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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 1:53 am 
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Barbra Voltaire wrote:
The adularescence is caused by scattered light passing through the exsolution lamallae, acting as scattering centers, creating a bluish hue. In the case of orthoclase moonstone, the layering of albite and orthoclase form an assemblage of layers varying between 50 and 1000nm in thickness. The thinner layers produce Rayleigh scattering.

So to recap in a simplistic, yet accurate way, why does some orthoclase feldspar display adularescence?
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In the case of orthoclase, the effect is caused by the interference of light resulting from layering with albite (end member of the plagioclase series). If the layering is thin, light interference produces a blue schiller. If the layers are thicker the sheen appears white. In order to get the best effect, the stone should be cut so that the plane of the base of the cabochon lies parallel to the plane of the layers.


So the scattering would result from the same structure as the interference (diffraction, interference... they are nearly synonymous), except smaller. But I'm confused by these last two paragraphs. The first paragraph seems to say the effect results from scattering, and the second paragraph seems to say the effect results from interference.

Knowing the structure, now I see why there is the claim that scattering is coherent.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 10:02 am 
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Hhmmm. I don't see diffraction and scattering as profoundly different. Diffraction is a special type of scattering that leads to large scale interference effects, no?

I can see that my usage of "interference" may be confusing as I was using it in the general sense to mean the act or an instance of hindering, obstructing, or impeding.
Which word would be the best word to scientifically describe that, Brian? I'll edit.

Please define coherent vs incoherent scattering.
Thanks.

PS I found a strand of 6mm orthoclase moonstone beads in a drawer. I checked to see if they were different in transmitted vs reflected light.
In a word.....no.
Perhaps I need more and larger moonstones. :|


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 3:23 pm 
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No, the diffraction and scattering are two different effects, but similar structure could cause either... in this case, namely, the "exsolution lamellae". I wish I had read pandora's first post more closely, and I curse Tim's second post for introducing the misleading possibility that "interference" could be the cause.

I really don't want to focus on the interference, which I realize now is not a possibility, because it would require a long, long post to prove a null result. The upshot is that if the "exsolution lamellae" did diffract the incoming light, since thickness of the "exsolution lamellae" extends from 50 nm to 1000 nm, the result would have to be iridescence, not a single spectral color throughout.

From pandora's second post
pandora wrote:
My main question though is to whether the cause is due to the lamellar structure or to scattering.

The answer is the blue light is caused by scattering from the exsolution lamellae. So it is caused by scattering and it is due to the lamellae. You can think of a plane of lamellae of given thickness as being composed of little cubes with the same thickness being stacked against one another. These little cubes are the scattering colloids.

A small thickness plane (~50nm) would be composed of cubes 50nm side length stacked side-by-side, and this is the sort of size range that scatters blue light. Then a medium thickness plane (~150nm) would be composed of cubes stacked side-by-side whose size range would scatter green light. And then a large thickness plane (~450nm) would be composed of cubes stacked side-be-side whose size range would scatter red light.

As you note, for a given amount of the correct-size scatterers, scattering of blue light is always stronger than green, which is always stronger than red. If the distribution of lamellae thicknesses was weighted toward the small end ~ 50nm, then the color seen in the stone would be blue because small particles only scatter blue light.

If the number of small, medium, and large thicknesses of lamellae was about equal, then the color seen in the stone would be light-blue, or blue plus some white light. The different thicknesses scatter red+green+blue = white, but the small particles scatter more blue light, so you'd add an additional portion of blue.

If the number of small, medium, and large thicknesses of lamellae was weighted toward the large end, then the color seen in the stone would be white. The more scattering done by the small particles is balanced by the increased number of large particles.

As for "coherent" scattering... let's go back to the picture of the cubes as our colloids. Coherent scattering arises when these cubes are close together, and incoherent scattering arises when the cubes are spaced far apart. Well, since these lamellae are composed of these cubes sitting side-by-side right next to each other, right in contact with each other, you can't get much closer than that. So each lamellae scatters coherently.

In contrast, atmospheric molecules are, on average, not very close to each other, relative to their size. So scattering in the atmosphere is incoherent.

And so what is the difference between coherent and incoherent scattering? For incoherent scattering of blue light, as you scanned a very small region of space containing this blue light, the brightness of the blue light would look essentially pretty even. Something like this...
Attachment:
incoherent.JPG
incoherent.JPG [ 1.63 KiB | Viewed 2084 times ]

But for the coherent scattering of blue light, as you scanned a small region of space containing it, you'd see very fine alternations in brightness from maximum blue to black (no blue) to maximum blue again. Something like this...
Attachment:
coherent.JPG
coherent.JPG [ 3.81 KiB | Viewed 2084 times ]

I think this coherent scattering is the origin of the "ghostly" effect... that is, the blue is half-there and half-not-there.

