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 Post subject: Re: chrome tourmaline
PostPosted: Wed Apr 20, 2011 10:37 am 
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Ok, so Bruce sent me quite the selection of tourmalines. Included was the tourmaline whose makeup is known (we'll call it the "vanadium" tourmaline) and three others that came from the same lot. He also included an usambara tourmaline... first time I've seen a nicely faceted one... it is mostly dark green, but it has neat red flashes. He also sent some others varying from yellow-green to yellow to yellow-brown.

Anyways, so I set up the laser (mine is a 454nm laser, rather than 400nm laser pointer, but shouldn't make much difference). And then I stick the "vanadium" tourmaline in the beam. And it glows red! What the?!?!

Time to break out the spectrometer and collect the spectrum of fluorescence emission (called a luminescence spectrum, among other names). Here is a luminescence spectrum of one of Conny's chrome tourmaline samples.
Attachment:
conny fluor.JPG
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This spectrum extends into the infrared in order to show a more complete picture. Recall that the border between visible and infrared is 700nm.

Examining the spectrum, we see it is composed of a narrow peak at 684 nm and a wide structure that peaks somewhere between 700 - 750 nm. We can assign the narrow peak to fluorescence of the Cr3+ ion. From what I've seen in other gem material, the chromium ion may produce one or more narrow peaks... and the 684 nm peak matches the absorption dip seen in some transmission spectra. But the broad structure... I don't know what it is? Is it also associated with the chromium ion? I don't think so. Let's look at another spectrum to see why. This is a luminescence spectrum of Bruce's usambara tourmaline.
Attachment:
bruce usam fluor.JPG
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Here we see the narrow peak's height is much larger than that of the wide structure. If the wide structure was also due to chromium, I would expect the relative heights of these peaks to stay the same across different samples.

Now let's look at the luminescence spectrum of Bruce's "vanadium" tourmaline.
Attachment:
bruce vanad fluor.JPG
bruce vanad fluor.JPG [ 16.72 KiB | Viewed 4230 times ]

In this spectrum, we see that the narrow peak's height is much smaller than that of the wide structure. So the relative height of the narrow peak is much smaller here than in the previous two spectra. If (and this is a very big "if") the unknown wide structure was independent of sample, and if that structure is unrelated to chromium, then one could interpret this as a reduced amount of chromium ions in the sample. Recall that Bruce's "vanadium" tourmaline is not devoid of chromium, only that the amount of chromium is approaching trace amounts.

So it seems that there are a couple reasons why the "violet laser test" is a failure for distinguishing chrome tourmaline in the field. Firstly, it appears to be sensitive to nearly trace amounts of chromium. And secondly, the laser causes some additional mystery fluorescence. Another question to ask is "can this mystery fluorescence exist without any evidence at all of the narrow peak?" I've not seen an isolated mystery fluorescence in anything I've looked at so far... Bruce's vanadium tourmaline was the only instance where the narrow peak comes close to being extinguished. But in private communication with Bruce, he mentioned seeing one of his copper-bearing tourmalines luminesce red under electron bombardment in a scanning electron microscope. Also I mentioned that Bruce sent along some yellowish tourmalines. I checked those guys in front of the laser for fluorescence, and most don't show any. But I recall one or two showing a weak red fluorescence. So maybe I should go back and examine their luminescence spectra.


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 Post subject: Re: chrome tourmaline
PostPosted: Wed Apr 20, 2011 11:31 am 
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Thanks Brian for the work, it is much appreciated.

I just wanted to add what my criteria was for sending Brian the set of tourmalines that I did. All of them have a significant absorption peak at the 600nm part of the visible light range. I think that this is unusual in a tourmaline that is principally colored by manganese, iron, copper and titanium. In fact I don't think that any of the gemstones I sent Brian are Elbaite, since I have found very little support for chrome or vanadium being a chromophore in Elbaite, the most common species of tourmaline in gemstones. Also the cathodolumenscence has been limited to copper bearing Elbaite that displays the reverse Alexandrite color change from purple in natural light to blue to blue/green in incandescent light. I call the new species Laurellite to cut down on needing to reiterate the description each time I write about it.

Bruce


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 Post subject: Re: chrome tourmaline
PostPosted: Wed Apr 20, 2011 12:59 pm 
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bruce_tourm wrote:
All of them have a significant absorption peak at the 600nm part of the visible light range.


And so in transmission spectra like I show on the first page of this thread, they should all show a valley centered at 600 nm. ;)

I'm still a bit bummed that the laser isn't useful for field work. :( I wonder what that crazy additional fluorescence is?


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 Post subject: Re: chrome tourmaline
PostPosted: Fri Apr 22, 2011 2:42 pm 
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Pheew... At least there are now stones, filter, UV laser, camera and myself in the same place. :)
So, I played a little with my "Wratten 4" filter and I've to say it's really helpful in cutting the (Ultra-)Violet spectrum: Looking at a Wood's lamp with the filter I cannot tell if the lamp is on or off (in an environment with a very moderate lighting (I've to see what I'm doing :D)).
It seems it's able to cut almost all of the direct laser beam (I didn't try to point the beam directly to my eye to check, I hope you'll understand ;) ).

It is essential in checking the real color of moderate fluorescence reaction and even more indispensable when looking for weak fluorescence. In the latter case without the filter it's easy not to see the fluorescence at all.

