July 21-28—ARDEN, NORTH CAROLINA: 16th Annual Western North Carolina Rockhound Roundup; Mountain Area Gem and Mineral Association; Camp Stephens, daily 8:30-5
Welcome to the GemologyOnline.com Forum
A non-profit Forum for the exchange of gemological ideas
It is currently Sun Jul 21, 2019 2:35 am

All times are UTC - 4 hours




Post new topic Reply to topic  [ 19 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 2:52 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1452
About a month ago now, Bruce Fry (Brucetourm) brought his tourmaline collection to my lab for a couple days. Not wanting to keep this opportunity to view a world-class gem collection to myself, I invited several people to come see it. Bruce was very gracious and accommodating and he made the viewings a rich experience for every attendee.

The nominal purpose of Bruce's visit was to verify spectra of some of his cuprian tourmalines. In particular, we examined the near-infrared (NIR) range from about 650 - 950 nm. These spectra don't contain color information. Instead, they can be used to identify the tourmaline chromophore, as described in this G&G article.

Here, I'll give a quick run-down of the identification described in this article. In the NIR region of a tourmaline's absorption spectrum, a broad peak appearing at wavelength just greater than 700 nm has been correlated to increased amount of iron in the stone, indicating that iron is the chromophore for that tourmaline. Similarly, in the NIR region of a tourmaline's absorption spectrum, a broad peak right at 700 nm and a (usually higher) broad peak at 900 nm has been correlated to increased amount of copper, indicating that copper is the chromophore for the tourmaline.

So, to begin with, Bruce shows me two yellow-green stones essentially matched in color. After looking over his collection, I see that it is no mean feat to match tone, hue, and saturation. But one of these stones has the je ne sais quoi. In a display of fifty tourmalines, that stone jumps out at you from the rest. Even under less than ideal lighting, this stone pops.

So we'll start with the ordinary yellow-green stone. Here is Bruce's photo of it.
Attachment:
yellow green noCu pic.JPG
yellow green noCu pic.JPG [ 27.79 KiB | Viewed 3572 times ]

And here is its NIR absorption spectrum.
Attachment:
yellow green noCu.JPG
yellow green noCu.JPG [ 17.02 KiB | Viewed 3572 times ]

In this spectrum we see a single broad absorption band peaking just beyond 700 nm. This identifies the chromophore as iron.


Last edited by Brian on Tue Sep 13, 2011 12:55 am, edited 5 times in total.

Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 2:55 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1452
And now we'll take a look at the je ne sais quoi yellow-green stone. Here is Bruce's picture of it.
Attachment:
yellow green Cu pic.JPG
yellow green Cu pic.JPG [ 30.5 KiB | Viewed 3570 times ]

We can see the similarity in color to the previous stone, but unfortunately the photo doesn't capture the "pop" exhibited by this stone.

Here is its NIR absorption.
Attachment:
yellow green Cu.JPG
yellow green Cu.JPG [ 17.25 KiB | Viewed 3570 times ]

In this spectrum, we see the first absorption band peaking just shy of 700 nm and the second absorption band rising up to peak at 900 nm. This identifies the chromophore as copper.


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 2:58 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1452
We set about looking at other cuprian stones. For example, Bruce has one stone that is the "windex blue" color exhibited by many cuprian stones. He didn't heat this stone to achieve the color, the rough just already had that color when he received it. A check of its NIR spectrum verifies the chromophore as copper.

Of course, we also investigated his purple tourmalines, which he calls Laurelite (sp?). These are the stones that, if heated, should produce the "windex blue" color people expect from cuprian stones. Here is Bruce's photograph of one of these stones.
Attachment:
purple Cu pic.JPG
purple Cu pic.JPG [ 23.83 KiB | Viewed 3569 times ]


And here is its NIR spectrum.
Attachment:
purple Cu.JPG
purple Cu.JPG [ 17.35 KiB | Viewed 3568 times ]

Again, clearly, the NIR spectra verifies the chromophore as copper.


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 3:10 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1452
Next we investigated a couple of red stones in his collection that seemed to have a special quality about them. Here is Bruce's photograph of one of them.
Attachment:
red Mn pic.JPG
red Mn pic.JPG [ 31.21 KiB | Viewed 3568 times ]

There is something very appealing about this stone. Bruce's spectrometer cuts off at 850 nm, so that he couldn't actually verify that the absorption band rising at higher wavelengths actually peaks at 900 nm. But the rising band is suggestive of copper. So using my spectrometer, we collected the following spectrum.
Attachment:
red question.JPG
red question.JPG [ 19.38 KiB | Viewed 3568 times ]

As the spectrum shows, the absorption band continues to rise past 900 nm! Now beyond 950 nm, the intensity of my light source is very weak. But after a bit of tweaking, I arrange to collect a useful signal out to 1000 nm, shown below.
Attachment:
red Mn.JPG
red Mn.JPG [ 20.82 KiB | Viewed 3568 times ]

As can be seen, not only does the absorption band peak somewhere beyond 900 nm, but there is also some narrow structure on top. The G&G article identifies this kind of spectrum, with an absorption band peaking beyond 900 nm and with some additional narrow peak structure on top, with a stone having an increased amount of manganese.

