FMF - Friends of Minerals Forum, discussion and message board

[John S. White]

For some years prior to 1976 there were many pyrite specimens from the Rico mining district, Dolores County, Colorado, that were being sold as pyrite with a very thin blue-gray coating of bravoite, thought to be a nickel-rich pyrite but now no longer regarded as a legitimate species. It is not known who is responsible for assigning this name to the coating. In 1976, in a paper in the Mineralogical Record by R. A. Kosnar and H. W. Miller (vol. 7, pp. 278-307) it was stated that "emission spectrographic analysis indicated the presence of copper only" in the film and the authors guessed that it must be chalcocite.

Recently Jordi Fabre acquired a specimen that was labeled bravoite coating pyrite from the Colorado locality and he was suspicious, so he had the film analyzed. It was analyzed by energy dispersive spectroscopy coupled with scanning electron microscopy (SEM-EDS).
The results of the analysis showed only traces of zinc, so the film clearly is not nickeloan pyrite and not chalcocite. Probably, the film is simply due to an incipient surface alteration of the pyrite. The presence of traces of Zn may be due to the presence of sphalerite micro-inclusions.

Gracefully submitted in the interest of advancing the understanding of the diverse mineral world on the part of the uninformed.

[Jordi Fabre]

Two images of the Pyrite incorrectly labeled Bravoite.

Jordi

[Pete Modreski]

John, & the Forum, I'll just add,
I was involved in helping to examine these supposed bravoite-coated pyrite crystals from Rico when we did research for our book, Minerals of Colorado (1997). Analysis of the nature of the surface of the pyrite crystals was difficult because the gray, metallic surface film was so very thin.

My results, as we described them in Minerals of Colorado, were that the surface contained Mo in addition to trace amounts of Pb and Zn, and an X-ray diffraction pattern showed faint lines of molybdenite. Our conclusion was that the shiny gray metallic surface film was a very thin coating of molybdenite on the pyrite.

(One factor that makes the recognition of the molybdenum by SEM-EDS difficult is that the strongest X-ray emission lines of sulfur and Mo almost coincide, so that the presence of Mo is easily masked by the sulfur in the underlying pyrite.)

Sincerely, Pete

[Pete Modreski]

P.S. to Jordi,
The specimens of which you just posted photos, look to be almost all pyrite; any surface coating seems nearly imperceptible. Others that I have seen have had a much more pronounced metallic-gray surface coating.

[Jordi Fabre]

Great, more information!. On our analysis the Mo didn't appear, but maybe this is just on my specimen. It comes from Folch collection; he purchased it in Tucson probably around 1979, and, unfortunately, there is no dealer name on the former label.

Anyway, the most important thing is that after your and my analysis, the thin coating for sure is not Bravoite.

Jordi

[John Jaszczak]

Pete,
Were you able to get a Raman spectrum? Moly and sulfur have some overlapping peaks in EDS, but the Raman spectrum of molybdenite should be fairly distinctive.
Cheers,
John

[Pete Modreski]

Hi John, picking up on this thread after a long lapse! No, I didn't have ready access to any Raman equipment at the time I examined that specimen, so I just tried the XRD, using a powder camera. Haven't tried anything further since; the specimen on which I examined the coating was one from the DMNS collection.

[John Jaszczak]

I may not have had access to Raman then either like I do now. I think I may have a Gilman sample. If I do, I'll see if I can check it out.

[Pete Modreski]

P.S., John, we're going to be in Houghton, at Michigan Tech, TWICE next month, for two different events! First the MESTA field conference, and then the following weekend, for a Michigan State geology alumni field conference. So, I'll hope to see you there, one or the other of those times! Catch you later,
Pete

[Kevin Conroy]

I was just wondering if the molybdenum coatings were confirmed for the gray stuff on all of the pyrite from the Rico area mines. I'm having a little trouble finding references online so I figured I would ask.

[Mathias]

Hi Regarding Bravoite....
I visited the Liege Mineral show (Belgium) last Saturday and I bought a small Bravoite specimen from Peru (for €12).... It has a metallic appearance, looks like Skutterudite from Morocco from the outside, but you can clearly see (on the sides) that it is a layer on Pyrite. I'll post some photos later.
Mathias

[Mathias]

This is the "Bravoite" that I purchased last week in Liege, Belgium

One can see that it is a Pyrite specimen with a thin layer (0.5 mm) of what is supposed to be "Bravoite". I purchased it because I have special interest in sulpides and sulfosalts, and I'd never heard of Bravoite before. The vendor had 3 specimens available and I bought the one I liked most. They were all in the very, very affordable price range.