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 Post subject: Confusing Chromophores Can Confuse Gemmos
PostPosted: Mon Apr 07, 2014 10:48 pm 
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Dr. George Rossman discussed chromophores at the recent Sinkankas Seminar. Although his emphasis was on peridot specifically, he pointed out that there is a presumption among many of us that a vibrant green always indicates the presence of chromium as a chromophore. This incorrect presumption may be reinforced by our Chelsea Filters, as vibrant green gems colored by other means can often appear red when viewed with a CF.

Even the most electric Burmese and Pakistani peridot is colored solely by iron. The only chromium present within these gems is found in chromite inclusions. But the chromium is not incorporated into the peridot itself.
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Indeed, chromium is responsible for the bright green seen in the following gems:
Emerald
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Demantoid
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Green Zoisite
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Tremolite
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Green Spodumene but there is more to the green spodumene story:
Scarodactyl wrote:
There are two distinct varieties of (pure) green spodumene:
1) irradiated green spodumene, which is caused by some sort of fancy charge transfer between Fe and Mg. This is universally unstable (usually very unstable, sometimes even in the dark) and shows strong blue-green pleochroism. As colorless spodumene is irradiated it apparently first goes pink and then green at a higher dose, and often the fading does that in reverse. That said, a couple I've tried have skipped the pink in between step while fading.
2) chromian green spodumene (hiddenite) whose color is caused by chromium (and possibly vanadium as well) which is completely photostable. It is inert to UV and also does not respond to radiation as other spodumenes do. I've had some chromian/vanadian Afghan material stress-tested for 2 weeks under a halogen lamp and it didn't fade at all. Similarly, North Carolina hiddenite does not fade.

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Jadeite
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But vanadium is responsible for bright green varieties of
Tsavorite
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DIOPSIDE:
Update directly from Dr. Rossman.
Dr. George Rossman wrote:
Much of the chrome diopside (e.g. that from Russia) has chromium as the chromophore. However, the material from east Africa (Tanzania, Kenya) often has vanadium as the dominant chromophore. The color differences are subtle at best.
I stand partially corrected!
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Tremolitie
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Titanite
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Nickel is responsible for the green in chrysoprase.
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Radiation* causes the green color in
Zircon (contains uranium and thorium)
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Ekanite
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Diamond
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Topaz
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Quartz var. Prasiolite
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*The radioactive decay of potassium (40K ) in the Earth's mantle ranks third, after Thorium-232(232Th) and Uranium-238 (238U), as the source of radiogenic heat)

[b]Lead cause the green variety of

Amazonite (as well as blue amazonite)[/b]
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Rare Earth Elements cause green in
Apatite (specifically praseodymium)
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Fluorite (specifically samarium)
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 Post subject: Re: Confusing Chromophores and Confused Gemmos
PostPosted: Mon Apr 07, 2014 11:34 pm 
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I think I remember reading that peridot with particularly fine color tends to have higher levels of nickel. I don't know how old that info was though.
As a side note, that green spodumene looks like the standard radiation-colored material rather than a true chromian spodumene. Can't say for sure of course, but the cut and color are indicative. Even with its fadeyness, green spodumene would definitely be an excellent example of radiation-induced green with its lovely (albeit temporary) hue.
I know that green diopside can be colored by vanadium (as in..."Tashmarine" {ugh}), but is it also the active ingredient in chrome diopside? Is that also the case with "chrome" titanite?
Another good example would be "chrome" tourmaline, which is actually uvite colored green by vanadium.

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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 08, 2014 12:02 am 
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Quote:
I think I remember reading that peridot with particularly fine color tends to have higher levels of nickel.

No one at the Symposium ever mentioned nickel as a component of peridot.


There is no chrome diopside nor chrome titanite.
According to Dr. Rossman*, it is a common misconception.

*And he is never wrong. :wink:

Green spodumene is colored by Cr3+ replacing Mg. It is not stable. True one can bring back the green, temporarily by irradiating it. Question...is the change green to pink the result of a valence change?

These pics are something I just dug up through my files. They were not part of Sinkankas. It makes a nice presentation. :D


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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 08, 2014 12:21 am 
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There was a time when it was thought that irradiated green spodumene and true hiddenite/chromian spodumene were colored by the same mechanism, but it has been shown that they different. There are two distinct varieties of (pure) green spodumene:
1) irradiated green spodumene, which is caused by some sort of fancy charge transfer between Fe and Mg. This is universally unstable (usually very unstable, sometimes even in the dark) and shows strong blue-green pleochroism. As colorless spodumene is irradiated it apparently first goes pink and then green at a higher dose, and often the fading does that in reverse. That said, a couple I've tried have skipped the pink in between step while fading.
2) chromian green spodumene (hiddenite) whose color is caused by chromium (and possibly vanadium as well) which is completely photostable. It is inert to UV and also does not respond to radiation as other spodumenes do. I've had some chromian/vanadian Afghan material stress-tested for 2 weeks under a halogen lamp and it didn't fade at all. Similarly, North Carolina hiddenite does not fade.

