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 Post subject: chrome tourmaline
PostPosted: Mon Mar 14, 2011 6:06 pm 
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Some time ago, Frank let me spectralize his usambara tourmaline, and the spectrum is shown here. At the time I thought it was peculiar that there was no narrow chromium absorption line in the range 680-700 nm. But I had seen another usambara spectrum that didn't show any narrow absorption, so I didn't give it much more thought.

Then in this topic Barbra stated that her usambara tourmaline DOES show narrow absorption. She sent me samples to spectralize, and so here is the first spectrum I collected:
Attachment:
barbra usambara 1.JPG
barbra usambara 1.JPG [ 19.59 KiB | Viewed 5726 times ]

This piece definitely shows a narrow absorption in the expected range. Other than that, though, its spectrum looks pretty much like Frank's.

Over the next few weeks, I will spectralize some chrome tourmaline that Conny sent me. Maybe I'll find another piece colored by chromium, but doesn't show a distinct narrow line?


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 Post subject: Re: chrome tourmaline
PostPosted: Tue Mar 15, 2011 3:52 pm 
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So the point of the previous post was to verify that the spectrometer would produce a narrow absorption feature in the spectrum of an usambara tourmaline that Barbra previously identified using a spectroscope. The spectrometer graph shows a definite feature that we can attribute to chromium ion. Checking another one of Barbra's pieces with the spectrometer, and yes that piece's graph shows the absorption just as distinctly. But then what about Frank's tourmaline, which shows no evidence of a narrow absorption line in red.

Let's look at different chrome tourmalines sent to me by Conny. He reported being able to see narrow red absorption in the spectroscope view of some of his (much larger number of) samples and not being able to see it in others.

Here is the spectrum from one of Conny's chrome tourmalines...
Attachment:
conny good absorption.JPG
conny good absorption.JPG [ 18.72 KiB | Viewed 5695 times ]

Clearly, a narrow absorption feature occurs in the same area as in the spectrum of Barbra's tourmaline. For this sample, I could easily find a narrow absorption in the red end of the spectroscope.

But then here is the spectrum from another of Conny's chrome tourmalines...
Attachment:
conny bad absorption.JPG
conny bad absorption.JPG [ 18.32 KiB | Viewed 5695 times ]

Do you see a narrow absorption feature around 680 nm? Maybe if I squint and cross my eyes, I can claim there is something there. I looked at this sample through the spectroscope, and I couldn't find any narrow absorption in the red.

I collected spectra from four other pieces. One of these other pieces showed absorption as strongly as in the first graph. Another piece showed weak to no absorption like in the second graph. The other two pieces showed absorption features somewhere in between.

So back to Frank's tourmaline. It is definitely usambara, as the pictures in this post show. And its spectrum has no narrow absorption. So is it possible that Frank's tourmaline can contain chromium and not show the tell-tale narrow absorption in spectroscope or spectrometer? After collecting spectra from several samples of chrome tourmaline sent to me by Conny, I think the answer is yes.

Also looking back and forth between this post and the previous one, we can see the ratio of peak height in green (around 525 nm) to peak value at the end of red (700 nm) is what separates an usambara tourmaline from an ordinary chrome tourmaline. That ratio is about 1/8 for the usambara and 3/8 for the chrome tourmaline.


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 Post subject: Re: chrome tourmaline
PostPosted: Tue Mar 15, 2011 10:23 pm 
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Nice graphs. I am working with Jeff to get pictures of a series of absorption graphs for tourmaline that goes from "chrome" green threw yellow green to a gem that changes from golden to green and finally a brownish yellow tourmaline that all show the absorption peak at 600 nm, up on the Forum. My controller for the spectrometer is not connected to the internet so it will take a little effort. The width of the peaks and their ratio certain change and I think that they all must be Dravite/Uvite because I don't think that chrome/vanadium has been shown to be a chormophore in Elbaite.

Bruce


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 Post subject: Re: chrome tourmaline
PostPosted: Wed Mar 16, 2011 1:27 am 
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Thanks Bruce. You don't want to know what I do to produce the graphs.

brucetourm wrote:
... that all show the absorption peak at 600 nm, up on the Forum.


Do you mean the absorption at ~680 nm? Interesting that it might not be chromium in the cases you mention. This reminds me of something else I should try... shine a blue or violet laser on these pieces to see if they fluoresce red. A reaction would be a strong indicator of chromium.

Barbra, you have that 405 nm laser pointer... why not point it at your usambara and see if it fluoresces red (or orange).


