Quartz Birefringence

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microb
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Quartz Birefringence

#1 Post by microb » Mon Feb 15, 2021 10:19 pm

If I'm looking perpendicular to the optical axis, I thought I would see more birefringence -- actually any.

Supposedly this is optical quartz. It has a hexagonal cross section, so that should define the optical axis as being perpendicular to the hex.

Am I missing something here?
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Scarodactyl
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Re: Quartz Birefringence

#2 Post by Scarodactyl » Mon Feb 15, 2021 11:23 pm

If you're not seeing birefringence you must be looking right down the C axis as you've guessed (as long as it's actually quartz anyway). Quartz crystals are hexagonal in outline but also (at least in projection, ignoring the 3d geometry)hexagonal when viewed from the side. If you're dealing with a slice from a large crystal or something sawn from a hydrothermal wafer the superficial geometry might be deceptive.
It's easy to tell if you have two polarizing filters. Illuminate from below and put the quartz between crossed polars. It will stay dark if you rotate it around the C axis, but it will blink clear and dark if you rotate around the a axes.

Placozoa
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Re: Quartz Birefringence

#3 Post by Placozoa » Thu Feb 18, 2021 4:31 am

I think what you would expect to see is two blue lines really close together. One would be polarizied one way and the other would be polarized 90° to the first, but your eyes cant tell that, and they would just look the same. You only get birefringance if the quartz is between two crossed polars. That is my experience viewing quartz under the microscope with and without the polarizing filters. I have seen large pieces of quartz before, but my eyes are better than my memory, so I might be mistaken.

Greg Howald
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Re: Quartz Birefringence

#4 Post by Greg Howald » Thu Feb 18, 2021 4:28 pm

Also remember that there are no perfect crystals in nature so you may have to play with it a bit. I usually do.
Greg

microb
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Re: Quartz Birefringence

#5 Post by microb » Thu Feb 18, 2021 5:36 pm

Greg Howald wrote:
Thu Feb 18, 2021 4:28 pm
Also remember that there are no perfect crystals in nature so you may have to play with it a bit. I usually do.
Greg
I'm going to polish opposite surfaces at a right angle to this. This is a piece I cut off a synthetic quartz, 8 inch long and 1 inch diameter. It's just that I had assumed that the axis was lengthwise, since the cross section was so hexagonal. But as Scarodactyl pointed out, the crystal can have a hexagon shape on the perpendicular cross sections, and this crystal does.

So I'll get some flats on it. And hopefully I'll see birefringence like calcite. Or confirm that I just bought some fused silica glass.

Scarodactyl
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Re: Quartz Birefringence

#6 Post by Scarodactyl » Thu Feb 18, 2021 6:49 pm

Are you just lookijg for visual doubling? Quartz has much lower birefringence than calcite. Eye visible doubling is limited to fine features looked at through a pretty thick section. You want to use crossed polars for this if you aren't already.

microb
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Re: Quartz Birefringence

#7 Post by microb » Sat Feb 20, 2021 3:35 am

So I cut and polished a section from a synthetic quartz rod.

Problems:
1) I don't see any blatantly visible birefringence. After basically making a box shape out of it with polished sides, you'd think there would be some visible doubling. (I'll get polarizers back out, my microscope set-ups are in boxes right now)
2) There's this square tube inclusion going down the length of the crystal rod (must be the seed, but #3 below is about the resulting odd crystal structure)
3) If I look at the sides, its flat, but there appears to be the cross sections showing of merged crystals -- so it's not an optical synthetic crystal

So I'm trying another supplier. If anyone has pointers to someone selling optical grade quartz, please let me know.
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Scarodactyl
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Re: Quartz Birefringence

#8 Post by Scarodactyl » Sat Feb 20, 2021 4:27 am

That is a single synthetic crystal pictured in the last image.
How are you detecting birefringence?

microb
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Re: Quartz Birefringence

#9 Post by microb » Sat Feb 20, 2021 4:53 am

Scarodactyl wrote:
Sat Feb 20, 2021 4:27 am
That is a single synthetic crystal pictured in the last image.
How are you detecting birefringence?
The short piece is cut out of one of those large ones. The seed is in the center, so at best I have four long crystals grow off the seed's four sides. Yet I'm wondering if the structure is even messier than that.

So far, I've been focused on just trying to cut and lap. I need to put together some optics to test things with.