Note that there's also greenish-yellow or yellowish-green spodumene which seems to be colored by iron. I've left some of it in the sun for a week or so and seen no fading, but I don't know how it reacts long-term. But that stuff is murk-green at best.

I can cite a stack of references on that, but it's been pretty thoroughly covered in other threads (and I should be working on my honors thesis :x )

Re: chrome diopside. There's no question that diopside can incorporate chromium into its structure. It's interesting that that doesn't affect the color where vanadium does.

I'm not surprised about the nickel. Nickel certainly can be a minor component in olivine, but the reference to it improving color was from an old book I read in our geology library so it's likely just an outdated hypothesis.

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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 08, 2014 11:38 am 
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re: Chrome diopside.
Trace amounts of chromium show up in LIBS analysis of diopside because of the presence of chromite inclusions.

According to Dr. Rossman, the Cr itself does not incorporate itself into the diopside structure in a way which would influence the color.

Analogy.
A LIBS analysis of sapphire with apatite and zircon inclusions will show trace amounts of Zr, Ca, P, F, Cl etc. which are not incorporated into the actual structure of the corundum.


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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 08, 2014 1:21 pm 
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I can certainly believe that vanadium is the active coloring agent. I have trouble, however, believing that all Cr in diopside is in the form of chromite inclusions.
Diopside is a clinopyroxene, and isostructural with kosmochlor, so it would make sense for chromium to fit in just as V does. And indeed, from a quick search in the literature it looks like diopside and kosmochlor's solid solution is well known and characterized. It's a much happier series at higher pressures but it certainly exists. I mean, heck, Cr and V occupy the same site in the V-endmember pyroxene natalyite (Na(V.75,Cr.25)Si2O6), so we can also be pretty sure they are at least somewhat happy to sit in the same sites.

I know Dr. Rossman is utterly brilliant and a huge authority in gemology and mineralogy and it's unlikely that he's wrong, but I am guessing that what he said had some extra qualifier on it.

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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 08, 2014 2:54 pm 
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I agree with you. I was raised believing in the reality of chrome diopside.
I'll send Dr. Rossman a note to see if I can find some clarification. :|


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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 08, 2014 3:47 pm 
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So it sounds like we all need to say "green diopside" instead of "chrome diopside". I didn't know that until now.

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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 08, 2014 3:58 pm 
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Weeeeeell, even if it's all colored by V, "chrome diopside" is a trade name at this point (which distinguishes the material from common, more murky green diopside colored by iron), and its appropriate use still helps the customer know what they're buying. So I would hesitate to toss it even if the chemistry it implies isn't accurate.

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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Mon Apr 14, 2014 8:39 pm 
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Update directly from Dr. Rossman.
Dr. George Rossman wrote:
Much of the chrome diopside (e.g. that from Russia) has chromium as the chromophore. However, the material from east Africa (Tanzania, Kenya) often has vanadium as the dominant chromophore. The color differences are subtle at best.
I stand partially corrected!
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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 15, 2014 3:00 pm 
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Aha. Now that makes sense to me. Based on their charge and atomic radii, I would suspect that Cr and V can exchange happily in almost any mineral (similar to Fe and Mg).
(That said, I still think the first post would be improved by fixing that spodumene issue. I know that true chromium spodumene isn't a major gem outside of collecting circles, but I'm from NC, and we are loyal towards our native Hiddenite :D )

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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 15, 2014 9:05 pm 
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What do you think of my update?
Personal question coming.....is Scarodactyl your actual name?


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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Tue Apr 15, 2014 11:54 pm 
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First: I appreciate that. I have a lot of fun with the irradiated stones when I can get them really cheap, but there's nothing like a real chromian spodumene. There's an excellent research paper with some cool experimental details that I read a while ago and will track down again when I get a chance, but the basics are covered in Colorimetric Investigation of Stable and Unstable Spodumene Colors, available as part of these proceedings: http://www.igc2011.org/#/abstract-proce ... 4540447685

Hehe, no it is not, though it's a handle I've used a few places. My real name is Stephen Challener.

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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Wed Apr 16, 2014 12:07 am 
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Pleased to meet you, Stephen.


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 Post subject: Re: Confusing Chromophores Can Confuse Gemmos
PostPosted: Wed Apr 16, 2014 1:08 pm 
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Steve, keep up the good work. :P


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