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 Post subject: Re: chrome tourmaline
PostPosted: Wed Mar 16, 2011 7:38 am 
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Hi Brian,

I hope to post the graphs soon so a discussion can take place. I have found that 600nm is generally absorbed relatively little in tourmaline that is colored by iron, copper and manganese. It certainly does not produce an absorption peak. I suspect that the 600nm absorption is vanadium rather than chrome, but I want to check the graphs for the about 680 nm peak. I didn't before and the graphs are out being photographed for uploading now.

With the "chrome" tourmaline that I mentioned with the 600 absorption, both the about 525 nm and the 600 nm peaks are broad and have a ratio of close to one. You can produce a better analysis when you see the graph. I had it analysised at University of New Orleans with a probe. This is it's numbers.

SiO2, 36.895 wt. percent

TiO2, 1.01 wt. percent

Al2O3 33.841 wt. percent

Cr2O3 0.005 wt. percent

Bi2O3 0.007 wt. percent

V2O3 0.094 wt percent

FeO 0.07 wt. percent

MnO 0.011 wt. percent

MgO 10.165 wt. percent

CaO 1.566 wt. percent

ZnO. 0.015 wt. percent

Na2O 1.135 wt. percent

K2O 0.012 wt. percent

F 0.987 wt. percent

You do see a reddish color when viewed threw the filter.

Bruce


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 Post subject: Re: chrome tourmaline
PostPosted: Wed Mar 16, 2011 7:53 am 
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Brian wrote:
Barbra, you have that 405 nm laser pointer... why not point it at your usambara and see if it fluoresces red (or orange).


My Usambara tourmalines do fluoresce red (even if I'm not Barbra :) ).
(I've one of those laser too, I could not resist to own one... :D )


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 Post subject: Re: chrome tourmaline
PostPosted: Wed Mar 16, 2011 10:12 am 
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hi Bruce

I understand what you are saying now... you are talking about broad absorption around 600nm. Those graphs will be interesting to see. Especially of the "chrome-like" green tourmaline that contains only trace chromium.

I like your mention of the fact that Chelsea filter turns red in a species that definitely doesn't derive its color from chromium.

Edited to add some comments regarding vanadium. Unlike chromium, I've not seen any narrow spectral absorption lines in the two species I have that I'm pretty sure contain vanadium: a few tsavorite and a single tanzanite. I'd love to get hold of a vanadium emerald and look there.

hi Marco

I'm happy when anyone accepts the occasional experimental challenge I post. And I'm pleased the fluorescence reaction occurred with the laser.

So I had to test the usambara from Barbra and chrome tourmaline from Conny with my violet LED flashlight. Unfortunately I only saw a definite reaction in a couple of Conny's samples, but I wasn't in a dark room. So only weak reaction, but a weak reaction with normal light can become a strong reaction with laser light!

So I went back and illuminated those tourmaline with the violet line from my selectable argon-ion laser. Boom! ... in every sample you could see the path of the laser beam through the stone as a bright red line. I may have to go back and measure the fluorescence emission spectrum one of these days.

So that is the simple, direct test for the presence of chromium (and absence of iron) in tourmaline... red fluorescence under violet laser light.


Last edited by Brian on Wed Mar 16, 2011 2:51 pm, edited 1 time in total.

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 Post subject: Re: chrome tourmaline
PostPosted: Wed Mar 16, 2011 10:52 am 
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So only weak reaction, but a weak reaction with normal light can turn into a strong reaction with laser light!

That's so true! I also have a LWUV lamp and there are some samples that do not react (in a perceivable-by-me way at least). The violet laser sure gives a good kick to UV fluorescence reaction. :) I've a few pieces of Usambara and all react with a red color. I've also a large faceted one, where the Usambara effect seems less pronounced and the fluorescence is darker/weaker.

I use that laser on ~everything I have and it's a lot of fun. :mrgreen:
(I should get a pair of proper safety goggles, anyway, not only for safety, but also to better see the fluorescence colors...)


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 Post subject: Re: chrome tourmaline
PostPosted: Wed Mar 16, 2011 11:07 am 
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just tried on my sample and it doesn't fluoresce if exposed to the cheap UV led (guess around 400) , nor to the 365 common lamp.
ciao
alberto

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 Post subject: Re: chrome tourmaline
PostPosted: Wed Mar 16, 2011 1:30 pm 
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Alberto wrote:
just tried on my sample and it doesn't fluoresce if exposed to the cheap UV led (guess around 400) , nor to the 365 common lamp.


Yep Alberto, that seems to be the case. The violet laser causes distinctive fluorescence but the violet LED (and the LWUV lamp) isn't bright enough to cause a noticeable reaction.

I've seen this occur before. One day I noticed that spessartine emits a distinct yellow fluorescence (due to the manganese ion if I remember correctly) when illuminated with a violet laser. So then I illuminated it with a violet LED and I saw no reaction. Just as Marco says, the laser gives the fluorescence reaction a big kick.

