrite with Chalcocite coating plus
Quartz, Milpillas Mine, Mexico (m) $55 |

#80852, Pyrite with Chalcocite coating,
Milpillas Mine, Mexico (m) $125 |

#75757, Chalcocite over Pyrite, Milpillas
Mine, Mexico (m) $32 |
|
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#75663, Pyrite with Chalcocite coating,
Milpillas Mine, Mexico (m) $30 |
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