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Zircon or Spinel?

Posted by Nick Gilly  
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Nick Gilly April 05, 2018 06:36PM
Hi again.

I bought the following specimen on eBay at the weekend, sold as a zircon, and it arrived today. It looks pinker in person than on the photos, particularly in daylight, and some parts with a slight orangey tint show up more orangey under incandescent light. The seller says it's from Nkasi District, Rukwa Region in Tanzania. Here's some pics from the auction:













It does have a few gemmy portions, and a nice glassy lustre. I'm not sure if the irregular form is due to parallel growth or etching.

To me it looks like a pinkish-red spinel, probably from Mahenge if it is from Tanzania. The weight is 58 grams. A pity I haven't got a UV light source. It looks like it should fluoresce, although the seller says it doesn't, which is possible if there is appreciable iron in there.

So, am I right? Is this a spinel? It's a lovely specimen whatever it is!

Thanks and regards,
Nick



Edited 1 time(s). Last edit at 04/05/2018 06:38PM by Nick Gilly.
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John Montgomery April 05, 2018 07:10PM
This is my spinel from Tanzania..
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Scott Rider April 05, 2018 08:00PM
It looks a lot like some spinel I've obtained that were from Burma. I couldn't tell you the locality is correct or not, but it does appear to be spinel. Especially with the compound octahedral shape that is made up of smaller octahedrons. One of the Burmese specimens I own has a very similar composition, I mean REALLY REALLY similar looking. But we all know how reliable visual IDs are...

And, I'm sure some others would like you to do some testing, hardness for sure, specific gravity, etc. so they can give you a much better opinion. My eye-ball ID says its spinel, but again, not too scientific of an observation...
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Nick Gilly April 05, 2018 08:46PM
Thanks guys.

It does scratch quartz, relatively easily too, so I'd estimate a hardness of 8. This would fit with the spinel ID. With zircon I have to press quite a lot harder to make a mark.

I'll have a look at it under the bug killer light at work. Any fluoresence should be fairly obvious I would have thought.



Edited 1 time(s). Last edit at 04/05/2018 08:47PM by Nick Gilly.
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Nick Gilly April 05, 2018 09:03PM
This spinel crystal is a deeper, redder colour, but otherwise very similar, with the phlogopite mica association:



It's also from Mahenge.



Edited 1 time(s). Last edit at 04/05/2018 09:04PM by Nick Gilly.
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Ian Nicastro April 05, 2018 09:32PM
Absolutely Spinel, very likely from Mahenge, Tanzania.
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Nick Gilly April 06, 2018 04:39PM
Thanks Ian. Yes my thoughts exactly. I did look at it under the bug-killer light at work but there was no obvious fluorescence. Bear in mind though that this is in a room with plenty of daylight and the lamp also puts out quite a lot of visible light, so weak fluorescence may have been masked.

John Montgomery Wrote:
-------------------------------------------------------
> This is my spinel from Tanzania..
>

John, that is a superb specimen! It also seems to have some other mineral adhering to it like mine, although mine is more orange/rust-coloured, which probably indicates iron oxides. This is apart from the phlogopite mica. One part of the specimen has a small inclusion of a unknown black mineral. Does yours fluoresce?

Actually, I was wondering if Owen Lewis would post his thoughts on this thread too, but I haven't seen any posts from him for a couple of weeks. I hope he's OK.



Edited 1 time(s). Last edit at 04/06/2018 08:40PM by Nick Gilly.
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Doug Daniels April 07, 2018 02:43AM
Note that "bug killer" lamps are more likely to give out long wave UV. You need short wave UV to get fluorescence in zircons. And, even if those lamps give off short wave UV, there may be too much visible light to see any fluorescence.



Edited 2 time(s). Last edit at 04/07/2018 02:47AM by Doug Daniels.
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Nick Gilly April 07, 2018 03:38PM
Some pics I have taken of this specimen, which show the pinker colour in daylight, and probably more informative than the auction pics:















- shows black inclusion













Edited 1 time(s). Last edit at 04/07/2018 06:21PM by Nick Gilly.
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Nick Gilly April 08, 2018 12:56PM
The guys at the FMF forum concur too - it is spinel.

I found another spinel that sold recently on eBay that is listed as being from Tanzania:



That has exactly the same association as my specimen: phlogopite mica, and patches of a rusty-looking substance clinging to parts of it. So assuming the country of origin is correct, almost certainly from Mahenge.
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Nick Gilly April 12, 2018 04:58PM
Update: I tried a ruby sample, which I know from previous testing to be fluorescent, with the bug-killer light at work and nothing was seen with this either, so this is obviously not a viable test.