So Marco, that's another stone to try out with the violet laser. Andy yes indeed, the safety glasses make the fluorescence much easier to see by removing the scattered and reflected laser light from your vision.


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 Post subject: Re: chrome tourmaline
PostPosted: Fri Mar 18, 2011 11:11 am 
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Brian wrote:
So Marco, that's another stone to try out with the violet laser. Andy yes indeed, the safety glasses make the fluorescence much easier to see by removing the scattered and reflected laser light from your vision.


Hmmm, I was looking for such a filter. I came out with some different options and I'd like an advice, since you sure have a good experience in the field. I'd like to get a sort of longpass filter that cuts UV laser light and all the violet shine... Here's the options I found:
1) common UV filters for photography: dirt cheap and easy to find (even if the transmittance spectra of some is quite worrying... basically some act just as common glass does), but they cut at too high energy, so they should be almost useless for my purpose, even the properly working ones. ;)
2) wratten "2A" filter: on the paper they should be almost fine, since they are cutting @ ~405 nm. My violet laser, if I remember correctly, is stated to emit 405+/-10 nm wl light. So, assuming it's correct (and a symmetric emission curve), I'd have half of the curve cut. Filters (glass) are available at ~14$. But I'd pass on this: Too much violet passing.
3) wratten "2E" filter: this one should cut at ~ 450 nm, that sounds good to me. But I can only find old gelatine filters (expensive too) and equivalent glass filters (coded as B+W021) starting @ 42$ and up. Still quite a bit of blue/some violet passing here, anyway.
4) wratten 4: this shows probably my favorite transmission spectrum, it cuts well the violet, but there's still some blue. It should be similar to 2E but from my info sources it's difficult to make a direct comparison. An old new gelatine filter (I'd prefer glass filters...) is available for ~8$ shipped. That's not bad at all, even if it is written these filters suffer some stability issues...
5) wratten 12 or B+W 022 or yellow #12: Cuts at ~500 nm, so we're out of the blue. :) That could be an good advantage, if looking for colors from green to red, but I fear to miss some of my nice fluorite glowing light blue... These glass filters seem popular for BW photography and some are available at a reasonable price. There's a low opening bid in UK now for an used one, and a tempting very discounted good one with "buy now" option, on ebay. I guess I'll get one of these anyway but...

...all in all: I cannot make my mind. :)


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 Post subject: Re: chrome tourmaline
PostPosted: Fri Mar 18, 2011 4:47 pm 
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Just any old diode laser is spectrally pretty narrow... maybe 10 nm wide from base to base of the peak. The peak does mode hop, but not beyond +/-5 nm range at the extreme. So any long-pass filter with cutoff at a wavelength longer than 420 nm should do. So options 3) - 5) all work.

Unfortunately these filters aren't really designed to stop laser light. The fraction of light transmitted through a filter is measured in units optical density (OD), and there is a logarithmic relation... OD = 1 transmits 10% of light, OD = 2 transmits 1% of light, OD = 3 transmits 0.1% of light, and so on.

Ordinary camera filters usually have OD = 2. In order to eliminate light from a laser with power in the tens of mA, safety glasses usually have an OD = 5 or greater. So you'd need to stack about three ordinary camera filters together to eliminate laser light. And thus, endure the three magnitudes of light reduction at other wavelengths.

Companies that sell lasers also sell safety glasses, but they are usually pretty expensive.


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 Post subject: Re: chrome tourmaline
PostPosted: Sun Mar 20, 2011 8:02 am 
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OK, thank you for the advice.


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 Post subject: Re: chrome tourmaline
PostPosted: Sun Mar 20, 2011 1:03 pm 
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Brian wrote:

Barbra, you have that 405 nm laser pointer... why not point it at your usambara and see if it fluoresces red (or orange).


Yes, I found my pointer and the material glows bright red.


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 Post subject: Re: chrome tourmaline
PostPosted: Mon Mar 21, 2011 8:52 am 
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The gelatine "wratten 4" filter is now flying to my mailbox... It had imho the best transmittance spectrum, it cost 8,44$ shipped :!: , and since it's a 2" square sheet I can cut it (in 2 or 4 pieces) in case I need a stack. :)
It should work very well for taking fluorescence pictures too. My only concern would be stability, but we'll see.

Anyway filters in lab equipment catalogs (thorlabs, edmund optics, etc.) look reasonably priced, compared to the ones in photography/astronomy catalogs (online sources for these)...

All in all, I found the cheapest solution. My requirements are not so strict after all. I can afford to be flexible. :mrgreen:


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