The solid solution series of Plagioclase Feldspar, which includes both of the varieties Laboradorite and Andesine has no fixed chemical composition but, rather, is of variable composition with Albite at one end of the scale and Anorthite at the other. Laboradorite is held to be that chemical composition that reasonably approximates to a 50/50 mix of Anorthite and Albite composition.
Laboradorite commonly looks nothing like Andesine. Where and when the name of 'Red Laboradorite' appeared I'm not sure but it would not surprise me to find that it was coined in the wake of the Andesine scandal in an attempt to differentiate branding. The reported RI ranges of Laboradorite and Andesine overlap in part. The reported (GT Pro ver 8.5) SG ranges do not overlap.
GT Pro also has a pic of a typical inclusion for Andesine (I can't say whether it's truly diagnostic) and here's a series of pics of the same inclusion type, all taken of the same stone, under different lighting conditions, and from my own little collection of study stones.
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Andesine 1-01b.JPG [ 66.82 KiB | Viewed 4908 times ]
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Andesine 1-10b.JPG [ 58.81 KiB | Viewed 4908 times ]
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Andesine 1-06b.JPG [ 65.91 KiB | Viewed 4908 times ]
Joined: Sat Nov 05, 2005 11:51 am Posts: 204 Location: France
Hello As I said on my website
Andesine is an intermediate member of the plagioclase series, close to Labradorite. The name depends on the ratio of Sodium and Calcium between Albite (Na(AlSi3O8) and Anorthite (Ca(Al2Si3O8).
Albite (Na 100%, Ca 0%) AlSi3O8 90-100% Ab; 0-10% An Oligoclase (Na 90%,Ca 10%) Al1-2Si3-2O8 70-90% Ab; 10-30% An Andesine (Na 70%, Ca 30%) Al1-2Si3-2O8 50-70% Ab; 30-50% An Labradorite (Na 30%, Ca 70%) Al1-2Si3-2O8 30-50% Ab; 70-50% An Bytownite (Na 10%, Ca 90%) Al1-2Si3-2O8 10-30% Ab; 70-90% An Anorthite (Na 0%, Ca 100%) Al2Si2O8 0-10% Ab; 90-100% An Other, rarer feldspars that we haven’t covered here do exist.
So, the chemical limit between andesine and labradorite is strict. The usual gemmological analyses are not enough to say if a stone is andesine or Labradorite.
When in 2002 I had the first red feldspar said coming from Congo, a few stones have been sent to Pr Fritsh in Nantes in France (published in gems and gemmology spring 2002) and to the CCIP the French laboratory in Paris). One more have been checked by AGTA and a lot by AIGS in Bangkok. They had all said Andesine, but near the boundary of Labradorite.
If you check an Oregon sunstone it is definitely a Labradorite.
So it was the first time that Andesine appeared on the gems market. At this time, of course, regarding the quantities, even if a red Labradorite and a red andesine looks the same, the andesine was rarest than a Labradorite. It is also why; some years after, when quantities of new feldspar came on the market they all was called andesine ore sometime "Labradorite-andesine" even if a lot was Labradorite. Saying "andesine-Labradorite" is not correct. We have to say only andesine or Labradorite, or if we do not know only red feldspar. But this limit between the andesine and the Labradorite is done my humans not by Nature. If we could have an other name (anderite ) for stones between 40 - 60% Albite, 60-40% Anorthite, we will no discuss any more on that point...
_________________ Dr Laurent SIKIRDJI http://www.gemfrance.com
Andesine is an intermediate member of the plagioclase series, close to Labradorite. The name depends on the ratio of Sodium and Calcium between Albite (Na(AlSi3O8) and Anorthite (Ca(Al2Si3O8).
Albite (Na 100%, Ca 0%) AlSi3O8 90-100% Ab; 0-10% An Oligoclase (Na 90%,Ca 10%) Al1-2Si3-2O8 70-90% Ab; 10-30% An Andesine (Na 70%, Ca 30%) Al1-2Si3-2O8 50-70% Ab; 30-50% An Labradorite (Na 30%, Ca 70%) Al1-2Si3-2O8 30-50% Ab; 70-50% An Bytownite (Na 10%, Ca 90%) Al1-2Si3-2O8 10-30% Ab; 70-90% An Anorthite (Na 0%, Ca 100%) Al2Si2O8 0-10% Ab; 90-100% An Other, rarer feldspars that we haven’t covered here do exist.
Thank you. That enlarges a little on what is given in 'Gems' 6th Edn.
Quote:
So, the chemical limit between andesine and labradorite is strict. The usual gemmological analyses are not enough to say if a stone is andesine or Labradorite. [
The IMA Mineral List (or my copy of) gives an identical formula for both Andesine and Labradorite. Would you be kind enough to give me the authority for the divisions you quote?