I'll have to get a new lamp, preferably one that does LW and SW UV.
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Alfredo Petrov April 13, 2018 12:40AM
Nick, many red spinels, and rubies, (and some secondary uranium minerals like autunite for that matter too) will fluoresce in ordinary blue light too; no need for ultraviolet. An ancient experiment: darken your room by covering the window, make a small hole for the sunlight to come in, put a round jar of copper sulphate solution in front of the hole; now you have a blue light source. Put your crystal at the focal spot of the jar, where the blue light is concentrated; if it glows red, it's fluorescing, as there wasn't any red light in the room. Primitive, but it works.
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Nick Gilly April 13, 2018 08:07AM
Thanks Alfredo. The pink is livelier in daylight, so maybe there is a bit of fluorescence going on.

I have some money left from a long service award at work and I think I'll buy a UV lamp with it.
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Donald B Peck April 13, 2018 04:29PM
Alfredo, I wonder if any UV will come through the blue copper sulfate in the jar?

Don
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Owen Lewis April 13, 2018 09:00PM
Hi Nick & all,

Sorry to have been quiet for a while. Been struggling a bit but the last couple of days have been easier and interest in matters more remote than my own anatomy is returning.

If you and Hannah would like to bring your zircon/spinel over on Sunday afternoon we can run a few tests and chat better informed with the results in hand. Thoughts for the moment are as follows:
- Especially from your own photos, I'd guess that your purchase is spinel rather than zircon.
- I have a 365/256 nm UV lightbox we can take a look at it in. It's always interesting but IMHO, UV fluorescence is generally supportive evidence of ID but is frequently short of diagnostic in the cases of zircon, spinel and many other gem crystal. We can play around with some other stuff here under UV too, if you are interested.
- SG. For obvious reasons, an SG test of your complete specimen is not likely to produce a trustworthy result.However, if you are up for it, breaking off a couple of small pieces of the pink crystal should give samples from which reliable results can be obtained. Samples in the range 0.5 - 0.1 ct wt will actually be more reliable (given the lack of crystal transparency) than anything larger is likely to be (given a first-class scale and density determination kit.. Two samples is enough, if the results agree closely. A third sample and averaging the results of all three determinations being necessary if there is appreciable 'spread' in the first two results.
- Hardness. I use the excellent MineralLabs Mohs 2 - 9 styli which (with just a little luck) should sort out zircon from spinel unambiguously..
- Absorption spectra. Worth a peek. The spectra of both a zircon and a pink spinell are usually quite striking and definitive.
- From the translucency of the crystal its likely that testing for anisotropy/isotropy is likely to fail - but always worth a shot.

Mail me if a p.m. Sunday meet is good for you; if not good, suggest something that will work better for you.
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Nick Gilly April 13, 2018 09:44PM
Thanks very much Owen. I have emailed you about this.

I'm 99.99% certain that it is a spinel but it would be good to get a confirmation.
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Alfredo Petrov April 15, 2018 12:40PM
Don, I'm not sure whether any UV light gets through a copper sulphate solution, or through the glass jar. SW UV can't go through common glass, but then SW isn't significantly present in sunlight at Earth's surface anyway, so the possibility of its transmission through Cu sulphate solution would be irrelevant to the experiment. As for LW I'm not sure - It would be interesting to check it with a spectrometer, although I imagine such data is already avaiable somewhere.
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Donald B Peck April 15, 2018 04:08PM
Alfredo, I forgot to consider the glass in the jar. But the LW would still be a possibility, I think. Just curious!

Don
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Fred E. Davis April 15, 2018 05:18PM
Absorbance (inverse of transmission) of copper sulfate:



Transmission of soda-lime glass (common glass) [blue line]:



Transmission of borosilicate glass (Pyrex glass):



From this information, it would appear that copper sulfate transmits UV pretty well. Borosilicate glass passes LW UV better than soda-lime glass, but both block SW UV.
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Douglas Schonewald April 15, 2018 05:54PM
I wonder if plastics block UV? a clear plastic bottle of copper sulfate might work very well to shoot UV to a specimen.
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Owen Lewis April 15, 2018 09:06PM
Doug,
YMMV but soda glass and some plastics are the common materials for making eye-protecting UV filters. I guess not too many gathered at this watering hole bother to use eye-protection from UV?
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Alfredo Petrov April 16, 2018 12:02AM
Fred, is that the transmission of copper sulphate solid (crystalline) or in solution? (or is there even a difference?) And what's the transmissability of the water? (I think LW does transmit through water, but not SW, if I remember rightly?)
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Fred E. Davis April 16, 2018 02:08PM
Hi Alfredo,

I believe the plot is for a copper sulfate solution (but I don't have hard evidence). Water appears to transmit UV pretty well, with attenuation the greatest at the orange & red end of the spectrum (making things look green under water).

Fred
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