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 Post subject: Cause of adularescence in Moonstone
PostPosted: Wed May 04, 2011 12:29 pm 
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Following a discussion I had with a friend last night, we agreed to put the question here to see if a consensus was forthcoming!

The question concerns in particular top blue moonstone - the ones with the super-transparent colourless body and strong blue schiller - from Sri Lanka. These particular moonstones contain higher levels of sodium that those of lesser colour and hence more albite than orthoclase.

Having read pretty much every article on moonstone I could lay my hands on over the last couple of months, most attributed the predominant cause of the schiller to the optical interference effect caused by the refraction and subsequent reflection of light through the albite and orthoclase lamellae - the albite having a higher RI than the orthoclase.

Some sources attributed the cause to scattering from colloidal particles within the stone. This was specified as coherent scattering. This cause was mentioned as a secondary to the lamellar interference cause.

Can anyone cast light on the current thinking on this? Also as to whether the scattering is coherent or not.

Apologies if I've messed up explaing this....


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 12:42 am 
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It has been my experience that the fine, transparent "moonstone" with spectacular blue irridescence we have seen on the market for the past several years is transparent labradorite, displaying polysynthetic twinning. The adularescence is caused by inter-grown layers of Na-feldspar and Ca-feldspar. This is a relatively new find, within the last 15-20 years and can also termed "rainbow moonstone" when the sheen is multi-colored.
Image

In the past, moonstone was defined as a potassium, aluminum silicate (orthoclase) with an adularescence caused by the intergrowth of alternating layers of albite and orthoclase. This material generally has a more translucent, foggy character:
Image

Remember, moonstone is NOT a mineralogically recognized term.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 5:26 am 
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The particular stones in question are definitely not Labradorite. They are the blues from the Meetiyagoda mines in Sri Lanka that were discovered in 1906. I have the full chemical breakdown of the stones from there if that would be useful but in short they are sodium potassium aluminium silicate.

The vast majority of the stones found there have more translucent body (a good proportion retain the blue schiller but most have a white schiller) but around 4% of the production is totally transparent.

Those blue labradorites are beautiful - what is the location - and how do they compare in performance with the 'moonstones'. I have a particular fondness for the very transparent 'rainbow moonstone' labradorites, they're not that easy to find.

My main question though is to whether the cause is due to the lamellar structure or to scattering. The reason being that the course notes at Gem-A have changed - the old ones attributing to the lamellar structure and the new ones to scattering. A fellow student and I were having a friendly argument as to which of the two was the truth and/or current thinking!


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 6:30 am 
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Hi pandora,

I don't think it's 'either thin film interference or scattering' but both: first the lamellar structure causes some interference to take place, (enhancing the blue or suppressing the rest or at least setting the conditions for the scattering to take place, dunno which one) and then scattering takes place, making the whole thing so unique 'ghost-looking'. At least, that's how my Gem-A notes (from 2008) explain the phenomenon... (plus a little free interpretation)

The interference part I grasp, I think :D , but the scattering part requires a bit of Brian... orrr... perhaps Alberto can be the savior here... Albè, that Nassau book I gave you, doesn't that cover adularescense in detail?


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 7:21 am 
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Tim, this is what I thought - with the scattering being the secondary cause of the two. The new notes attribute the cause to light scattering off the albite particle in one part and then in another section it's attributed to thin film interference (thank you for that, I was trying to find that phrase!).

I'm also fine on that part, but the scattering part I'm a tad hmmm on my comprehension - especially as to the differences in coherent and incoherent scattering. My, probably erroneous, understanding being that the scattering in moonstone is 'coherent' so that the reflected light is all in phase and so is seen as blue rather than as rainbow colours. But then someone told me that the colour seen is dependent on the chemical composition of the particle the light is hitting.

Eugh, would be very grateful if someone who is a master of this stuff could come and do the Ladybird Book of Optical Phenomenon (ages 3-6) for me! :D


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 8:04 am 
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pandora wrote:
Eugh, would be very grateful if someone who is a master of this stuff could come and do the Ladybird Book of Optical Phenomenon (ages 3-6) for me! :D


And me!

I'm still kicking myself for dropping physics after 4 years of getting it during high school. I was tricked into dropping it by my s.o.b. teacher, Mr Krumm... I was part of a combo class with the Gymnasium kids (the smart ones, his favorites) which had just two of us Atheneum dumbos in it. I was a massive thorn in his eye, bad with homework, scoring c's and b's at tests and way more interested in what was happening in the schoolyard than on his blackboard. At the end of year 4 he approached me and promised me to give me a B on my end list if I promised to drop the subject. I did... he didn't... he gave me a C... the bastid... :evil:


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 10:27 am 
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pandora wrote:

Those blue labradorites are beautiful - what is the location - and how do they compare in performance with the 'moonstones'. I have a particular fondness for the very transparent 'rainbow moonstone' labradorites, they're not that easy to find.



The labradorite moonstones are coming from India. They are the finest examples I have see in what we loosely call moonstone. :D

The adularescent effect is not a mystery in either the orthoclase nor labradorite.

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.

For further information on the physics of the phenomenon,see:
"Rayleigh Scattering", George Rossman and E. Fritsch, G&G, Summer 1988

Hope this helps

The effect can also be seen in a glass of watered down milk. The glass must be clear and the milk watered down enough so that it is translucent. The milk has a bluish tint when illuminated in reflection. When illuminated in transmission, light seen coming through the milk looks reddish.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 12:37 pm 
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Barbra Voltaire wrote:
For further information on the physics of the phenomenon,see:
"Rayleigh Scattering", George Rossman and E. Fritsch, G&G, Summer 1988

Hope this helps

The effect can also be seen in a glass of watered down milk. The glass must be clear and the milk watered down enough so that it is translucent. The milk has a bluish tint when illuminated in reflection. When illuminated in transmission, light seen coming through the milk looks reddish.


