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 Post subject: A strange Feldspar
PostPosted: Fri Sep 04, 2015 6:21 pm 
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Hello everybody,
I am new to this forum. I would like to introduce myself with a stone, which is really nothing special and yet very, very rare. In itself a nice guesswork. But never mind that. It is an light green Amazonite from Vietnam, 5.55 ct and fully crystallized, very small inclusions and absolutely not for sale. 8)

Unfortunately the picture is not so successful. It's a little too dark. Maybe it's the cold light?


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 Post subject: Re: A strange Feldspar
PostPosted: Fri Sep 04, 2015 11:52 pm 
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This material was short lived, awesome, rare and highly collectible.
I missed the boat.

When I first saw it, I suspected some exotic treatment. I was wrong.
Here is some from Pala Gems 2013:
Image
Pala wrote:
Amazonite feldspar suite: above, 12.19-carat pear shape, 18 x 14 x 10 mm; below, left to right, 2.79-carat cushion, 0.97-carat round, 2.17-carat pear shape, 2.22-carat cushion. (Photo: Mia Dixon)

Pala acquired a rough parcel of some of these exceptional amazonites, and had them tested by Prof. George Rossman up at Caltech to make sure they were 100% natural. These gems were cut from a parcel that can be traced back to a gem pocket in Luc Yen, Vietnam. This pocket was uncovered in 1997 and contained many high quality tourmaline crystals along with some of these gemmy amazonite feldspars. The color is reminiscent of neon-green paraiba tourmaline, showing fluorescent-like properties in daylight. This is fairly common mineral transformed into extremely rare and beautiful faceted gems.


Wowsers!


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 Post subject: Re: A strange Feldspar
PostPosted: Sat Sep 05, 2015 12:40 am 
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At Tucson, one of the dealers showed me a picture of a green madagascar orthoclase. It was....I think fist sized? Not the super nice green of the vietnamese material, but amazing stuff (which was apparently bought up so quick it didn't hit the general market to a big extent). I'd love to have a smigeon--guess I'll just have to jump fast next time there's a find.

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 Post subject: Re: A strange Feldspar
PostPosted: Sat Sep 05, 2015 11:52 pm 
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I actually saw a few pieces, but it was so wildly expensive I didn't even think about buying it. I think it was two gem shows back...


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 Post subject: Re: A strange Feldspar
PostPosted: Sun Sep 06, 2015 2:03 pm 
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Hi,

Your gem is very clean and large for amazonite from Vietnam.

Maybe a plagioclase from Kenia?

http://www.gia.edu/cs/Satellite?blobcol ... inary=true

page 84 and 85

I have seen ( fair priced ) some in St. Marie - small and included but very attractive. A nice green to bluishgreen color with a slight neon touch.

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 Post subject: Re: A strange Feldspar
PostPosted: Wed Sep 09, 2015 3:39 pm 
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Hi Marlow,

I didn't buy that stone without detailed gemological report.

The gemologist has examined the stone with refractometers, immersion microscope, spectroscope and ultraviolet light and came among others to the following conclusion: "The Amazonite is a microcrystalline variety of the feldspar group and is known only translucent / opaque, but the present stone shows a fascinating transparency."

I have obtained a second opinion with the same result.

Best regards
Chris


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 Post subject: Re: A strange Feldspar
PostPosted: Thu Sep 10, 2015 1:48 am 
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Chris.
Would you please post your detailed gemological report?
I would be interested in criteria used for determination.


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 Post subject: Re: A strange Feldspar
PostPosted: Thu Sep 10, 2015 2:13 am 
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chrisP wrote:
ultraviolet light

I believe UV is useless in feldspar identification. It's honestly useless in most cases, though it is fun in quite a few.
chrisP wrote:
The Amazonite is a microcrystalline variety of the feldspar group

And here's where you know he doesn't know his feldspars. Amazonite is by no means microcrystalline. It's just a potassium feldspar with a pinch of lead giving it color (varying between oxidation states and possibly in some relationship with OH).
In a technical sense, the Vietnamese material is not actually amazonite, because it isn't a K-feldspar (it's oligoclase iirc), but I think it's actually more sensible to just quietly extend the definition of amazonite (since what we really care about is feldspar colored blue-to-green by lead, and it just happened it was all k-spar till now I guess).

chrisP wrote:
and is known only translucent / opaque, but the present stone shows a fascinating transparency."

Transparent blue-to-green oligoclase with stunning color is also known from Canada. See:
Image
And material with decent blue/green color does indeed come from Kenya:
Image

chrisP wrote:
I have obtained a second opinion with the same result.

The question isn't whether he's right about it being lead-colored feldspar, just of whether it's from Vietnam in specific. I don't know how that would be determined, but it does seem greener than the material I've seen out of Kenya. Unfortunately for me though, the Kenyan find was a bit before my time of having a budget so I have only seen scraps.

