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Bornite coated pyrite Milpillas Mine

Posted by Ben Grguric  
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Ben Grguric April 14, 2016 11:27AM
Recently some specimens labelled 'bornite coated pyrite' from the Milpillas Mine, Sonora, Mexico have come on the market (including Tucson 2016). The pyrite crystals are lustrous gun-metal grey to dark purplish grey in colour.
I made some fragments of this material into polished mounts and examined them in a reflected light microscope in order to check the identity of the sulphide coating the pyrite. In the examples I looked at the coating was a thin (5-10 micron) layer of chalcocite or djurleite, a classic supergene zone association. No bornite was observed. The scale in the x-direction on the attached photo is 600 microns.

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Uwe Kolitsch May 29, 2016 05:24PM
Jordi Fabre kindly sent me the analyses done by Marcus Origlieri (who wrote "Both samples showed significant Fe content, which is consistent with a member of the bornite-digenite series. Chalcocite-djurleite and other minerals in the Cu-S system are excluded").

Two analyses one gave (At.%):
Cu50.3, Fe6.9, S42.9
Cu58.1, Fe4.2, S37.7

While Fe-poor bornites exist, an ideal bornite should have about Fe10.
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Ben Grguric May 30, 2016 04:10AM
Thanks Uwe,

My problem with these analyses are:

1. They don't really plot on the compositional mixing lines for the bornite-digenite solid solution (assuming end member digenite has 1 at% Fe).
2. Intermediate bornite-digenite compositions have not been confirmed in nature, or didn't at the time I did my PhD on the series.
3. The colour of the phase does not resemble quenched compositions in the bornite-digenite solid solution, given the analyses presented. They would be the pink-brown more similar to the bornite end member, rather than the blue we see in polished section.

The coatings on these pyrite crystals are very thin, generally <20 microns, so thin in fact that I was unable to detect any Cu sulphide peaks in powder XRD after scraping the surface and only got diffraction peaks of the pyrite substrate.
I suspect, given the thin nature of the coatings, the EDS or WDS analyses were "contaminated" with underlying pyrite. To back this up the analyses do plot on a mixing line between pure chalcocite and pyrite consistent with about 10% pyrite contamination in one and 20% in the other.

Cheers,

Ben.
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Uwe Kolitsch May 30, 2016 12:43PM
Jordi Fabre will have his samples re-analysed.
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Dennis McCoy June 01, 2016 11:33PM
Could this coating be stannite?
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John Sobolewski June 02, 2016 12:21AM
Interesting. I had always wondered if the orignal Bornite identification had been contaminated by Fe from the Pyrite given that the "Bornite" film is so thin. John S.
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Uwe Kolitsch June 02, 2016 12:05PM
The analyst kindly wrote me:

"Ben is right and the beautiful picture shows clearly chalcocite. Also, the chalcocite coatings on pyrite are common in secondary enrichment zones and to me makes more sense than bornite-digenite. When Jordi asked me first, I thought the sample were bornite on chalcopyrite. Given the characteristic of the sample, the iron observed by Marcus is probably an artifact. The analysis will require a more appropriate technique for surface analysis."

Will change the photos/coatings to "copper sulphides" (while refering to this thread), until a definite ID will be obtained.
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Ben Grguric April 25, 2017 11:05PM
Is there some new data on this one? Three more specimen images from here labelled bornite coating pyrite (e.g. specimen CPV-N67) were uploaded onto Mindat by Lavinsky in November 2016 with the statement "the bornite has been X-ray confirmed by M. Origlieri."
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Reiner Mielke April 26, 2017 12:21PM
Must have been challenging to get enough material for XRD. I'd sure like to see the results.
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Uwe Kolitsch March 03, 2018 02:51PM
New report (thanks to Cesar Menor-Salvan): clearly NOT bornite, but chalcocite coating on pyrite.

http://mineral-analysis.es/the-case-of-the-milpillas-chalcocite-covered-pyrite-an-example-of-bad-practice/
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Marcus J. Origlieri March 03, 2018 10:53PM
Rob

I may have screwed up! In March 2015, I performed some SED-EDS analysis on the coating for a friend. The results showed Cu, Fe, and S. However, without additional data, the identification of bornite is questionable. There are many species in the Cu-S and Cu-Fe-S systems. And unlike the caption states, I never performed X-ray diffraction. (I do not have ready access to a thin film setup). As Dr. Menor-Salvan states, SEM-EDS is not sufficient to determine the identity of this thin coating.