Barbra Voltaire wrote:
PS I found a strand of 6mm orthoclase moonstone beads in a drawer. I checked to see if they were different in transmitted vs reflected light.
In a word.....no.

If you didn't do so before, be sure you set up the stone like the piece of blue glass at the bottom of the page of this Wiki article, so that the light transmitted through is projected onto a white sheet of paper. If that doesn't work, then yes you need a larger moonstone.


Last edited by Brian on Sat May 07, 2011 1:10 am, edited 1 time in total.

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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 3:39 pm 
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Quote:
Perhaps I need more and larger moonstones. :|


We all need more and larger moonstones Barbra :D

Good thread though


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 6:00 pm 
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Thanks for all of this - way more than I expected.

I'm going to try an rephrase what I think is the final answer - just so that I can be sure that I have got it right!

Adularescence in orthoclase moonstone is NOT caused by thin film interference where light is reflected from the alternating orthoclase and albite lamellae.

Instead it is the result of coherent scattering from the incident ray interacting with the multiple colloidal particles in each of these exsolution lamellae.

The colour of the adularescence is dependent on the size of these colloidal particles - the smaller the particles the bluer the colour, the larger the particles the whiter the colour.

The strength of the adularescence is dependent on the thickness of the stone and the height of the cabochon.

This was a diagram that I had drawn to explain the thin-film interference which I guess is now not the right answer. :cry:

Image

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This is a new one which I hope shows the answer???

Image

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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 6:51 pm 
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pandora wrote:
Adularescence in orthoclase moonstone is NOT caused by thin film interference where light is reflected from the alternating orthoclase and albite lamellae.

Instead it is the result of coherent scattering from the incident ray interacting with the multiple colloidal particles in each of these exsolution lamellae.

The colour of the adularescence is dependent on the size of these colloidal particles - the smaller the particles the bluer the colour, the larger the particles the whiter the colour.

This is okay, except I'd replace the phrase " coherent scattering from the incident ray interacting with the multiple colloidal particles in each of these exsolution lamellae" with "coherent scattering from the ten-hundred nanometer thick planes of exsolution lamellae." It is easier to think of scattering from individual cubes or balls or whatever, rather than planes. So one can think about the planes being divided up as separate cubes or balls to understand the scattering. But they aren't actually separate cubes or balls.

pandora wrote:
The strength of the adularescence is dependent on the thickness of the stone and the height of the cabochon.

The strength of the blue adularescence is dependent upon the number of small thickness lamellae present in the stone. The presence of white adularescence is dependent upon the number of larger thickness lamellae present.

The diagram doesn't make a good or useful representation of scattering. And you can't use light rays to represent the coherent scattering. Sorry


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 7:05 pm 
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Thank you!

And please, don't say sorry about the diagrams - much better to know that they are not the right thing!

So regarding the planes versus cubes thing, does that mean that the plane (ie the nano-layer of albite or orthoclase) is the actual colloidal particle or is one considering the layer at an atomic level with each atom or clump of atoms acting as the colloidal particle?


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 8:12 pm 
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Brian, that is brilliant, clear and understandable. I totally get it.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 11:53 pm 
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pandora wrote:
And please, don't say sorry about the diagrams - much better to know that they are not the right thing!

Here is sort of a ray diagram depicting the scattering. Red and green rays pass through. Blue rays bump around in random directions and end up filling all the space of the stone. It isn't useful to show the lamellae layers, because they are smaller than seeing. But one can infer their presence in the diagram.
Attachment:
scattering.jpg
scattering.jpg [ 35.34 KiB | Viewed 2050 times ]

As I mentioned, you cannot represent coherence with a ray diagram, because that is strictly a wave phenomenon.

pandora wrote:
So regarding the planes versus cubes thing, does that mean that the plane (ie the nano-layer of albite or orthoclase) is the actual colloidal particle or is one considering the layer at an atomic level with each atom or clump of atoms acting as the colloidal particle?

The plane of alternating layers is the scatterer. The alternating layers may stack up to form a 50 nm thick plane that scatters blue light, or they may stack up thicker and scatter other colors.