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 Post subject: Re: A strange Feldspar
PostPosted: Thu Sep 10, 2015 5:15 am 
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Stephen I disagree with you. We should leave Amazonite on the K side of the feldspars and, if this blue-green gem variety is indeed a plagioclase, find its place and maybe a name for it on the calcium/sodium side of the feldspars. Blue sunstone ? Skystone ? Whatever.


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 Post subject: Re: A strange Feldspar
PostPosted: Thu Sep 10, 2015 10:40 am 
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Isi, that is a bit of a sidebar, as I agree with Stephen, that the identification of the pictured stone by ChrisP
is nebulous at best. Let's sort out positive ID before assigning it a tradename. :wink:
I'm looking forward to having my opinion changed, so.......ChrisP post the report you describe, please.


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 Post subject: Re: A strange Feldspar
PostPosted: Mon Sep 14, 2015 2:58 pm 
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Sooooo, if I understand this correctly, "amazonite" is a varietal name for the mineral microcline, of the feldspar group.

Unless it's blue and transparent, (and also very rare) in which case it's actually oligoclase?

Gemology really, really, needs some more mineralogical scrutiny.

The blind acceptance of thousands of trade names and varietal names is torture to anybody scientifically minded.

A thousand thanks to the poster of the link to the 7ct Canadian oligoclase. That's an incredible stone. Usually very small and very, very pale blue, and not at all expensive in these parts.


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 Post subject: Re: A strange Feldspar
PostPosted: Mon Sep 14, 2015 4:41 pm 
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Quick correction: I am wrong! Sorry about that, I totally misremembered the proper variety. The Vietnam material is in fact orthoclase--which we know thanks to Barbra: viewtopic.php?t=4442. Orthoclase and microcline are both potassium feldspars with some slight structural differences. Microcline is triclinic, slightly less ordered than monoclinic orthoclase (though the difference in angles is small). To complete the trio, sanidine is a k-spar that is also monoclinic but is disordered in a different way, in how the potassium is packed within the crystal structure.
Oligoclase is a plagioclase on the sodium-rich side of the spectrum. The Kenyan and Canadian stones are oligoclase. At some point between when I read that 2007 thread and now my brain crossed wires and decided that the Vietnamese material was as well. Just goes to show I should do more fact checkin' before I post.

I don't know how much easier this makes it to sort a vietnamese orthoclase from a kenyan oligoclase with standard tools. It would be trivial with advanced ones, of course.

The differences between the k-spars have some geologic significance, and they often look different in gem form because of the different contexts that produce each polymorph. But when it comes to relevant gemological properties the differences are essentially irrelevant--same hardness, same kind of cleavage in almost the same directions, overlapping optical properties. Even a skilled gemologist like Barbra had to resort to advanced methods to tell the vietnamese stones were not microcline.

Sorry if I'm going too off-topic here by giving my thoughts on amazonite in general. If you look up amazonite it is defined as being lead-bearing blue-to-green microcline. But it is important to ask why. From what I can tell, it's because we only ever saw microcline bearing lead and being blue-to-green, so we thought that microcline was the only one that did. But as it turns out, we were wrong, it's just really rare in the other feldspars. Since we wrote the definition that way, do we stick with it, or do we reconsider the way we defined it?
Personally, I think the microcline definition is more a matter of excessive precision. It wouldn't be the first mistake we made with the name--after all, it's not actually found in the Amazon either. I think a reasonable broadening of the definition would be similar to our acceptance of omphacite as a jade mineral or of sunstone varieties across the entire feldspar spectrum. A part of me is admittedly unsatisfied, because the Kenyan stones are so pale that they don't really resemble traditional amazonite. But it's better than leaving out the Vietnamese and Canadian stones. I know not everyone will feel that way, it's just my opinion.

Sorry for for the long post on this--I know it doesn't really matter, since the trade is going to call it amazonite whether we approve or not.

Finally, here's a link to some green madagascan sanidine. I wish I could tell what color they actually are, though... http://www.mindat.org/photo-521534.html

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 Post subject: Re: A strange Feldspar
PostPosted: Mon Sep 14, 2015 7:24 pm 
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Looks like ChrisP may be a drive by.


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 Post subject: Re: A strange Feldspar
PostPosted: Sat Dec 12, 2015 4:14 pm 
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Is faceted Blue Green sunstone from Kenya rare? I bought some years ago and had a couple of stones faceted. The rough was in the same batch as confetti sunstone and some of the rough had both the blue green and confetti in the same stone with a clear demarcation line like bicolor tourmaline.

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 Post subject: Re: A strange Feldspar
PostPosted: Wed Jan 06, 2016 9:02 pm 
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Sorry for posting again but I never got an answer and thought I would ask again and bring to the top. Is Tanzanian VVS Faceted Greenish Blue Orthoclase rare? I have a 9ct round and a 4.5ct trilliant I had cut from rough I bought about 10 years ago from Thiagem when they first started online sales. They had listed it as Oregon Sunstone Rough at the time.

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