New Mineral Named After GIA’s John Koivula
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 Post subject: Re: confessions
PostPosted: Fri Aug 25, 2006 3:11 pm 
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Precision Gem wrote:
I must confess, when I first started cutting I cut some pretty cheap stuff. I bought some very cheap white topaz that was just as you described yours, water worn. I always just oriented the rough for the best yield, and never had any problems with a cleavage plane. Maybe I was lucky.


I did the same and have had no problems to date. I often 'proof' previously untried designs in white topaz. I did discover you can often determine cleavages visually just through careful study of the rough with a 10X loupe. There are often incipient cleavages near the surface.

Rick Martin aka ROM


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PostPosted: Fri Aug 25, 2006 6:09 pm 
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While Doos is doodling, I'll think out loud here for a minute.

Even if the optic axes aren't parallel to the crystallographic axes in topaz or some other biaxial stones, in the case of topaz where cleavage may be a problem, the optic axes should remain constant and at a consistent angle to the cleavage plane.

By examining a stone with cleavage and orientating it to an optic axis, you should be able to do the same with worn stones that show no cleavage. The geometry should be consistent.

I mean, why wouldn't it be?

Maybe those who told pt to use the polariscope, were thinking the optic axis is the c-axis, and coincidentally the axes are close enough together that the cutting orientation works without cleavage problems.

Doesn't seem like it should be that difficult.


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PostPosted: Fri Aug 25, 2006 9:09 pm 
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I posted this same question to the usfgfacetersdigest list. I'll see what answers that group comes up with.

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PostPosted: Fri Aug 25, 2006 9:38 pm 
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Peter,
I looked this up in my Vargas 3rd Edition. Knowing the direction of the C-axis in topaz is important because the basal cleavage plane is on the A-B axis lying at right angles to the C. Stones should be oriented so the A-B is not less than 10 degrees away from the table plane. But I can't find the answer to your question.

The Vargas's instructions for locating the optic axis are to place the rough (either uniaxial or biaxial, they say) between the crossed polars and to tilt it in all directions while continuing horizontal rotation. A point will be found where the stone remains dark or relatively so throughout the rotation. Place a mark at "the uppermost central portion of the piece." Then turn the piece to the opposite side and repeat. The optic axis lies between the two marks.

That seems well and good for uniaxial stones but it doesn't make sense to me in regard to biaxials. It doesn't account for the second optic axis and the only way it makes sense is if one optic axis is parallel with the C-axis. Quick searches through my books and on-line haven't shown me any illustrations of optic axis locations in topaz.

I have some stream-tumbled Nigerian white topaz nodules. I'll dig them out and see if I can learn more when time allows. Meanwhile I learned of an interesting cutting trick. Some overseas cutters cleave topaz and use the perfectly polished cleavage as the table, saving polishing time. It's okay unless the stone needs recutting and then the repair person has a real problem.

Rick Martin aka ROM


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PostPosted: Sat Aug 26, 2006 7:49 am 
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Here is my crude attempt to explain with some images.

1: situation in uniaxial stones like Quartz.
The c-axis is parallel to the optic axis and a simple interference figure is produced around the optic axis. The coloured circels are due to birefringence (also influenced by the thickness of the material).
OA = optic axis.

Image

2: situation in biaxial positive stones like Topaz (only for the orthorhombic system).
There are two optic axes (OA - red lines) and they do not align with the c-axis. The 2V is the angle between the two optic axes (in Topaz that is between 48° and 68°).
In the top view the isogyres (black lines running through the "eyes") are parallel to the polarizer and analyzer of the polariscope. So a 0° rotation.

Image

If you find an image like this, you are set and know exactly where the c-axis is ..
As this is not a perfect world, you will probably very rarely find it.

3: situation with the stone rotated 45°.
The isogyres transform into hyperboles on rotation with the greatest curvature at 45°. This is a situation you can use as the c-axis will be exactly between the two centers of the hyperboles.

Image

Again this is not a perfect world, so you will be very lucky if you can find such an image. The reason is that the 2V values are mainly too large (the optic axes are far apart).
No problem for us though, we can easily outsmart the stone.

4: situation with the stone rotated 45°, but only focusing on one hyperbole (optic axis).
Only one hyperbole is examined, you can imagine where the other is and the c-axis will still be in between them.
It is VERY VERY important that you turn the stone to the point where the curvature is at maximum, otherwise the c-axis will not lay in between the optic axes.

Image

All this is done under the polariscope with a conoscope rod. You will want some magnification to see the interference figure more clearly. Oh and a lot of practise :)

Hope this helped and sorry that my drawng skills are limited.

p.s.: The last 3 images are for biaxial stones with a positive optic sign that form in the orthorhombic system.


Last edited by Doos on Sat Aug 26, 2006 3:11 pm, edited 1 time in total.