I can send you these samples. But I'm worried that making DIC prism from this will be a challenge.

Scarodactyl
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Re: Quartz Birefringence

#10 Post by Scarodactyl » Sat Feb 20, 2021 5:24 am

Do you have a pair of linear polarizing filters?

microb
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Re: Quartz Birefringence

#11 Post by microb » Sat Feb 20, 2021 5:48 am

Scarodactyl wrote:
Sat Feb 20, 2021 5:24 am
Do you have a pair of linear polarizing filters?
I have to hunt them down. Microscope stuff is in boxes right now.


But you see the seed in the center. So I don't think I can cut out a flat blank of any useful size.

microb
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Re: Quartz Birefringence

#12 Post by microb » Sat Feb 20, 2021 6:00 am

Scarodactyl wrote:
Sat Feb 20, 2021 5:24 am
Do you have a pair of linear polarizing filters?
Assuming there is a chance of a good continuous structured crystal, put the crystal in between to cross-polarized filters that are extinguishing the light. If there is birefringence, there should light showing again as if a quarter waveplate were inserted.

But you don't think any visible line double would show like calcite.

Scarodactyl
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Re: Quartz Birefringence

#13 Post by Scarodactyl » Sat Feb 20, 2021 7:04 am

Quartz's doubling is much more subtle, nothing like calcite's. A thick line like that, especially with subtle texturing, will obscure the doubling effect pretty well. You need sharp details with sharp edges, and better yet would be viewing it through a microscope. Believe me, the doubling will definitely show up if you try to look at features inside or behind a chunk of quartz at higher magnification without a polarizer--it is not a fun experience, at least if you're not looking down the optic axis.
This will be one continuous crystal, that's just how they're grown.

microb
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Re: Quartz Birefringence

#14 Post by microb » Sat Feb 20, 2021 1:46 pm

Scarodactyl wrote:
Sat Feb 20, 2021 7:04 am
Quartz's doubling is much more subtle, nothing like calcite's. A thick line like that, especially with subtle texturing, will obscure the doubling effect pretty well. You need sharp details with sharp edges, and better yet would be viewing it through a microscope. Believe me, the doubling will definitely show up if you try to look at features inside or behind a chunk of quartz at higher magnification without a polarizer--it is not a fun experience, at least if you're not looking down the optic axis.
This will be one continuous crystal, that's just how they're grown.
Thanks. I've been focused on surfacing these and still have to experiment some more. I'll test it to see if it undoes cross polarization.

But does a crystal like this have just one consistent optical axis? Or is it four crystal structures merged together against the seed, each with a different direction for the OA? Because cutting that and getting big enough pieces is going to be tricky.
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microb
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Re: Quartz Birefringence

#15 Post by microb » Wed Feb 24, 2021 10:03 am

It looks like the I can count on a consistent atomic crystal structure. I was worried it grew with at least four merging.

The left blue and left red are letting some light through, but it is cross-polarized and the same light level shows without the prism, basically very dark – and black if the LED is dimmed.

So when the prism is placed between cross-polarized Olympus slides, the 45 degrees of the blue and red axis let light through while the 0 degrees do not. All angles on the green let light through.
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microb
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Re: Quartz Birefringence

#16 Post by microb » Fri Feb 26, 2021 6:02 pm

So c-axis and z-axis are the same when looking around for things about quartz. Z-cut means a flat optic with the z as the normal.

Heppy
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Re: Quartz Birefringence

#17 Post by Heppy » Tue Mar 23, 2021 8:40 am

microb wrote:
Sat Feb 20, 2021 3:35 am
So I cut and polished a section from a synthetic quartz rod.

Problems:
1) I don't see any blatantly visible birefringence. After basically making a box shape out of it with polished sides, you'd think there would be some visible doubling. (I'll get polarizers back out, my microscope set-ups are in boxes right now)
2) There's this square tube inclusion going down the length of the crystal rod (must be the seed, but #3 below is about the resulting odd crystal structure)
3) If I look at the sides, its flat, but there appears to be the cross sections showing of merged crystals -- so it's not an optical synthetic crystal

So I'm trying another supplier. If anyone has pointers to someone selling optical grade quartz, please let me know.
You said you were looking for a reference to someone who sells optical grade quartz. Artifex Engineering sell custom optics and they have a tutorial on birefringent materials where they also talk about quartz specifically (https://artifex-engineering.com/article ... on-optics/).

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