Both 'Gems' 6th Edn and the Gemology Tools database state that Andesine and Labradorite have different (but contiguous) SG ranges. However GEMSDAT reports differently. What is your view? Can the two varieties be differentiated by SG and, if not, for what reason, if their chemical composition is differentiated substantially?
I consider Dr. Rossman to be the final word in the discussion of plagioclase.
Thanks Barbra; that's a very interesting thread. Did Dr Rossman ever publish his presentation for distribution and from where might a copy be available? So many lessons can be drawn from this matter and I should very much like to have a copy for myself as reference material.
But back to the very basics of Andesine/Labradorite differentiation. Good secondary sources give an arbitrary division of the Plagioclase series into several varieties, according to fixed ranges of mixed Albite and Anorthite composition and it would follow that this is the essential differentiation beween Andesine and Labradorite. What is the authority for this arbitrary differentiation? IMA? Where can one find it published by the issuing authority?
Given this means of differentiation, it follows that Andesine can reliably be differentiated from Labradorite by SG alone in most cases. This is a simple inference to be drawn from fig 12.4.(a) in the current edition of 'Gems'. There is a clear inference from the same page too that Andesine can be differentiated from Labradorite by RI alone and with certainty up to +/- 2% of the arbitrary borderline of Ab50 - An50 composition. Sadly, 'Gems' does not cite the primary source for the described linkages of RI and SG to composition. Again, if anyone can direct me to the primary source(s), I should be grateful.
If the statements in secondary sources are borne out (as one surely expects that they will be), this would lead to some conclusions as follows: 1. In most cases, Andesine can be differentiated from Labradorite by use of classical gemmological instruments alone. 2. The range of uncertain Andesine/Labradorite differentiation by classical instruments ( for specimens of near-borderline composition) is determined. 3. Some results reported in major gem reference databases must be incorrect. In such cases one might find that the errors result from uncertainty as to the actual composition of the sample(s) on which the report(s) were based. 4. The foregoing would all seem to be true whether or not Andesine and Labradorite are red in colour.
This line of thought doesn't assist in consideration of possible diffusion treatments but there is some reporting of useful work with other stones (e.g. Emerald) in linking differences in SG to locality of origin. It would be interesting to know if this line of enquiry has been tested in respect to the places of origin of Plagioclase samples.
Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
Also, this month, the fall issue of G&G will be released culminating the decades of research Dr. George Rossman has devoted to gem plagioclase and finally clearing up the misinformation, rumors and fraud associated with this tortured and maligned calcium aluminum sodium silicate.
I did read the articles, and am totally confused. They don't seem to draw conclusive conclusions. But you said "you betcha."
I guess my question is: Is there red facet grade material mined in Tibet that is not treated?
Do the articles answer this?
There are reputations - careers even - to be broken here. Not to mention a class action law suit running and (who knows?) how many others lurking covertly in the wings. Thus, if you find knowledgable experts being a little careful in their public statements at this time.... well, what else would you expect? What is unexpected and therefore may be of note is, after some years of riding red Tibetan Andesine = diffusion-treated Mexican yellow Feldspar wave, ISG seems very silent at this time, though that wave was substantially of its creation.
I think it looks like the IGS is silent, because I believe they are not allowed on this board. Anyway, I find the whole topic rather amusing since I have no vested interest these stones at all. This is my somewhat forgetful experience with the topic.
About 9 years ago I was approached by a guy in Buffalo at a show, who claimed to be the US representative for Tibet Red Sunstone. He gave me his card, which I lost, and showed me a flyer with pictures of beautiful red faceted stones, finished jewelry etc. I asked him if he had facet rough I could buy, and was informed that only finished stones and jewelry are available.
Never thought about the stones again, until I started seeing them for sale on TV (JTV, GemsTV etc). Suddenly these guys were talking this stuff up and getting high prices for it. People seemed to be buying in like crazy. One customer of mine told me she "invested" $10,000 of her grand daughters college fund into these stones because they told her the prices were going to sky rocket.
I guess around the same time the AGTA had tested some and was issuing certs of this stuff as natural. I would look around in Tucson, and never see any rough, and very little cut, seemed only the TV shopping channels had this stuff.
Sometime later people started to question these stones, the ISG jumped in the mix claiming they are treated, and gets ridiculed by the "experts", JTV eventually admits they are treated... R. Hughes digs some up in Tibet under a bush that don't look like facet grade, and is not allowed to see the actual mine ... there is a scramble by people to try to save face... the "experts" publish findings that in the end don't seem to conclude anything. Still I have yet to hear of anyone actually having any facet quality rough, the TV gem shopping people seem to have totally dropped out.
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