So two of Bruce's red stones show this manganese spectrum.

So all in all, fun stuff. Sometime soon I'll come back and add some details about another broad absorption peak and the chromophore(s) associated with it.


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 4:28 pm 
Offline
Site Admin
User avatar

Joined: Fri Apr 17, 2009 3:53 pm
Posts: 2048
Location: Sweden
Thanks Brian, this is most interesting.

_________________
_____________
Conny Forsberg


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 4:53 pm 
Offline
Valued Contributor

Joined: Wed Oct 22, 2008 5:00 am
Posts: 344
Location: Finland
Thanks Brian. It seems tourmaline alone makes me want to spec the next spectrometer up to 1000 or 1100nm. I'm interested to learn about data plotting method you use. Do you plot multiple points per wavelength to indicate variation or are you presenting subsequent points without line?

_________________
GemmoRaman GemmoFtir GemmoSphere


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 5:26 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1452
Thanks Conny. I haven't had much reason to explore the NIR, so Bruce's visit provided a fun opportunity.

mikko, I use a commercial plotting software called Origin, which is then converted to jpg in order to post here. I always plot individual points for experimental data, reserving lines or curves for theoretical fits and predictions. Putting 3000 data points into 300 pixels, basically every ten data points gets stacked on top of each other.

One could produce a "cleaner" looking spectrum by replacing each ten data points with a single averaged point (boxcar or binning or smoothing, as it is called). But I hate to throw away information. So although the stacked data points don't create a smooth curve, they do provide some idea of the statistical uncertainties in the measurement. And the occasional outlier that shows up here and there makes the measurement feel more "real" to me.


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 5:58 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 19720
Location: San Francisco
Fascinating.

One issue I'm curious about is what influence the quantity of an element plays in the coloration of tourmaline.

What is the elemental percentage NIR will detect?

It seems that analytical procedures (like LA-ICP-MS) to quantify Cu or Fe or Mn content would also be needed to determine positively the chromophore in tourm.

Tourmaline is typically an elemental kitchen sink, no?


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 6:33 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1452
Barbra Voltaire wrote:
Tourmaline is typically an elemental kitchen sink, no?


Heh, kitchen sink is an understatement.

The G&G article shows the use of a VIS-NIR spectrometer to separate cuprian tourmalines from their ferrous brethren. This is done by correlating LA-ICP-MS measurements of elemental content to absolute spectral absorption measurements. By absolute measurement, I am referring to the fraction listed along the vertical scale. The result, then, is that an absolute spectral measurement of these absorption bands should predict the same elemental content as the LA-ICP-MS measurement (within each measurement's uncertainties). So if the peak fraction in the absolute spectral measurement predicts 0.5 wt. % of CuO, the LA-ICP-MS measurement should produce a result somewhere in the neighborhood of that value.

In contrast, my spectral measurements are only relative... i.e., you can shift the vertical scale up or down to some degree. So when I see the twin peaks of copper absorption in my spectra, with shapes identical to those seen in absolute spectral measurements, I can only estimate that CuO content is somewhere between 0.1 and 1.0 wt. %. Enough to make it worthwhile to send to a lab for certification or whatever it is that they do.


Last edited by Brian on Mon Sep 12, 2011 9:28 pm, edited 3 times in total.

Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 6:41 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 19720
Location: San Francisco
Gotcha... :smt023


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 8:13 pm 
Offline
Gemology Online Veteran

Joined: Thu Sep 25, 2008 10:33 am
Posts: 832
Location: Mars PA
Thanks Brian for putting up the pictures, data and your insight.

I was warned by the author of the G and G article that manganese plus three can produce an absorption peak in the near infrared. I tried to limit my judgments of copper being in red/pink tourmaline to an indication of its presented by a significant 700nm curve and a strong change in slope of the farther from visible infrared absorption curve. As we found out a spectrometer that is limited to 849nm is not able to distinguish between the manganese +3 peak with an iron 700nm peak and copper. We also work with other red/pink tourmalines, one a gem I cut 45 years ago that turned out not to have copper and a couple others that turned out to have copper. The determinations depended on a more powerful sour of infrared than I have for my spectrometer and/or the ability to work with lower frequencies of infrared.

I of course have to find something interesting and provocative about the picture of red/pink tourmaline that is unfolding.

1, Why does only a few per cent, out of hundreds of red/pink tourmaline show the indications of the manganese +3 absorption curve in the infrared. (I have now looked at all the tourmaline that are suitable in my 1,000 plus collection). It certainly does not depend on tone levels or saturation or hue in my opinion.

2, Why can I pick out many of the red/pink tourmalines by eye as exceptionally bright, mirroring the different between the yellow green tourmalines that Brian and I investigated. (I still feel confident that I can distinguish other cuprian colors than red/pink from iron etc. since those colors do not contain a large amount of manganese plus 3).

