January 24 Through February 4—TUCSON, ARIZONA: Annual show
Welcome to the GemologyOnline.com Forum
A non-profit Forum for the exchange of gemological ideas
It is currently Tue Mar 19, 2024 6:09 am

All times are UTC - 4 hours




Post new topic Reply to topic  [ 9 posts ] 
Author Message
 Post subject: Pollucite
PostPosted: Tue Mar 02, 2010 6:33 am 
Offline
Moderator
User avatar

Joined: Thu Dec 13, 2007 3:42 pm
Posts: 4091
Location: the Netherlands
Last weekend I was having fun ID-ing rocks at a dutch gem fair when one of the customers submitted a colorless stone for an ID. Clean gem, somewhat topazy look, you know, that silvery shine.

Results:
polariscope: SR, no strain
Refractometer: 1.518 SR

ok, no need to look any further... I've been taught that when you encounter a transparent, single refractive stone with a RI between 1,5 to 1,7 you can safely state it is glass (or obsidian). I knew that there were a few exceptions to the rule (leucit came to mind) but they are all extremely rare minerals that are very rarely encountered faceted.

So I informed the gentleman he was the proud owner of a piece of faceted glass. He returned to the vendor he bought if from to get his money back; after all it was sold to him as pollucite, which in my head was listed as DR. I should have checked with my Tables of gem ID book... pollucite is one of those SR exceptions... The vendor stuck to his story: it was pollucite, no doubt about it. Some japanese lab in Bangkok had ID'ed it as such.

OK, fair enough... possible... but not convinced...

Günther lists pollucite as fluorescing orange or pink. This piece was inert to both LW and SWUV. Has anyone ever run into this mineral and found it to be totally inert?

cheers,


Top
 Profile  
Reply with quote  
 Post subject: Re: Pollucite
PostPosted: Tue Mar 02, 2010 10:25 pm 
Offline
Gold Member

Joined: Mon May 14, 2007 11:00 am
Posts: 1133
Location: Monterey, CA
Tim wrote:
OK, fair enough... possible... but not convinced...

Next time, try the Hodgkinson Method. :D
Pollucite shows less dispersion than quartz, while glass ususally has much greater.


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Tue Mar 02, 2010 11:04 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 21600
Location: San Francisco
This is a tough separation (glass vs. pollucite), RI, SG are very similar and the birefringence in pollucite is ridiculously low, if present.
Here is a blurb from GIA on some pollucite and petalite from Afghanistan

Their pollucite was also inert to both LW and SWUV.

Sadly, a flame test would be completely inappropriate for gem ID, but it could be an interesting parlor trick for a bunch of gemmos on a long rainy night after a few cocktails.

Pollucite:Cs0.6Na0.2Rb0.04Al0.9Si2.1O6•(H2O)

One can place a sample of pollucite in steel tweezers.
Dip it in HCl and then insert it in an alcohol flame. :twisted:
Cesium and rubidium will color the flame violet;
lithium produces a red color
sodium a yellow/orange color.

Sodium's yellow masks violet. Therefore a cobalt blue glass plate might be handy to block the yellow so the violet can be seen.

PS All components must be super clean!


Top
 Profile  
Reply with quote  
 Post subject: Re: Pollucite
PostPosted: Wed Mar 03, 2010 4:23 am 
Offline
Moderator
User avatar

Joined: Thu Dec 13, 2007 3:42 pm
Posts: 4091
Location: the Netherlands
Bill Hanneman wrote:
Tim wrote:
OK, fair enough... possible... but not convinced...

Next time, try the Hodgkinson Method. :D
Pollucite shows less dispersion than quartz, while glass ususally has much greater.


That does help, the gem in question showed virtually no dispersion at all.

Quote:
This is a tough separation (glass vs. pollucite), RI, SG are very similar and the birefringence in pollucite is ridiculously low, if present.
Here is a blurb from GIA on some pollucite and petalite from Afghanistan


"Can be weakly anisotropic" (from that GIA blurb) How does that work with isometric xls?

Quote:
Their pollucite was also inert to both LW and SWUV


This piece was indeed said to come from Afghanistan, so it being inert makes sense; if it's from the same deposit I mean.

Thanks for your input dr Bill and Brabra!


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Wed Mar 03, 2010 11:55 pm 
Offline
Gold Member

Joined: Mon May 14, 2007 11:00 am
Posts: 1133
Location: Monterey, CA
Tim,
The point I was trying to make and which was convincingly confirmed by that GIA report is that the determination of the numerical value of dispersion is not considered a part of standard gemological testing. Today no one (even you) ever thinks of measuring it even though it can be easily done using either the very inexpensive Hanneman Refractometer or the Hanneman-Hodgkinson Refractometer, or estimated by the Hodgkinson Method.

