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PostPosted: Mon Aug 28, 2006 8:54 pm 
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Dang Doos, you are one smart mutha-FGA when it comes to gemology! I have learned more on Gemology online than I have in my GG course. Doos , JB and several others here have the best explanations , photos and diagrams. Have you though of a best of book from here Barbra? You certainly have the stuff to do it. I would buy one. :smt042

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PostPosted: Tue Aug 29, 2006 10:17 am 
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Thanks Daniel.

JB, did you take a spot reading on your conoscope and you got that image through the ocular?


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PostPosted: Tue Aug 29, 2006 10:42 am 
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It's just looking through the analyzer no magnification. I haven't tested the RI on it. It feels lightweight, more like plastic. There are some funny inclusions in it as well, that I have never seen in plastic or glass. Nothing natural looking though.

The reason I noticed it was, I was trying to find an image in a piece of topaz that resembled your drawing of the separate hyperboles when I discovered there was one on my conoscope. I examined it by itself and discovered the hyperboles merged into a cross figure with a 45 deg. rotation. I've seen this happen with uniaxial stones.

I figured it's just ADR of some sort. The loupe does the same thing, but the hyperboles never merge into a cross shape upon rotation.

I really never use the conoscope for optic figures. I find it much easier using the loupe. That's just my personal preference.


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PostPosted: Tue Aug 29, 2006 10:52 am 
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Ah okay, I was thrown off a bit because you said refractometer.
There are pseudo-biaxial images in uniaxial minerals (as Beryl), and there are pseudo-uniaxial images in biaxial minerals.
Never would have thought that to happen in amorphous substances. Very interesting.


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PostPosted: Tue Aug 29, 2006 10:59 am 
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Yeah, I made a few bloopers in that post last night, must of been tired. Didn't go back to change any of them.


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PostPosted: Wed Aug 30, 2006 8:19 am 
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O.k., I got one very good reply from the faceter's digest. To be honest, much of this is over my head (as with Doos above), but I'm glad for the challenge.

The replies below seem to overlap the Doos explaination above, but there is some new stuff as well -- and a link to some incredible (and huge) images.


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Re: Polariscope and the c-axis
Posted by: "Dr William B. Amos" bradscopegems
Date: Fri Aug 25, 2006 7:07 pm (PDT)

Dear Peter,
I would not recommend the use of a polariscope in the
case of topaz.
With some materials, such as quartz, corundum and tourmaline, it
works fine: you just put the rough into the polariscope and rotate
it. It goes dark in four positions and the c axis is parallel to the
polariser axis in two of those positions and perpendicular to it in
the other two. The problem reduces to a choice of two positions for
the axis. If you have no clues in the crystal form, you just turn
the chunk all ways until you find a direction of view where spinning
it around the line of view leaves it dark (or at least constant in
brightness) all the time. You are now looking along the c axis,
which in these three minerals corresponds to looking along the long
axis of the hexagonal prism. These minerals are called uniaxial:
they have only one optical axis and it corresponds to the
crystallographic c axis. Topaz, however is biaxial: it has two
optical axes and you cannot discover where they are by the simple
rotation tests. Moreover, they do not have any simple relation to
the crystallographic axes and the angle between the two varies quite
a lot from one topaz specimen to another. Vargas (p68) does not
explain any of this. Having experimented with topaz crystals of
known crystal facet morphology I promise you that you will drive
yourself crazy trying to do it. I finally persuaded Frank Norman to
make a little sphere of topaz so that I could demonstrate just how
complicated biaxial crystals are when viewed in the polariscope. If
you want to get into this, you can download some articles I wrote:
http://homepage.ntlworld.com/w.amos2/Brad%20Amos's%20Website.


(See especially "List of PDF files" then "photographs of topaz sphere..." low bandwidth users beware...)

Quote:

Posted by: "Dr William B. Amos" bradscopegems
Subject: Re: Polariscope and the c-axis
Date: August 30, 2006 3:16:31 AM EST

Dear Peter,
You are welcome to post my reply on the gemmology
online forum.

