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NIR absorbance spectra of tourmaline
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Author:  mikko [ Fri Oct 28, 2011 12:21 pm ]
Post subject:  Re: NIR absorbance spectra of tourmaline

Congrats for beating the nasty IR stray Albe =D>

Yes, Cu2+@ 900nm is obvious and if one looks careful enough the ~700nm Fe2+ / Cu2+ is also detectable as a shoulder. Small peak at ~410nm is possible sign of Mn2+

Author:  Alberto [ Fri Oct 28, 2011 12:29 pm ]
Post subject:  Re: NIR absorbance spectra of tourmaline

mikko wrote:
Congrats for beating the nasty IR stray Albe =D>


thx mate.

Quote:
Yes, Cu2+@ 900nm is obvious and if one looks careful enough the ~700nm Fe2+ / Cu2+ is also detectable as a shoulder. Small peak at ~410nm is possible sign of Mn2+


yup, checked the stone in different positions, this one shows more distinctively the 900 feature, others displayed a little better the 700 shoulder....

ciao
albé

Author:  bruce_tourm [ Fri Oct 28, 2011 2:03 pm ]
Post subject:  Re: NIR absorbance spectra of tourmaline

I believe that the curve indicates copper in a blue/green tourmaline. I just looked at an absorption graph done by a Swiss Laboratory on my best blue/green cuprian tourmaline (the gemstone has been tested by LA-ICP-MS and shown to be copper bearing) and one of the axis (a/b)shows a similar curve) while the c axis has a higher peak at 700 nm and the 900 nm peak is increased some, but not as much as the 700 nm peak (ratios would be different between the 700nm and 900 nm depending on axis). If the gemstone was purple or red/pink I would have some reservations about the gemstone definitely being cuprian because of the Mn+3 absorption around 900nm. It would be interesting if you could get absorption curves for the c axis and a/b axis to see the differences in them. All my cuprians that have been tested by outside laboratories have shown different curves even when I can not see any difference by eye. I have not made any effort to change axis with my set up. My best blue/green cuprian is not dichroic by eye.

Keep up the good work and good luck with tourmaline.

Bruce

Author:  Brian [ Mon Oct 31, 2011 3:02 pm ]
Post subject:  Re: NIR absorbance spectra of tourmaline

mikko wrote:
Congrats for beating the nasty IR stray Albe =D>

I too think it must be a bit of stray light getting into the spectrometer that had not first passed through the stone. Such stray light definitely causes anomalous structure to appear in a spectrum.

Another indication that stray light not passing through the gem is the cause of the problem can be seen in the 900-1000 nm region of the two spectra. From my recent experience, incandescent and halogen lamps produce very low intensity beyond 900 nm, so that the spectral data should jump around quite a bit in this region. We can see this is exactly what happens in Alberto's second graph. But in his first graph, the data appears as smooth in the 900-1000 nm region as it appears in the 400-900 nm region. Just another reason not to use data-smoothing routines like boxcars. ;) Good job Alberto.

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