When I introduced the idea of dividing up the plane into little cubes that have no relation to actual atom positions or crystal structure, I should have held off until I began talking about coherence, because that picture is primarily meant to help explain why the scattering is coherent. So just transpose that picture a few lines down to just before the coherence discussion.

Barbra Voltaire wrote:
I totally get it.

Very good! Knowing the actual lamellae structure (most importantly its possible range of thicknesses), and reading pandora's second post (ahem, I only read the last couple posts the first time around), and sleeping on it, everything was in place when I woke up.

Oh yeah, and Tim... I'm cursing the post, not you! But it is always preferable to have too much information, rather than not enough. A key experimental concept is sorting through noise in order to lock onto signal, and this is a good example of that. ;)

And Frank... glad you are paying attention.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sat May 07, 2011 5:52 am 
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Barbra Voltaire wrote:
Brian, that is brilliant, clear and understandable. I totally get it.


=D>

You should take up writing text-books! I am really grateful for all your explanations - hope other people have enjoyed it too...


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sat May 07, 2011 9:07 am 
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Hi!
Very interesting reading! I’ve always wondered exactly what caused the ‘moonstone effect’.
Just wondering Barbara, are you suggesting that the scattering we observe from Moonstone is from exsolution lamellae, or layers?

My understanding is that optically, scattering occurs from particles. I’m very interested to know how exactly how layers cause scattering.

If part of the Moonstone effect can be attributed to scattering, then I assumed this meant that Moonstone must have colloids or particles, of the correct size to scatter blue light. As we have already read, the blue colour could also be caused by the ‘thin film interference effect’, layers of the correct size such that there is constructive interference of blue light.
In summary;
Attribute the Moonstone effect to scattering: it must have particles, (could also have layers)
Attribute Moonstone effect to thin film interference: it must have layers, (could also have particles or colloids)
Attribute Moonstone effect to both: it must have both layers and particles

Is there any research on the exact structure of Moonstone?

Thanks again for an interesting chat!


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sat May 07, 2011 10:44 am 
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Hi,

Apologies, I missed this statement:
'You can think of a plane of lamellae of given thickness as being composed of little cubes with the same thickness being stacked against one another. These little cubes are the scattering colloids', which explains the particle or colloidal structure of the Moonstone layers that gives rise to scattering.

I'm trying to imagine how this lamellae can be composed of little cubes? So is each cube a very long thin cube, dimensions being depth of the lamellae and length and width of the stone? Or is each lamellae composed of many ‘cubes’.
If these layers can form a cube structure, (ie each layer composed of many cubes that act as colloidal particles for scattering) then there must be something quite different about the chemical structure/composition of each layer such that cubes are formed, or to light the structure of the layers appears as cubes. (I guess, if not, then these layers would be normal layers like we see in soap films that give rise to interference).

Do we know why the layers behave as cubes, (if we’re taking big flat cubes, or many small cubes along each layer), for scattering?

I can understand why they do not behave as layers for interference, there is not the uniformity in depth to purely give blue light by interference.

Thanks again!


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sat May 07, 2011 10:45 am 
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Quote:
Just wondering Barbara, are you suggesting that the scattering we observe from Moonstone is from exsolution lamellae, or layers?

Yes.
This is not an original thought on my part.
It comes from research done by Kurt Nassau on moonstone in:
The Physics and Chemistry of Color

Dr. George Rossman and Emmanuel Fritsch:
An Update on Color in Gems, Part 3: Color caused by Band Gaps and Physical Phenomenon

Information obtained at the Sinkankas Feldspar Symposium: George Rossman and Skip Simmons both discussed phenomenon seen in feldspar varieties.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sat May 07, 2011 7:43 pm 
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And Frank... glad you are paying attention.


Brian, I read everyrything you post....some of them require two or three readings :D

If you ever feel the need for a chat to explain anything you post about please just announce it...promise I'll be there...or dead...or in the middle of a family thingy :D

Chats have been non existant lately due to a bout of illness on my part followed by a lack of students...

You always gave me a least a week of thinking from any chat...miss you... :D announce one soon please....might I suggest coherent and non coherant scattering :D

hope to chat soon

Frank


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sun May 08, 2011 7:04 am 
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I just returned from a little trip...

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Oh yeah, and Tim... I'm cursing the post, not you!

:D

Don't even curse my post... the interference part comes straight out of the Gem-A course notes. These guys tend to know what they are talking about so I'm hesitant to ditch the interference idea altogether.

The scattering part has been explained brilliantly, sanks for that!


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