Those who have studied crystallography will say that it is possible to
do it. This depends on having some way of viewing the so-called
conoscopic pattern between crossed polarisers. There are three ways of
doing this. One is to make a sphere, like the one Frank Norman made
for me. This is hardly practical if you have an irregular piece of
rough and just want to do quick check. The second is to hold a small
spherical bead of glass against the sample and look through both the
bead and the sample between crossed polarisers.. This is often
recommended in gemmology texts. The other is the geologist's way,
which is to cut a section of the gem, polish it on both sides, put it
in a polarising microscope and add an extra lens called a Bertrand
lens, which allows you to see the back focal plane of the objective.
With topaz, all these methods give a curious pattern with two
'eyes' (melatopes). If you can orient the specimen so that you have a
dark line running right across it parallel to either the polariser or
analyser axes and then turn it, keeping the line in view, you will see
the melatopes on the line. The conoscopic pattern is a kind of
angular map, and it tells you the angular position of the optical
axes, which run through the centre of the melatopes. In the centre of
the acute angle between the two optical axes, the topaz will look dark
(zero degrees in my pictures of Frank's sphere). At this position, you
are looking along the c axis.
It is quite tricky to find this position with even a smooth
semi-polished topaz pebble, because the angle between the melatopes is
so large and you are lucky to see even a single one of them. That is
why I don't recommend the method for beginners. However, I do
recommend trying this on a sheet of mica or even a sheet of 'Mylar'
plastic film. The melatopes are worth seeing, though not for people of
a nervous disposition, since they look like devil's eyes! A really
easy way to see them is to make a sandwich of a piece of mica between
two crossed polarising films, hold it close toyour eye and gaze at a
large white area such as a wall or a sheet of paper. The melatopes
will loom up at you.
The final twist is that in some specimens of topaz the
angle that is normally acute apparently becomes greater than 90
degrees, so the 'acute bisectrix' then becomes perpendicular to the c
axis. I have not seen this, but it is yet another difficulty to
contend with.
Post this bit too, if you wish.

Brad

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PostPosted: Wed Aug 30, 2006 11:29 am 
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Hi all,

I'm just reading up on some of the posts I've missed while I've been away and have come accross this beauty...nice thread guys...excellent diagrams as usual doos...I'll pop em into my "doos' diagrams" folder :P

I've been continuingly surprised by the polariscope since I've started studying. It seems that the more you learn about it the more tricks there are to learn... it seems such a simple uncomplicated piece of equipment when you first start to use it...

GREAT thread guys :)

Frank


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PostPosted: Wed Aug 30, 2006 12:51 pm 
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Peter,

The part Brad posted is exactly what we were explaining. The geological way is not suited for us as we don't want to slice a 30 micron piece of the rough (if we can do that atall).

He is correct that you need a lot of patience finding the interference figures, but with some experience you should be able to find them.
As of writing I can find them in 100% of all the uniaxials I tried and about 30% in all biaxials. I expect the latter to go up when I have some more time for experiments.. Immersion may help finding them.

Another technique I get good results with is to keep staring down the conoscope (the sphere ) while searching for the figure, rather than applying it after you found the flash. That will enable you to find interference figures more easily in my tests.


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PostPosted: Wed Aug 30, 2006 6:26 pm 
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Quote:
Never would have thought that to happen in amorphous substances. Very interesting.


Doos, regarding the photo, I actually went back and cracked a book :) and there it was, fully illustrated.

It is a common ADR reaction for transparent glass or plastic. Probably enhanced by the spherical shape of the bulb.