At the broadest level, it is fascinating that absorption by either copper plus 2 or manganese plus 3 in the infrared can effect the brightness of tourmaline in the visible world even when the tourmalines do not have different hues.

I have been looking over my absorption curves and there is still fodder for adventures in tourmaline to come. Even with a more complex world than I expected, the spectrometer is still a very useful tool.

Bruce

A footnote: as the G and G article explain there are a number of limitations on quantitative measurements of copper in tourmaline with the spectrometer. I think its greatest use is to quickly screen out the vast majority of tourmaline from being worth testing for copper. I have also found rhodolite and neon blue apatite parading as tourmaline.

Bruce


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Mon Sep 12, 2011 10:18 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1452
bruce_tourm wrote:
Thanks Brian for putting up the pictures, data and your insight.

Happy to be some help.

bruce_tourm wrote:
I was warned by the author of the G and G article that manganese plus three can produce an absorption peak in the near infrared.

Recall that the assignment of Mn3+ to the absorption that peaks beyond 900 nm is only tentative. In the G&G article, that absorption band is only shown for one pink stone that happens to have a large wt. % of MnO. But then also, they do have some external evidence for the assignment.

Also from close reading of the article, it seems the 700 nm absorption appearing in that pink stone's spectrum is mislabeled in the figure. The graph assigns it to Fe2+, but the text assigns it to Mn3+. And checking the wt. % of FeO in that pink stone, it is very small compared to other ferrous examples.

Anyways, I think you'd agree that it would be interesting to see LA-ICP-MS results for your two special red stones.

bruce_tourm wrote:
Why does only a few per cent, out of hundreds of red/pink tourmaline show the indications of the manganese +3 absorption curve in the infrared. (I have now looked at all the tourmaline that are suitable in my 1,000 plus collection). It certainly does not depend on tone levels or saturation or hue in my opinion.

... I was thinking the same thing. Tourmalines exhibiting this absorption feature seem to be even more rare than the cuprian tourmalines.

bruce_tourm wrote:
At the broadest level, it is fascinating that absorption by either copper plus 2 or manganese plus 3 in the infrared can effect the brightness of tourmaline in the visible world even when the tourmalines do not have different hues.

It is fascinating; a mystery enhanced by its eye-catching nature and rarity of occurrence.


Last edited by Brian on Tue Sep 13, 2011 1:07 am, edited 1 time in total.

Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Tue Sep 13, 2011 12:12 am 
Offline
Gemology Online Veteran
User avatar

Joined: Thu Feb 22, 2007 10:23 am
Posts: 863
Location: GemClub
Thank you Brian, very useful stuff! some questions,
when you use UV-VIS-NIR do you focus the light beam on the culet area of the sample?

_________________
Farshid Roshanravan
http://diamondclub.us
Blog: http://roshanravan.tumblr.com/


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Tue Sep 13, 2011 12:46 am 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1452
roshanravan wrote:
when you use UV-VIS-NIR do you focus the light beam on the culet area of the sample?

Yes. more or less. The light from the fiber optic isn't focused (although this could be done), but it enters faceted gems from the culet side.


Top
 Profile  
Reply with quote  
 Post subject: Re: more tourmaline NIR
PostPosted: Tue Sep 13, 2011 7:43 am 
Offline
Gemology Online Veteran

Joined: Thu Sep 25, 2008 10:33 am
Posts: 832
Location: Mars PA
I certainly think that quantitative data for trace elements should be obtained for the red/pink tourmaline we have tested. I have worked very hard to obtain support for tourmaline research, mainly with Laurellite, but have had only limited success.

I do have some more tourmaline to toss into the pot to see if we can make a richer body of data. While working with Brian, I asked myself if I might have some red/pink tourmaline that would show the indications of the manganese +3 ion's absorption in the infrared, but not have the 700 nm absorption (therefor not be copper +2 based) curved assigned by the article in G and G to either iron +2 or manganese +3. Well I do have a few examples of such tourmaline. Now mangeses +3 in tourmaline seems to be a magical element that can produce a 720nm peak to meet your needs for pink tourmaline, a plus 900 nm absorption curve in the near infrared to confuse the identification of cuprian tourmaline with a 849nm limited spectrometer, but can it do all this while sometimes producing a 700nm absorption peak and at other times not producing a 700nm absorption peak? WOW!!!!!!! Not only do we have an element that likes to find a home in tourmaline, but it seems to like to get around in a disorderly fashion.

The point Brian brought out about the G and G article having only one pink tourmaline that was very rich in manganese, but very low in iron (still showing the 700nm absorption curve) included in the data base brings me to a problem I have with many early papers on cuprian/Paraiba tourmaline. The very limited number of samples tested. The compexity of tourmaline precludes easy answers about chromophores that are in tourmaline of very similar if not identical hues, let along sweaping conclusions drawn from a mear handful of gemstones, perhaps arbitralily determined to respresent the full range of possibilities presented by tourmaline. The wonderful world of tourmaline.

Bruce


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 19 posts ]  Go to page 1, 2  Next

All times are UTC - 4 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
Gemology Style ported to phpBB3 by Christian Bullock