For strongly-fluorescent materials there's no big difference as seen in this Vietnamese ruby.
Image

I got this crystal in a tourmaline parcel (from Africa per vendor), but I fear it's an apatite. Look how the filter makes the real fluorescence color appear. This is a better (almost decent) pic too, but keep in mind that I was shooting pics with a point&shot camera.
Image

The same goes for my jumbo usambara tourmaline (as you can see it was NOT faceted by me :D )
Image

This is a neat example of the case when the filter makes the difference between seeing and not seeing the color (this is a bad case also because the cotton glows with a whitish color)
Small unheated Montana sapphire (ID ~100% sure)
Image

Then I quickly checked many mineral samples and I got some nice info: I've green grossular glowing orange (and some not glowing at all), plagioclase (from both Mexico and Oregon) weakly glowing yellow-orange (almost impossible to see without filter), V-chrisoberyl orange-red, I've a "sunset" (or similar) tourmaline (pretty sure about the ID) glowing weakly dark red (and this was a surprise),... plus the many other minerals I've that fluoresce and the filter really improves my visual experience, as Brian said :) . I think this (and/or similar) filter is a "must have" for the photographer wanting to take good UV pics. Too bad they are relatively expensive for the casual user.


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 Post subject: Re: chrome tourmaline
PostPosted: Fri Apr 22, 2011 3:24 pm 
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Excellent demonstration, Marco.

maialetto wrote:
This is a neat example of the case when the filter makes the difference between seeing and not seeing the color (this is a bad case also because the cotton glows with a whitish color)

I've found that leaving a gemstone on white cotton can really mess up the fluorescence reaction. A while ago, there was a thread about a benitoite that appeared not to fluoresce under SWUV. The problem was it was sitting on white cotton. Taken off the cotton, apparently it fluoresced beautifully. I tried it out myself... benitoite on and off white cotton.

maialetto wrote:
I've green grossular glowing orange (and some not glowing at all)

With regard to the green grossular, have a look at this page. There is a link on that page to another thread where I spectralized the grossular fluorescence.

maialetto wrote:
plus the many other minerals I've that fluoresce and the filter really improves my visual experience, as Brian said :) .
:D


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 Post subject: Re: chrome tourmaline
PostPosted: Fri Apr 22, 2011 4:21 pm 
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Brian wrote:
I've found that leaving a gemstone on white cotton can really mess up the fluorescence reaction.

Paper is very bad too. That's also why I took most of the pics with the sample onto a black, matte, surface. :)

Brian wrote:
With regard to the green grossular, have a look at this page.

That precisely describes what I was looking at (and it's the answer to what I was looking for)

I spent a few minutes in the dark and I was enjoying my RED-glowing blue kyanite (and this was a surprise), bright, pure, strong reddish-orange hackmanite and tugtupite, super-strong green uranocircite and opal (with U-bearing stuff embedded), I've found another orange-brown tourmaline (100% sure ID) that weakly fluoresces (reddish-orange). Yellow topaz glows a very pretty orange color too (very similar to yellow apatite and yellow scapolite!).

Yup, 8.44$ well spent, after all. :idea: Even with the Wood's lamp looking at the fluorescence is more pleasant (and a bit less tiring for my eyes).


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 Post subject: Re: chrome tourmaline
PostPosted: Thu Jul 21, 2011 6:11 pm 
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Just got back from a great trip visiting people I met on the inter net. My last stop was with a person who is very interested in Usambala tourmaline. I showed her my small, (too small to show the color change) green emerald cut gemstones with the red flash that I sent Brian. She got out some of her pieces of rough and you could see it change color depending on its thickness. Then we ran our test.. Using fiber optics and stacking my little green tourmaline on top of the rough, I saw my Usambala fire for the first time. Bands of bright red confirmed it's hidden nature. This sight makes me smile about the complexity of color in tourmaline and that I was not sold the tourmaline as Usambala. Searching for something different in tourmaline can be such a a trip.

Bruce


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 Post subject: Re: chrome tourmaline
PostPosted: Thu Jul 21, 2011 9:43 pm 
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Hi bruce-tm & Brian
Very interesting data on tourmaline.
What sort of spectrometer are you using to generate the absorption graphs?
Also, in the analysis, what is Bi203 ?typo?
Cheers
Allano


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 Post subject: Re: chrome tourmaline
PostPosted: Fri Jul 22, 2011 7:11 am 
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The spectrometer I am using is made by StellarNet which has a web site that you can find for more information on their equipment. My model is called Black. You can get various confirgurations. Mine uses fiber optics and no moving parts.

Bi2O3 is my primative way of writting Bismuth Oxide on my limited keyboard. It is an example of how an element in an analysis is reported when it is in a high enough concentration to be more than a trace amount. Bi2O3 as I have stated here is reported in weight per cent rather than parts per million weight for elements in trace amounts.

Bruce


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 Post subject: Re: chrome tourmaline
PostPosted: Fri Jul 22, 2011 9:05 am 
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bruce_tourm wrote:
Bi2O3 is my primative way of writting Bismuth Oxide on my limited keyboard.


If you write Bi2O3 and then highlights the 2 and press the "button" named "sub" above. Do the same with the 3 and when published it will look like this: Bi2O3
Any nice soul has a "cheap" Spectrometer for a fellow feelin' the urge for one? ;)

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 Post subject: Re: chrome tourmaline
PostPosted: Fri Jul 22, 2011 3:58 pm 
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allanotaylor wrote:
What sort of spectrometer are you using to generate the absorption graphs?


I have three Ocean Optics USB4000 specs: one covering 200 - 850 nm range, another covering 350 - 1100 nm range, and a third that covers 400 - 700 nm range with resolution sufficient to clearly separate ruby chromium lines.

I prefer transmission graphs (upside down version of absorption graphs). So that is what I tend to post.


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