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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 9:10 pm 
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I also believe that to explain things simply you need to understand them in depth - even if you never use half the information in an exam answer.


Wise words, pandora.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 9:35 pm 
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In that case...

pandora wrote:
Adularescence in orthoclase moonstone is caused by the coherent scattering of light from the incident ray off the colloidal particles in nanometer-thick planes of alternating orthoclase and albite lamellae.

Replace "the incident ray off the colloidal particles in" with "50 - 1000"

pandora wrote:
The colour of the scattered light is dependent on the thickness of these exsolution lamellae planes: a small thickness plane of around 50nm scatters blue light, a medium thickness plane (150nm) scatters green light and a large thickness plane (450nm) scatters red light.

Replace "a medium thickness plane (150nm) scatters green light and a large thickness plane (450nm) scatters red light." with "and thicker planes scatter longer-wavelength light."

pandora wrote:
The shorter wave-length blue light scatters more than the light of longer wavelengths so the larger the distribution of small thickness planes in the stone, the more blue will be seen.

If the distribution of plane thicknesses are about equal - scattering red, green and blue light (the mix of which results in white light) - the stronger effect of the blue light scattered by the smaller planes will result in a pale blue colour. If the distribution is weighted towards the thicker planes then the colour will be seen as white as the blue light’s natural advantage is cancelled out.

This part is ok, but the type scattering needs to be identified. There are many different types of scattering, just as there are many different types of interference effects.

pandora wrote:
The ‘ghostly’ phenomenon seen in moonstone is due to ‘speckle interference’ in this coherently scattered light. Speckle is a granular pattern of intensity produced when light waves from a coherent source (in this case the light reflected from the lamellar planes) interfere with each other: if the waves are in phase with each other they combine and accentuate the crests and troughs of the wave resulting in a bright point. If the waves are out of phase and the trough of one wave meets the crest of another then they cancel each other out resulting in a dark point.

Because speckle is an interference phenomenon it appears to move as the stone is moved or viewed from different angles.

This is way too much detail, compared to the rest of the discussion. Keep only the first sentence.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Tue May 10, 2011 6:14 am 
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Brilliant! Thank you Brian, that should fit my

With the speckle bit, I just used the first sentence and stuck the rest into a footnote - reason being that speckle isn't something we cover and so if I don't explain it there'll be a query as to whether I actually know what it means or have I just copied a sentence out of a book...


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Wed May 11, 2011 10:31 am 
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Hi!

Thanks again to everyone for this interesting chat.

I've also noted that at times the Gemmology defintion differs from the classis science defintion for some terms. Having taught (secondary school) physics for 15 years I initially found this quite frustrating when I started studying gemmology; (just finishing the diploma course now). Now I have my gemmolgy definitions and my science definitions.

Another area where there is a difference is in 'play of colour' in Opals, which the GemA notes attributes to 'diffraction'. From a science point of view this not strickly correct, there is in fact diffraction, the bending and spreading of the waves, and interference, the intference of these diffracted waves that results in the colours seen.

Another simple term used differently is: diffusion. In science this is used quite differently.

in the GemA notes, interference is used both to refer to the interaction of two waves as well as to specifically refer to thin film interference. If we were to write that the the colour effect we see from a coated Topaz is due to interference, then according to our GemA notes we are talking about thin film interference. On the whole in our GemA notes interference=thin film interference.

As to Moonstone, the effect in the new notes, (where it has been changed from the old notes) is attributed to scattering due to particles:
From the new GemA notes: " This is also the cause of the blue colour in fine quality moonstones; the scattering is caused by particles of albite,"

I love to learn new things and I've really enjoyed reading Brians comments about coherent scattering from layers in Moonstone.

Just wondering, based on our GemA notes; does anyone know if in fact moonstones does have particles of Albite? Or prehaps they mean layes composed on closely paked particles?

thanks again!


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Wed May 11, 2011 10:41 am 
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The "particles" exist within layers.
To quote from Brian
Quote:
Adularescence in orthoclase moonstone is caused by the coherent scattering of light from the incident ray off the colloidal particles in nanometer-thick planes of alternating orthoclase and albite lamellae


From the first posts in this thread, I have found the use of the term "particles" to be misleading. The effect results from alternating layers of orthoclase and albite.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Wed May 11, 2011 11:46 am 
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Hi!

Just to clarify my question.

Since the GemA diploma notes write:
" The strength and shade of blue is dependent on the size, arrangement and shape of the particles and this scattering effect is most nticeable when the particles are smaller than 400 nm in size. This is also the cause of the blue colour in fine quality moonstones; the scattering is caused by particles of albite. This effect when seen in moonstone is sometimes called adularescence, sheen or schiller." Page D7-31

As well as the layer structure that we know about, (and it has been shown that this can cause the moonstone effect), I'm wondering if at GemA they have found research on the structure of Moonstone to show that it also have a particles, (of Albite as they say). I'm wondering what they exactly mean by 'particles of Albite'?

Thanks again!


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Wed May 11, 2011 12:33 pm 
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Correction to what I wrote previously.
In the colour chapter, of the new GemA notes, play of colour in Opal is attributed to a combination of diffraction and interference.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Wed May 11, 2011 2:07 pm 
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The explanation for the color in opals is reasonable. Not perfectly clear, mind you, but reasonable.

I would guess that "particles of albite" and "layers of albite" are describing the same thing.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 12, 2011 3:33 am 
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Thanks again.