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PostPosted: Sat Aug 26, 2006 8:29 am 
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Quote:
By examining a stone with cleavage and orientating it to an optic axis, you should be able to do the same with worn stones that show no cleavage. The geometry should be consistent.

If you need to cut no less than 10 degrees as ROM said, then orientating on one of the optic axes is a safe way indeed. It will be atleast 24 degrees away from the c-axis (2V = atleast 48 degrees).


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PostPosted: Sat Aug 26, 2006 10:04 am 
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ROM -- thanks for digging that up. Based on what you describe, I understand why faceters simply refer me back to Vargas & don't try to explain!

Doos -- WOW, thanks for the diagram and expanded explaination! After reading it, it's clear that I need to read it a couple more times. I'm impressed with your grasp of how this all works and clarity in explaining it! Thanks!

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PostPosted: Sat Aug 26, 2006 10:28 am 
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Quote:
Hope this helped and sorry that my drawng skills are limited.

Talk about modesty! Your post is fit for a minerlogical digest, Doos. They are excellent illustrations of your concept.
Very well done!


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PostPosted: Sat Aug 26, 2006 11:22 am 
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Excellent drawings Doos.

I'll throw this out for your consideration. Notice how the 2 optic axes intersect with the C-axis at a given point. I contend that if you are looking exactly down the C-axis, you will see the typical biaxial optic figure at the point of convergence. At least with topaz. This will tell you you are looking parallel to the c-axis and perpendicular to the cleavage plane.

As long as the both angles between the optic axes and the c-axis are equal, this should be true.

Yes/no


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PostPosted: Sat Aug 26, 2006 11:41 am 
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In the ideal world, Yes.


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PostPosted: Sat Aug 26, 2006 12:57 pm 
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Doos wrote:
In the ideal world, Yes.

Since this is the real world, Doos, I'll continue looking for visual cleavage plane clues and forget the polariscope when cutting topaz unless I'm working with extremely valuable Imperials, which usually show a crystal form anyhow.

Meanwhile your diagrams and explanations are top-notch! Thank you for them -- they're exactly what I was searching for. I echo Barbra's praise. I've printed them out and have sandwiched them into page 68 of the Vargas book for future reference.

The Vargas books have helped many beginning faceters but I think the authors out of their area of expertise in this matter.

Rick Martin aka ROM


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 Post subject: Erratum
PostPosted: Sat Aug 26, 2006 3:09 pm 
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I forgot to mention that this applies to biaxial minerals with a positive sign that form in the orthorhombic system.
In the monoclinic and triclinic systems the c-axis doesn't align with the Bxa. I think I mentioned that before, but forgot to include that in the images explanation. That is corrected now.

The Vargas explanation on how to find an optical axis is correct, the excercise here was to find the c-axis in Topaz. You can't do that with the Vargas version unless you find both optic axes in Topaz.


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 Post subject: Re: Erratum
PostPosted: Sat Aug 26, 2006 3:19 pm 
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Doos wrote:
The Vargas explanation on how to find an optical axis is correct, the excercise here was to find the c-axis in Topaz. You can't do that with the Vargas version unless you find both optic axes in Topaz.


Just for the record, that was my point Doos. Thanks for the additional information.


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PostPosted: Sat Aug 26, 2006 11:15 pm 
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JB wrote:
Excellent drawings Doos.

I'll throw this out for your consideration. Notice how the 2 optic axes intersect with the C-axis at a given point. I contend that if you are looking exactly down the C-axis, you will see the typical biaxial optic figure at the point of convergence. At least with topaz. This will tell you you are looking parallel to the c-axis and perpendicular to the cleavage plane.

As long as the both angles between the optic axes and the c-axis are equal, this should be true.

Yes/no


Scratch that theory, after a little more thought, it makes no sense to me at all. Don't know what you would see where optic axes intersect (maybe nothing) but Doos needs more to think about. :wink:


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PostPosted: Mon Aug 28, 2006 8:05 pm 
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OK,
This thread has been very interesting and I have tried to find images (to photograph) to duplicate the excellent drawings that Doos composed so we can put real life images together with what you actually see using the polariscope in a normal fashion.

So far using only faceted topaz, all I ever get, with little effort, is a standard textbook biaxial figure. Maybe the optic axes are too far apart, as Doos said, to get the images depicted in the drawings, using the polariscope in the standard teaching methods. Other biaxial minerals may exhibit that better.

To add a little more interest, below is the images of my conoscope under the polariscope. One at approx. 90 deg. orientation and one at approx. 45 deg. orientation.
Image
The material of the conoscope is either plastic or glass, so I'm not sure why I'm getting this reaction.

Haven't spent to much time thinking about this, but this is a fun thread that I think we can expand on and make some distinctions between theory and what one can actually expect when using instruments in a normally taught fashion. Clear up some confusions. :)


Last edited by JB on Wed Aug 30, 2006 2:18 pm, edited 1 time in total.

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