Until the gemological teaching organizations recognize and teach the value of considering the B : D (Birefringence:Dispersion) ratio in gemstone identification, I shall continue to maintain they are "cheating" their students. I believe it is long past the time for these orgnizations to drop their concept that if a technique has not been "invented" by themselves it is not worthy of being taught. :D :D

Of course, when one has an expensive spectrophotometer and an extensive data base, one doesn't need to know anything about "optical" gemological properties to identify anything. In fact, one doesn't even need to know anything about gemology, itself. Big woo, but of no help to the practicing gemologist out in the real world. :D :D


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 04, 2010 10:17 am 
Offline
Moderator
User avatar

Joined: Thu Dec 13, 2007 3:42 pm
Posts: 4091
Location: the Netherlands
Hey dr Bill,

Quote:
The point I was trying to make and which was convincingly confirmed by that GIA report is that the determination of the numerical value of dispersion is not considered a part of standard gemological testing. Today no one (even you) ever thinks of measuring it even though it can be easily done using either the very inexpensive Hanneman Refractometer or the Hanneman-Hodgkinson Refractometer, or estimated by the Hodgkinson Method.


And point taken. Of course there is a 'but' though. I'm lacking the experience to rightfully apply the Hodgekinson method to the point that I dare to identify a stone on observations made by observing the B : D ratio by peering through a stone. There is further limitations to the method that prevent me from locking myself in a room for a month with a point light and a few thousand gemstones to master the experience: crappy cuts. The pollucite I was presented, for instance, was a step cut pavilion stone with a belly thicker than eh... (insert famous person with an unhealthy diet). It further was just a little over a carat and had a 2mm table facet. Using your refractometer concept would have posed me with a few problems as well i think.

That said, I don't own a single tool that doesn't have limitations and discarding a method or apparatus because of it's limitations seems silly. It certainly has its uses. I could, and should, have checked the dispersion rate of the stone in question.

Quote:
Until the gemological teaching organizations recognize and teach the value of considering the B : D (Birefringence:Dispersion) ratio in gemstone identification, I shall continue to maintain they are "cheating" their students. I believe it is long past the time for these orgnizations to drop their concept that if a technique has not been "invented" by themselves it is not worthy of being taught.


There is a few more things lacking in present gemology education :D I got the chance to compare a 15 year old course with mine last year and am sad to say that there was little progress, in fact, the 15 year old course was more extensive than mine.

Quote:
Of course, when one has an expensive spectrophotometer and an extensive data base, one doesn't need to know anything about "optical" gemological properties to identify anything. In fact, one doesn't even need to know anything about gemology, itself.


perhaps not to identify stones, but to advance the study of gemology (with or without the aid of advanced equipment) a firm understanding of physics, geology and chemistry is essential. We may be able to teach a monkey to extract Raman graphs from a stone but that monkey isn't going to develop new hypothesis, interprete data collected from experiments and draw conclusions which will advance our knowledge.

Quote:
Big woo, but of no help to the practicing gemologist out in the real world
.

The real world? Hmmm... In my real world I often stand powerless regarding treatment detection and the occaisional gem identification with my classical gem lab. I can't wait to be 20 years further; a 'classical gemlab' in 2030 will have a Raman, UV-Vis and FTIR spectrometer. A few grand will buy us nice second hand units by that time, hence making many of the methods used now obselete. Is progress a bad thing?


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Thu Mar 04, 2010 11:39 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 21600
Location: San Francisco
It has been my observation that dispersion in glass can vary substantially.

For instance, the brilliance of lead crystal relies on the high refractive index caused by lead oxide. Ordinary glass has a refractive index of n = 1.5, while the addition of lead produces a range up to 1.7. This heightened refractive index also raises the correlating index of dispersion.

Glass with RI of 1.5 can potentially have a very low dispersion (crown glass), therefore, observation of the dispersion would not consistently provide a reliable separation between pollucite and glass.

I dare say, without the presence of diagnostic inclusions visible with microscopic observation, one would need sophisticated instrumentation for a positive identification.
:D


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Fri Mar 05, 2010 7:58 pm 
Offline
Gold Member

Joined: Mon May 14, 2007 11:00 am
Posts: 1133
Location: Monterey, CA
Quote:
I dare say, without the presence of diagnostic inclusions visible with microscopic observation, one would need sophisticated instrumentation for a positive identification.

I will dare say that without such sophisitcated instrumentation, one cannont positively identify anything. :D

However, "Practical Gemology" is based on the process of elimination of the most likely possibilities. In this case, the question is whether the stone in question is pollucite or glass.

For the answer, simply touch it to your tongue. I can attest my pollucite feel cold, as do all crystalline materials, while amorphous glass does not (relatively speaking). :D :D


Top
 Profile  
Reply with quote  
 Post subject:
PostPosted: Sat Mar 13, 2010 8:22 am 
Offline
Gold Member

Joined: Sun Jun 28, 2009 9:56 am
Posts: 1284
pollucite is soluble in boiling hydrochloric acid while glass is not, here's your answer :twisted:

...just kidding, at least one could check if it is in the right SG range (pollucite SG is 2,80 to 3.0 and usually 2.85 to 2.94 for gem material)

Now if you were in the right SG range that could still be glass...

Bill's idea sounds smart (provided that you haven't been handling the stone between your warm fingers for long)


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 9 posts ] 

All times are UTC - 4 hours


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
Gemology Style ported to phpBB3 by Christian Bullock