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 Post subject: Raman/PL results: Spinels
PostPosted: Sun Sep 23, 2012 10:47 am 
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OK, decided with Albé we start to share our results with Raman/PL spectroscopy here.

Let's start with spinel. Identifying spinel can be really easy or extremely difficult. If natural spinel contains characteristic inclusions it is relatively easy to identify by careful microscope examination. However, if the stone is free of inclusions the job gets really tough. Most Verneuil synthetic spinels are easy to separate from natural by R.I. and other typical characteristics. Flux melt spinels are whole another story, so here Raman/PL spectrometer comes very handy.

Figure 1 shows the characteristic photoluminescence reaction of chromium in natural spinel crystal lattice. Strong emission peaks at red (starting from 630nm) are known by gemologists as 'organ pipes'. Almost all spinel colors contain sufficient amount of Cr3+ for showing this luminescence reaction. If one looks at the spectra more carefully it is clear that this reaction is very consistent in peak positions and their relative intensity for varying spinel colors. This spectrum is so easy to memorize that usually gemologist know it's natural spinel long before the Raman spectrometer has accomplished the library comparison.

Figure 1: PL (532nm) spectrum of Natural unheated spinel (#117 and #136 are shifted up for clarity)
Image

If well ordered spinel crystal lattice receives heat above 800°C it starts to disorder. Disordering shows up in the PL spectrum so that emission peaks start to broaden, merge and shift. In other words, the spectra in Fig1. proves both the natural origin and the fact the stone has not received high temperature heat treatment for improving it's color.

Verneuil synthetic spinel has been "heavily heat treated" during the flame fusion process, so the crystal lattice is never in very good order. While color variations and chromophores used in manufacturing of these synthetics are endless, they almost always have high enough chromium content for triggering the PL reaction. Disorder of the crystal lattice broadens the main peak (the so called zero phonon line) significantly and also shifts it's position. Minor peaks have been broadened and merged. Please note this kind of spectrum is NOT indicative for Verneuil method as it is possible to obtain these results from natural heated spinel as well. It is mandatory to continue testing with a refractometer and microscope for making a final decision. It is also worth noting that chrome bearing tourmaline has a very similar PL spectrum. It is a good idea to perform Raman fingerprint analysis to make sure one is looking at spinel instead of tourmaline.

Figure 2: PL (532nm) spectrum of synthetic spinel, flame fusion method (#142 and #127 and #129 are shifted up for clarity)
Image

Flux melt process temperatures create disorder in a spinel's crystal structure as well. These stones can be extremely difficult to distinguish from natural spinel by traditional methods, so here PL spectrometer provides a fast and easy solution. These two stones are red (Cr) and blue (Co, Cr) flux synthetic spinels manufactured in Russia.

Figure 3: PL (532nm) spectrum of synthetic spinel, flux melt method (#126 shifted up for clarity)
Image

If spinel does not contain enough chromium to make a separation, one can focus on Raman fingerprint peaks of spinel instead of PL reaction. These peaks broaden and shift as well as a consequence of heat in the crystal lattice. Additionally, excess amount of aluminum in Verneuil spinel introduces new peaks to the Raman spectrum. Distinguishing natural unheated inclusion free spinel from synthetic spinel is simply a matter of looking at the line width and position of the 405 cm-1 Raman peak.

Figure 4: Raman fingerprint (532nm) spectrum of natural unheated spinel vs. flame fusion synthetic spinel (#126 shifted up for clarity)
Image

More reading for those interested: Distinguishing Heated Spinels from Unheated Natural Spinels and from Synthetic Spinels

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 Post subject: Re: Raman/PL results
PostPosted: Sun Sep 23, 2012 11:36 am 
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Very interesting Mikko, thanks for the article! =D>

It's a great idea to prepare such articles with "case stories" for most important gemmological problems solved with GemmoRaman, maybe to publish in a special section of your site too!..

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 Post subject: Re: Raman/PL results
PostPosted: Mon Sep 24, 2012 4:49 pm 
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Thanks Egor. I wish we can eventually include them all in to the Gemology Project.

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 Post subject: Re: Raman/PL results
PostPosted: Mon Sep 24, 2012 4:54 pm 
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I was just going to suggest that. Cosmic coincidence. :wink:


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 Post subject: Re: Raman/PL results
PostPosted: Mon Sep 24, 2012 5:14 pm 
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Wiki editor on standby!

Good stuff Mikko!


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 Post subject: Re: Raman/PL results
PostPosted: Tue Sep 25, 2012 5:16 pm 
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mikko wrote:
OK, decided with Albé we start to share our results with Raman/PL spectroscopy here.


A very interesting story. Looking forward to the future storylines.


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 Post subject: Re: Raman/PL results
PostPosted: Thu Sep 27, 2012 3:39 pm 
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It'd be just great to have this and similar articles on the GP. :)


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 Post subject: Re: Raman/PL results
PostPosted: Thu Sep 27, 2012 5:32 pm 
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Nice work Mikko,
and thanks to you and Albé for sharing this material with us.
This article is of particular interest to me as identifying a Spinel ( which has yet to be accomplished.) started my journey of learning Gemology.

Look forward to more =D>


Regards
Harold


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