I hope I'm not making things too much more complicated by asking this next question.
There is a statement in Gems 5th Edition by R. Webster that I had always wondered what it exactly meant.

p 665
Opalesence
Another optical effect due to internal reflection is the scattering of light by particles of matter making a visible 'beam' or 'colud' such as is caused by a a ray of light illuminating particles of dust floating in the air of a room. This is called the Tyndall effect and is the cause of the milky opalescence in some Opal, and in opalascent glasses and some plastics. It is also causes in part the optical effect in moonstone."

Again, can we assume Webster is referring to layers, in the case of moonstone, causing Opalesence?

On the previous page, the other cause that attributes the moonstone effect to is the 'intergrowth of albite and orthoclase in alternate layes", "reflection can occur at differing refractive index within the stone and such an effect is called sheen."

So, also, what exactly do they mean by opalesence in gemmology and how does it deffer from scattering in gemmology?


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 12, 2011 3:13 pm 
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The text doesn't use the words, but it is referring to opalescence as resulting from multiple Mie (or Debye) scatterings.** "Tyndall effect" is a not quite synonymous term for this type scattering. This scattering occurs from particles whose size is equal to or larger than the wavelength of light. If you shine white light on a volume containing Mie scatterers, the overall volume is filled with white light.

In my mind, Tyndall effect is not a perfect synonym for Mie scattering. Instead it usually refers to a slight enhancement of blue scattering by Mie particles. But it is a very small effect. The scattered white light may have a slight blue tint to it, but this blue tint usually isn't visually noticeable. Milk (undiluted by water) is full of Mie scatterers. When you see a glass of milk, do you see white, or do you see white with a blue tint?

So I'd re-write the text to say the whiteness seen in opalescence is due to Mie scattering.

Mie scattering could play a small role in white moonstone, since some lamellae in those stones could have size equal to or larger than light wavelengths. It has nothing to do with blue moonstone.

Now for the description of moonstone given on the previous page of the text. In this case, without using the words, the text is referring to thin-film interference. You can go back through this thread and read my opinion on thin film interference in moonstone.

**Not that it makes any difference, but my research specialization is Mie scattering by single particles.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 13, 2011 9:33 am 
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Hi Brian,
Thanks for your patience and can I please check a couple more things
The type of scattering, (eg Rayleigh, Tyndall, Mie) that occurs, amongst other things, depends on the wavelength of the wave compared to the size of the scattering particles.
Mie scattering, which is the type of scattering that Read refers to when he talks about opalescence, can be used, (in part) to explain the white reflection effect seen in moonstone. Is this type of scattering in moonstone due to the presence of single, spaced apart particles? (This is what I’ve been wondering about, have any Gemmologist found a small round spaced apart particle structure, for any moonstone with a reflection effect, possibly also with a layered structure as well).
The blue reflection effect we see in moonstone is due to scattering from the layers or lamellae. This is where I get a little stuck. Thinking about the wave theory of light, I find it hard to visualise how this actually occurs. Am I safe to think of this type of scattering occurring due to the particle structure of each layer? Then if I think of light as particles or photons I can imagine the light particles interacting with their ‘preferred’ or ‘correctly sized’ particles in the layered structure and as a result scattering occurring.

Thanks again, I appreciate that I’ve gone beyond the scope of this forum. The moonstone effect has been something that has bugged me for some time and I have not been able to find anyone to ask about it,


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 13, 2011 10:11 am 
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Am I safe to think of this type of scattering occurring due to the particle structure of each layer?


It has to do with the thickness of the layer.
Using the term "particles" causes misunderstanding.
The layers are microscopic lamellae of albite.
Adularescence is caused by Rayleigh scattering of light through alternating exsolution lamallae of orthoclase and albite.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 13, 2011 1:03 pm 
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diataon wrote:
The type of scattering, (eg Rayleigh, Tyndall, Mie) that occurs, amongst other things, depends on the wavelength of the wave compared to the size of the scattering particles.

These are physical theories of scattering whose mathematics define their range of applicability. So yes, the range of applicability for Rayleigh theory is that the characteristic length of particles must be smaller than the wavelength of light. And the range of applicability for Mie theory is that the characteristic length of particles must be equal to or larger than the wavelength of light. Tyndall effect is not a scattering theory.

diataon wrote:
Mie scattering, which is the type of scattering that Read refers to when he talks about opalescence, can be used, (in part) to explain the white reflection effect seen in moonstone.

The characteristic length of scatterers in a white moonstone can cover a range that overlaps the boundary between Rayleigh and Mie theory. The characteristic length of scatterers in blue moonstone lies squarely in the Rayleigh theory.

diataon wrote:
Is this type of scattering in moonstone due to the presence of single, spaced apart particles? (This is what I’ve been wondering about, have any Gemmologist found a small round spaced apart particle structure, for any moonstone with a reflection effect, possibly also with a layered structure as well).


Please... anything at all can be described as a particle. The Empire State Building can be described as a single particle, or it might be described as being composed of particles we'd call "floors." I am definitely not saying that the scattering in moonstone is due to well-defined little balls nicely separated from each other. All I am saying is the characteristic size of scatterers determines the applicable scattering.

The shape of the scatterers then determines other effects. In this case, the planar shape of the scatterers in blue moonstone give rise to coherent scattering, and the planar shape of scattering in opals give rise to thin-film interference effects in the scattering.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 13, 2011 1:53 pm 
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ooo Brian...so many types of scattering...and it's your current research topic...are you sure you don't want to do a chat (series of) on scattering??? (Just to get it all straight in your mind you understand)

Fascinating topic / subject


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 13, 2011 2:48 pm 
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Hi Brian,

Thanks for taking the time to help clear this up for me and apologies if I'm taking some time to grasp it.

"the planar shape of scattering in opals give rise to thin-film interference effects in the scattering."

I think I'll just leave than one!!


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