Joined: Tue Sep 19, 2006 6:04 am Posts: 33 Location: Melbourne, Australia
Hi Guys,
I would just like clarification of something I was told last night. The bullseye figure of quartz seen through a conoscope is only seen in untwinned quartz. Twinned quartz will have the usual uniaxial interference figures (isogyres meeting in the centre to form a black cross). I looked up Read's "Gemmology" but it just to refers to quartz in general. Does anybody have a reference I can look up regarding this please?
Sorry for the late reply, but I don't think it has to do with twinning atall. I could be totally wrong of course.
To the best of my knowledge it has to do with enantiomorphism (left-right handiness) of quartz. This causes the rotation of the structure, resulting in a typical bull's eye.
When a quartz is cut very thin, you might vaguely see the isogyres meet in the middle.
"Quartz" by Michael O'Donoghue might be a good read on that although he doesn't go into it very deep.
You might also google a bit for "optical activity".
I thought the bullseye was due to the crystal structure of quartz which causes the O and E rays to polarise in a rotational manner rather than in flat planes.
The enantiomorphism (left-right handiness) mentioned by doos will determine whether the polarised rays within the crystal will rotate in a clockwise or counter clockwise direction.
Don't have much more info on this but I'd be interested in learning more if anyone has anything to add
The standard description of the origin of the "bull's eye" figure of quartz is exactly as Doos and Frank outline, the light's response to molecular asymmetry of the unit crystal, or at least planes of single crystals.
I'm trying to build a picture in my head of why the bull's eye occurs, and the related question, why it doesn't occur for uniaxial stone. I can almost do it, but I require a little experimental input.
I'd like to know what is observed with the typical uniaxial stone, and with the quartz, when the analyzer polarization filter is removed. I can guess that the isogyres disappear completely for both cases, but I'd appreciate experimental confirmation.
Or better yet, what is observed when the analyzer polarization filter's axis is parallel to the first polarization filter's axis?
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Another thing I am wondering is a bit of a digression. For the usual conoscope figure of an uniaxial stone, is it possible that there exists a dim spot of light directly at the melatope? This spot would probably be very dim and pointlike, so that it would be difficult to observe in the best of situations.
I have observed a similar light-diffraction based phenomenon with a round penny, known as Arago's spot... A beam of parallel, coherent light shining across the front surface of a penny and beyond should cast a circular shadow with a spot of light in the very center. But that is very difficult condition to set up with a normal white light source: has to be a parallel and coherent beam (when most light sources radiate light incoherently in all directions) and has to be quite bright to observe the spot.
A difficult condition to achieve with any normal white light source, but easy to achieve with a monochromatic laser. And so this is what I had my students arrange... A beam of red HeNe laser light, expanded with a Gaussian telescope to diameter larger than the penny, shines across the front of the penny. On a screen behind the penny, one sees the circular diffraction pattern of alternating dark and light rings (called the penumbra shadow if the source is white light), completely merging to a dark circle (called the umbra shadow), and at the center of the umbra shadow there is a spot of red light... quite the definitive experiment in identifying the wave nature of light. However two-slit interference, since it is an interference phenomenom, is usually is cited more often as the primary demonstration of wave nature of light.
Another thing I am wondering is a bit of a digression. For the usual conoscope figure of an uniaxial stone, is it possible that there exists a dim spot of light directly at the melatope? This spot would probably be very dim and pointlike, so that it would be difficult to observe in the best of situations.
Brian,
In thin slices of quartz you could see that, but I doubt if that is what you mean.
For the rest I really have no clue of course.
Leigh,
I had a few hours that I needed to spend in which I could do little else than read mostly and I found a great book which proves your source right.
The bull's-eye is indeed not caused by twinning but is due to enantiomorphism (as said earlier).
In twinned quartz (Brazil twinning) you may observe 4-fold "Airy spirals" which are very different from the classical bull's-eye, BUT it can be very difficult to see the difference in cut stones (especially if the optic axis cuts through small facets). You will see this type mostly in amethyst.
Source: Edelsteinbestimmung - by Godehard Lenzen ISBN 3-9800292-2-0 (a copy is obtainable through amazon.de).
Reminds me of David Golub the smartest and youngest kid in my highschool graduation class of 800 in 1968.
He originated (as a high school senior) the "enantiomorphic helical field theory" which of course refers to the fact that everyone gets screwed one way or the other.
Never thought I would EVER get a chance to repeat that joke. Thanks fellas!
Everyone gets screwed atleast once in a lifetime, the trick is to not let it happen again.
Enantiomorphism goes both ways, so smart people will recognise it after the first incident and don't let it happen again.
Pretty people let it happen over and over.
As it happened to me twice, does that mean I'm smart and pretty?
I remember a joke I learned in second grade that I still haven't told anyone. Probably because it isn't very funny. But I've got a couple more years before reaching this 39 year record. Maybe by that time, my son will be in second grade, and I can tell it to him.
I've updated the polariscope page on the gemology project to show Airy Spirals.
Those are computer generated images, so they might look too perfect.
I'm not sure if this knowledge is useful, but what the heck. Maybe it could be a means of seperating heated citrine (amethyst) from naturally occuring citrine, but I need an expert opinion on that.
Does Brazil twinning occur in natural citrine?
If anyone has an image of the 4-fold Airy Spirals in amethyst, please let me know (and use it). No need for a retardation plate to observe that.
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