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PostPosted: Wed Aug 30, 2006 8:23 pm 
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I am very puzzled by JB's photos of the glass or plastic bead. It is correct that an optically isotropic (i.e. non-birefringent) sphere does contain a dark cross between crossed polarisers, with the arms of the cross parallel to the analyser/polarizer axes.
This is brought about by differential reflection losses from the four quadrants, which results in a net rotation of the plane of polarization by the sphere. This is, in fact used as a test that microscope objectives and other optical devices are strain-free. If there is strain-induced birefringence, you get two dark arcs (as in the picture on the right), even if the strain is uniaxial.
This means that the little sphere in JB's picture is undoubtedly birefringent, which makes it useless as a conoscope.
However, the really amazing thing is that the centre of the dark cross is missing (i.e. brightly coloured). This suggests that the bead is in this case made of quartz. Quartz has the peculiarity that it rotates the plane of polarization of linearly polarized light passing along the optical axis, and does this by a different angle for different colours. As I show in my article, a quartz sphere , oriented so that you are looking along the optical axis, looks just like JB's conoscope sphere, with the centre bright or coloured and four dark arms. Please try the same experiment with a different sphere, preferably one known to be made of well-tempered glass!


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PostPosted: Wed Aug 30, 2006 9:09 pm 
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"brad," welcome,

Thanks for that information. I apologise for the poor photograph that depicts the dark crosses with a bright center. In person there is no interruption of the cross, just some sort of light reflection in the photo.

The two dark arcs are present as well as the dark cross on rotation and I also believe that the cross arms represent the orientation to the polarizing axes.

I can confirm that the conosphere is either plastic or glass and I did suspect quartz briefly, but discarded that notion after closer examination.

I think it is no more than ADR that I have seen illustrated although not fully explained.

Maybe every one can examine their conoscopes under the polariscope and report back. I'm sure we will get to the bottom of this.

It could very well be that this is a cheap conoscope exhibiting strain that isn't suitable for polariscopic examination. Although, I do get similar reactions with my "optically corrected" loupe. :?:


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PostPosted: Wed Aug 30, 2006 9:56 pm 
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I haven't been this interested in optical stuff since my thin film coating class. No one teaches you this stuff in lens design - now THIS stuff is FUN!

gotta start buying some equipment here!! :D


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PostPosted: Fri Sep 01, 2006 4:36 pm 
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JB, What is ADR?


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PostPosted: Fri Sep 01, 2006 5:43 pm 
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Hi brad,

ADR is short for anomalous (false) double refraction. Sometimes an isotropic gem will have an anisotropic reaction when tested with the polariscope.

The reaction can be a dark to light blink, dark snakebanding or a cross figure or any other dark to light interruption when rotated 360 deg.

When this happens, we do a confirmation test by turning the stone to it's lightest position under the crossed polars and the quickly turning the analyzer to the open position. If the stone becomes "noticeably lighter" it confirms it as singly refractive. Anything else which includes staying the same or getting darker, confirms it as doubly refractive.

ADR reactions are typical in a number of stones including garnets, some syn. spinel, glass and plastic to name a few but can occur with other gems as well.

So it's always best to do the confirmation test any time you have a reaction other than staying dark or staying light when fully rotated between crossed polars.

That's how we were taught anyway. One other thing you may want to do is, when testing the stone, test it at a couple different orientations to be sure you're not looking down an optic axis of a DR stone.

When we are keeping records or worksheets on a gem and run across this reaction in an isotropic material we note it as SR/ADR.


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PostPosted: Fri Sep 01, 2006 8:11 pm 
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ADR Thank you for this clarification. If this is an established term, we can't do much about it, but from a crystal optics point of view it is a bit confusing, since anomalous double refraction (or anomalous dispersion of birefringence) already exists as an accepted term with a very specific and different meaning.

The meaning is that when you plot the two indices against wavelength, instead of running parallel as they do in, for example, quartz in the visible spectrum, the two lines converge and may even cross. At a crossover wavelength the crystal becomes isotropic, so where quartz would give you a dark band in the polariscope the anomalous crystal would be, for example, strongly blue, if blue happened to be the crossover wavelength. If possible, it would be good to change your term to 'False double refraction'. The term would then be quite acceptable for a number of phenomena, including the dark cross with dark centre that any isotropic sphere or lens shows, for the reason I gave before, which is totally unconnected with birefringence. Minerals such as corundum and apophyllite show some anomalous birefringence and this shows in that the fringes in a wedge are very similar, and remain black instead of showing the colours of Newton's series.


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