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Red material on copper?

Posted by Emile Osborn  
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Emile Osborn September 14, 2018 10:50PM
Hi. I picked up a piece of native copper (listed as from Michigan) online, that when magnified appears to have some sort of red material on the surface. It looks slightly transparent and glassy, but not as reflective as the copper. I was wondering if anyone might have an idea what this could be? It seems to show up on the copper in a variety of locations on the specimen, and does not look like it crosses over to the grey stone-like material (basalt matrix?). The material is not really obvious to the naked eye. I guess it is safe to assume that the copper has been cleaned and possibly varnished/lacquered, based on how bright or "new" it looks? The stone-like surface does look a bit glossy when magnified. There is a post here (https://www.mindat.org/mesg-19-32500.html) that mentioned copper could be cleaned with muriatic acid, copper brite, TarnX, and other materials. The copper does look uniform in color (not iridescent, as could possibly be caused by Tarn-x). I reached out to the seller to ask about the item and have not heard back from them. Could the red material be caused by some sort of reaction with a cleaning agent? In the thread above a person mentioned that copper could be dipped into a solution that would add a thin coating of cuprite on the surface (like a red film) that would slow down the tarnishing. Any ideas about what might be going on here? As far as other materials on the specimen, there are some small white crystals (calcite?) and possibly tiny green crystals (epidote?).

Photo 1: Entire specimen, with a box around general area where photos 2-5 were taken. The deeper red color is more prominent in this photograph than it is when viewing it with the naked eye. Perhaps this can be attributed to the narrower color gamut of a camera sensor+monitor, I don't know.
Photo 2: Stacked image showing the red material (about 3.75X). (There is an X at the same spot on photos 2-5 for general orientation).
Photo 3: Stacked image of the same area as photo 2, under different lighting (i.e., doesn't seem to be an optical effect).

Photos 4-9 continued below.

Thanks for your help!
Emile



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Emile Osborn September 14, 2018 10:52PM
Photo 4: Single frame from same region as photos 2-3, under a third lighting configuration.
Photo 5: View of the region through the camera viewfinder.
Photo 6: Calcite(?) crystals.



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Emile Osborn September 14, 2018 10:54PM
Photo 7: Green (epidote?) crystal. There was probably less diffusion on the lighting here, making the surface higher in contrast.
Photo 8-9: A few different shots of the copper and matrix.



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Scott Rider September 15, 2018 12:50AM
Emile, very good images and beautiful specimen!!

My guess is that it could be an alteration of copper into cuprite (https://www.mindat.org/min-1172.html). Sometimes cuprite can be very lustrous and gemmy. I think I see what you are seeing in the 2nd and 3rd images. But I don't see it in the others. Photo 7 is interesting, there is a botryoidal, rounded mineral there. I wouldn't venture a guess to the calcite or epidote. Others will probably ask you to test to verify those, but it looks like the specimen may be too small for that...

I can't tell you if it was cleaned or not, it does look like it was prepared in some way or another. However, I think it could be natural. You never know unless someone analyzed it, which I would not do, in my opinion it looks good the way it is currently. But copper oxidizes in air so the red areas are probably an oxide like cuprite.



Edited 1 time(s). Last edit at 09/15/2018 12:53AM by Scott Rider.
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Łukasz Kruszewski September 15, 2018 12:55AM
Cuprite
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Paul Brandes September 15, 2018 12:55AM
Welcome to Mindat, Emile!

I would agree that is a Michigan native copper specimen. The red material “coating” the specimen, to me, looks like a combination of different reflection angles because the copper is so well cleaned, and some odd optical illusions during the photo stacking process. I suppose a way to confirm this would be to use a needle under a microscope and see if any of the red can be scraped off. The white crystals in Photo 6 are indeed calcites that have been severely etched by acid. The gray matrix is basalt. The green crystals are most likely epidote. I also see a little blue on a couple of the shots that could be calumetite.
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Emile Osborn September 15, 2018 03:15AM
Excellent, thanks for the feedback guys! I'm at a rookie knowledge level here so this is all very helpful. So one idea is that assuming this stuff is indeed on the surface and not an optical illusion, the specimen could have been cleaned with acid (and possibly other materials) and then the bare copper reacted with the air to form these dendritic like cuprite forms? Is the oxidation process slow enough that the red material will continue to grow over time (years?) until the surface is covered, or it is possible that the copper has already been treated with something to seal the surface so these forms are kind of frozen in time?
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Doug Daniels September 15, 2018 05:54AM
All good questions. Answers not so good. The "stuff" is definitly on the surface of the specimen. The specimen may (most likely) or may not have been cleaned with acid, or something else (if there's no info to say, then....). The oxidation process kinda depends on the individual copper specimen, and if acid-cleaned, how well it was rinsed of the acid. Some do nothing for years, others, well.... Whether it's been sealed or not, yeh, don't know how to tell that. Another caveat - hopefully your body isn't like mine. Depending on your body chemistry, be careful handling the specimen. my sweat/oils etc tend to be a bit destructive to copper (and brass). So, be careful with it - nice specimen, anywho.



Edited 2 time(s). Last edit at 09/15/2018 05:57AM by Doug Daniels.
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Emile Osborn September 15, 2018 09:17PM
Ok, thanks for the info/recommendation Doug, good to know!
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