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 Post subject: spudomene
PostPosted: Fri Jun 13, 2008 5:29 pm 
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Ok, so this discussion up in "colored stones" section interested me... how can you tell the difference between treated Kunzite and Hiddenite? Somewhere in that thread someone mentioned that the green of Hiddenite arises from chromium.

Recall that the chromium ion causes a distinctive absorption line in the visible spectrum of emerald around 680 nm. Somewhere in the "research" section, I posted visible spectra showing this feature. Recall also that the chromium ion causes a distinctive fluorescence peak in ruby somewhere around 694 nm. Again, I posted some spectra showing this feature.

Thus I reasoned: If chromium is causing the green color of hiddenite, then it should create some kind of feature in the hiddenite visible spectrum in the 680 - 694 nm range. And since the green kunzite presumably doesn't have chromium, then we'll see no feature in its spectrum in the 680 - 694 nm range.

Of course one might consider this too simple... surely everyone would already know this... but what the heck, we'll try anyways.

A reminder... when the curve seen in the transmission spectrum graph is vertically higher, it means a larger fraction of light with that wavelength is transmitted through the stone. When the curve is lower, more light is being absorbed. Sharp, narrow dips in the spectrum are "absorption lines".

First up... the donated collection in my dept. has lots of pink kunzite. I don't have any of the green treated stuff, but oh well. So here is the VIS-IR transmission spectrum of pink kunzite. (Click on the icon to call up a full scale picture)

Image

All in all, not a whole lot going on. Note however, how the transmission really takes off in the infrared beyond 900 nm. But most importantly, nothing at all happening in the 650 - 730 nm region, as can be seen in this close-up:

Image

Next up... the donated collection in my dept. has a very small sliver of hiddenite. It was acquired decades ago, it is green, and it has been left out in the sun for days and days (just to check) and it is still green. So here is the VIS-IR transmission spectrum of hiddenite.

Image

Looking closely at the red region of the spectrum and BANG! There is a definite absorption line at 690 nm. Here is a closeup of the region:

Image

I can't believe my simple reasoning actually worked!

Now, you say, all in all that is a pretty small feature at 690 nm. But I looked at the hiddenite through my prism spectroscope before I collected the spectrum, and I thought I saw a line... enough for me to take the time to set up and make these measurements. Of course now that I know an absorption line is there, it is easier to convince myself that I see it. Originally I was thinking maybe the absorption line stood out as much as the equivalent line in emerald. But I checked the emeralds again, and decided their lines were easier to see. Even so, if I were looking to see if green spudomene was hiddenite, I would definitely use the spectroscope.

But also, take a look at one other interesting feature in the hiddenite spectrum. In the infrared above 900 nm the signal nose-dives, unlike the kunzite where it takes off. The hiddenite is strongly absorbing 900-1000 nm light, whereas the kunzite strongly transmits that light.

This gives me another idea. Your bog-standard IR LEDs emit light with peak at 940 nm, and these are always matched to IR detectors. So you could easily make a LED emitter-detector setup... stick the stone in between, and if you get a reading from the detector then you know you don't have hiddenite. Of course I'd have to check a "treated" green piece of kunzite to see if it behaves the same as untreated kunzite in the 900-1000 nm region. But I bet it does... and bob's your uncle, there's another way to separate hiddenite from treated kunzite.


Last edited by Brian on Wed Apr 29, 2009 11:52 am, edited 1 time in total.

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PostPosted: Mon Jun 16, 2008 12:31 am 
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Ok now I've spoken about what is different between the hiddenite and kunzite spectra, how do I know that both are the same basic material? Well obviously... because their labels said so!

Actually though, I notice a couple bumps in the IR region of the kunzite spectrum, around 790 and 880 nm, that also seem to show up in the hiddenite spectrum. So the beryls show similar features in the IR spectrum, and the spudomenes show similar features there too. Interesting.

But hey, what causes that little absorption line in the hiddenite around 440 nm? Anyone?


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PostPosted: Mon Jun 16, 2008 6:32 am 
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Absorption around that area is usually due to the presence of iron...

Here's some more research on it with some slightly different results:

http://www.minsocam.org/ammin/AM66/AM66_118.pdf


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