I would like to know if someone could measure reflectance for the coating.



Edited 1 time(s). Last edit at 03/03/2018 10:59PM by Marcus J. Origlieri.
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Ben Grguric March 03, 2018 11:18PM
Great report Cesar, but in science it is generally considered good practice to reference previous work. This article makes no reference to the fact that a. I first highlighted the ID issue in April 2016, b. I performed reflected light microscopic work that showed the coating wasn't bornite, and c. noted that based on analysis of the EDS data, contamination of a copper sulphide with underlying pyrite would yield the analysis obtained.
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Rob Woodside March 05, 2018 12:11AM
Thanks Marcus. This clears it all up. We'll fix the captions.

That's a good question about reflectance. Chris Stanley would know. The coating might be too thin?



Edited 1 time(s). Last edit at 03/05/2018 12:13AM by Rob Woodside.
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Rob Woodside March 05, 2018 12:57AM
OK Pictures cleaned up and messages sent
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Ralph Bottrill March 05, 2018 02:33PM
Thanks Rob.
I have updated the locality page and the bornite locentry.
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Cesar Menor-Salvan March 05, 2018 04:25PM
Ben Grguric Wrote:
-------------------------------------------------------
> Great report Cesar, but in science it is generally
> considered good practice to reference previous
> work. This article makes no reference to the fact
> that a. I first highlighted the ID issue in April
> 2016, b. I performed reflected light microscopic
> work that showed the coating wasn't bornite, and
> c. noted that based on analysis of the EDS data,
> contamination of a copper sulphide with underlying
> pyrite would yield the analysis obtained.

Sorry, I didn't knew any previous reference until now (Sorry, I usually do not follow forums and rarely I count it as reference for some reasons, but sometimes are really referable) and I didn't found anything in the literature about that. My report is just a 'report' of results, not a paper. Probably with your reflected light microscopy picture, I would saved the time spent doing polished sections and optical microscopy. But, I didn't knew ever it was published.



Edited 2 time(s). Last edit at 03/05/2018 04:35PM by Cesar Menor-Salvan.
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Ben Grguric August 14, 2018 10:45PM
An update on this. John Betts, NY mineral dealer has gone to the trouble to document the ID process of these coated pyrites on his website.
http://www.johnbetts-fineminerals.com/jhbnyc/milpillas.htm

Good to see a dealer presenting thing sort of thing.
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W. Richard Gunter August 14, 2018 11:24PM
Hi Ben:

I am glad to see John has done this. It is nice to know that the sample you are buying has enough data to support the label. A texture I have a question about is the bornite overgrowth over euhedral chalcosite that is seen in some 19th Century samples from the Levant Mine in Cornwall, U.K.. Have you seen any data as to whether the bornite and chalcocite are what they are supposed to be, given that so many more phases are now present in the Cu-Fe-S system?
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W. Richard Gunter August 14, 2018 11:29PM
That did not work very well. The min id I was trying to post is 423589
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Ben Grguric August 15, 2018 10:05AM
Hi Richard,
In theory you could have a bornite overgrowth or replacement of chalcocite but I can't recall an example in the polished sections I've looked at. That's not saying much however. It's usually the other way around in supergene systems. I don't know of any detailed characterisation studies of the Cornish material you show in the photo.
Copper sulphides with chalcocite-like chemistry are difficult things to work with as they transform so easily, e.g. some change structure when finely powdered for XRD work and they often transform in the electron beam if you use TEM. Even with basic microprobe analysis you need to use a de-focussed beam otherwise metals can diffuse and modify your stoichiometry the longer you expose them to the beam.
If you had a fragment I could make up a polished section and look at the textures of the phases but I don't expect you to break up a nice historic specimen.
Cheers,
Ben.
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Dale Foster August 15, 2018 01:39PM
W. Richard Gunter Wrote:
-------------------------------------------------------
That did not work very well. The min id I was trying to post is 423589


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W. Richard Gunter August 15, 2018 02:30PM
Hi Ben and Dale:

Thanks for the photo post. I somehow misspelled the posting formula. This is only a boxed thumbnail specimen so there is nothing to detach to send you. There must be other similar samples that the Mindat people have from this find. that could shed a fragment or two. I think in this case the "bornite" and "chalcocite" were convenient designations for tarnished bronze and dark grey sulphide respectively.

I don't know how old the sample is. I purchased it in the 1970's without any older labels.
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