Native copper inclusions in charoite

Images made through a microscope. All subject types.

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Scarodactyl
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Native copper inclusions in charoite

Post by Scarodactyl »

I've recently been having some fun working on these challenging mineral inclusions, three-dimensional flakes of native copper floating within charoite.
Charoite is a lesser-known gem. It is a rare mineral with odd chemistry, and gem grade material is only found in one location in Russia. It is found as a rock composed of fibrous translucent crystals of charoite (which is purple to lavender), tinkasite (which is orange), aegirine/tschermakite (which is black to the eye, dark green to the microscope) blended in with feldspar and scattered with little bits of other stuff too.
Each piece is different and fascinating, and it was one of the first things I looked at with my first stereo microscope. It's fun to photograph just for its silky texture and mix of colors (click for bigger).
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But I kept seeing these little metallic specks of copper mixed in. They are mentioned in the geology literature as an indication of the weird, highly reducing conditions the rock must have undergone during formation, but they're rarely mentioned in the gem literature (I say 'rarely' but I mean 'I have never seen any mention but if someone cites an example I don't want my ignorance to be revealed). Part of that is probably because gem microscopy is almost always done with a stereo microscope, and these inclusions are small specks even at the high end of stereo magnifications. On top of that they're shiny, crinkly, and embedded in a medium which is far from transparent. Charoite's RI is about 1.55, basically the same as your typical coverslip glass, but the silkiness adds its own issues to the mix. As you try to chase them with higher magnifications the challenges add up quick.
Anyway, I was rooting around in my jewelry business's back stock to find something to photograph and came across my old bag of charoite tumbles, and figured it was time to have another go at this. Last time I tried was probably back before 2020, with much more limited equipment and much less experience.
Step 1 was to pour them out under my Leica Z6 with a ring light on and see if I could spot any copper. In this bag it seems like 80% of the stones have visible flakes of copper.
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Yup, found one!
Oblique lighting of these is challenging with my fiber optic dual illuminators. The reflective crinkles are very unforgiving and require a ton of diffusion. Coaxial illumination helps on some but sometimes gives a poor look. On many my tiny fiber optic ring light ended up being a pinch hitter, adding some nice top illumination without the same issues as coaxial (but some of its own).
Here's that same flake shot with my Mitutoyo 10x and heavily cropped (click for bigger):
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There's certainly more detail there but in spite of my best efforts at diffusion there are some hotspots.
For these inclusions immersion can be extremely helpful. Here's the same inclusion shot with my water immersion 40x (shooting at 44x, stitched from 4 stacks) solely with coax illumination.
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My 60x oil objective does significantly better, though this flake was just way too big to shoot with it so here's a different one I could fit into one stack:
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One interesting and very convenient phenomenon is that coaxial illumination gives much less flat results at higher NAs, likely thanks to the range of angles involved.
What I like about getting such a closeup view of these is the story they tell. You can see how they started as single grains of a copper-bearing mineral which then got the crap reduced out of it, setting the metallic copper free to fill cracks and push up against the fibrous grains of the surrounding charoite, giving it a detailed texture.
I'm still poking at these--It's probably time to prioritize picking out the prettiest and easiest to light ones I can find to get some more aesthetical images, but it's been a very fun exercise so far. Also, I don't recommend hand stacking with a 1.4 NA objective.

Incidentally, while doing all this I noticed this on the surface of some of these stones:
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In spite of them being beautiful examples of charoite some jerk tossed a bit of purple dye on them, likely during tumbling #-o Just a little bit and it should come off easily with some acetone, but come on!

Lou Jost
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Re: Native copper inclusions in charoite

Post by Lou Jost »

Beautiful results, especially with immersion. How much working distance did you have? Such a 3-d subject is challenging at NA=1.4! These look great.

Scarodactyl
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Re: Native copper inclusions in charoite

Post by Scarodactyl »

Thanks! There is not much working distance but enough for shallow inclusions, .37mm total with the oil objective (since there is no glass) and similar for the 40x water objective. I'd love to get a long working distance oil objective in spite of the logistical issues with keeping it immersed but they're generally rare niche objectives and priced ruinously.

J_Rogers
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Re: Native copper inclusions in charoite

Post by J_Rogers »

Very nice photos of inclusions! Was that last photo stitched from multiple stacks as well?

Scarodactyl
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Joined: Sat Apr 14, 2018 10:26 am

Re: Native copper inclusions in charoite

Post by Scarodactyl »

Thanks! The 60x shot was not stitched, just a much smaller copper inclusion. Even with decent overlap stitching the 40x image didnt go that well--it has some real corner shading in coaxial illumination. I'll have to tinker with that some more.

SW_Perspective
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Re: Native copper inclusions in charoite

Post by SW_Perspective »

Very nice results. I really enjoyed seeing the subject material. Thanks
David Moerman
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Olympus BH-2, BHTU
NFK relay lens, Splan PL objectives
Nikon D7000, D7200

Bob-O-Rama
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Re: Native copper inclusions in charoite

Post by Bob-O-Rama »

Hi,

That is a really interesting and nicely shot find that would be easy to overlook.

-- Bob

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