A pretty good example. I can think of another. Rayleigh scattering is the answer to the question "why is the sky blue?" And it also answers the related question... If sunlight is white, then why do little kids grab the yellow crayon when they want to draw a smiley-face sun?

First a little background. The white light from the sun is composed of all colors; we can divide that color range into three rough ranges... red, green, and blue. Red light is characterized as low frequency light, blue light is high frequency light, and green light is mid-frequency light. White light is a combination of all frequencies, which we can write as an addition: white = red + green + blue. A little more simple color addition of light: red + green = yellow (try overlapping a red spotlight and a green spotlight on a white surface... amazingly you see yellow... I love that demonstration). Also red + blue = magenta and green + blue = cyan.

Now, light from the sun traveling through the atmosphere tends to hit molecules in the atmosphere. Let's look at what can happen in this sort of collision. If the light is propagating straight down before colliding with a molecule, then after the collision there is some probability it will continue propagating straight down (it is not scattered) and some opposite probability that is will propagate in some random direction that is not straight down (this is scattering). For visible light, small molecules or structures composed of only a few atoms scatter high frequency light (blue light). Molecules or structures composed of a few more atoms scatter mid frequency light (green) and those composed of even more atoms scatter low frequency light.

The molecules in our atmosphere... nitrogen, oxygen, water vapor... are composed of two or three atoms... a very few atoms. So these molecules scatter blue light. How do you locate the sun in the sky? Light traveling from the sun through the atmosphere that isn't scattered follows a straight line down into your eye. What color light isn't scattered by the atmosphere? Red + green = yellow. So when you locate the sun, you see a yellow spot. What happens to the blue light from the sun? It is scattered, which means it is sent in every random direction. Thus blue light is spread throughout the atmosphere. You look up somewhere where the sun isn't and you see blue light coming at you, thus the entire atmosphere (the sky) appears blue.

Note that the red and green light from the sun that gets into your eye along a straight-line path hasn't been scattered. But the blue light from the sun that gets into your eye has been scattered at least twice... once to direct it to some other portion of the sky, and once again to direct it from that portion to your eye.

Why are clouds white? Well they are combinations of water molecules, and these combinations cover all size ranges... small, medium, and large. So they scatter red + green + blue = white. So all colors of sunlight are scattered through the cloud and it looks white. Why do thicker clouds look darker and darker grey? Well, that is because the thicker cloud starts absorbing light rather than scattering, and that is a different topic.

Ever closely look at smoke rising from a cigarette? It looks blue. What size molecules are in that smoke? Small molecules. Ever see cigarette smoke exhaled by a person? It looks white. Why the color change? Smoke molecules passing in and out of a person can trap water molecules, so that some bits remain small, some acquire a water molecule or two and grow medium, and some acquire more than a few water molecules and grow large.

You see smoke coming out of some industrial chimney, and it is yellow = red + green. What size particles are coming out of the chimney? Medium and large.

Referring back to Barbra's example... you shine white light on the milk. There are all sorts of molecule sizes in the milk suspension, but small molecules tend to predominate. So the milk scatters all colors (making it look white) but scatters more blue light than the others (giving it a bluish tint). Light making it straight through to the other side has had more blue light subtracted from it, so it has a yellowish tint.

Ok, so here is your first test question. After climbing to the top of Mt. Everest, you notice the sky looks a lot darker blue than when you were at sea level? How come?

Ok, so here is your second test question. You are standing on the moon. During daytime, what color is the sky? What color is the sun?


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 4:05 pm 
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Thanks Brian...

to be a good student:

1. thinner atmosphere up there, gravity allows only the lightest (read: smallest) molecules to float around at that altitude-> less scattering of green -> a purer blue.
2. black - white (no atmosphere)

In the case of milk, smoke & air it's clear as a whistle... lot's of stuff floating around in there, but what's causing the scattering in layered albite/orthoclase?

Barbra wrote:
Quote:
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.


Apparently there is more than just thin film interference... the question is what the role of the scattering is and where it's coming from.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 5:30 pm 
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Tim wrote:
1. thinner atmosphere up there, gravity allows only the lightest (read: smallest) molecules to float around at that altitude-> less scattering of green -> a purer blue.
2. black - white (no atmosphere)


1. Close. The atmosphere is thinner up there, but also the layer of atmosphere between you and empty space is much thinner. So... not as much "sky" between you and the edge of space to fill with blue light. So darker blue means less blue light (and the sun would look whiter up there)

2. Correct. It is the extension of question 1, where you move to no atmosphere at all above you.

Tim wrote:
In the case of milk, smoke & air it's clear as a whistle... lot's of stuff floating around in there, but what's causing the scattering in layered albite/orthoclase?
...the question is what the role of the scattering is and where it's coming from.


Scattering happens from very small structures. In the case of the atmosphere, the small structures are air molecules embedded in "nothing". In the case of the milk, the small particles are milk structures embedded in liquid water. If we were talking about small water structures, like molecules, then the water could scatter. But liquid water is a bulk structure and doesn't scatter. It is the fragmentation of milk into minute bits that does the scattering.

So for your stone, we can ignore the bulk structure material. It is some very small scale structure... the colloidal particles pandora refers to... embedded in the bulk material that does the scattering. "Colloidal particles" is secret code for "very small bits of stuff usually on the nanometer to hundreds of nanometer size," which is exactly the size objects you need to scatter light. What the particles are, I don't know.

So just as there are little molecules floating around in the air, and there are little colloids of milk whey and/or fat floating around in the water, apparently there are little particles floating around in the stone... they just don't move around quite as much as the other floaty particles.

The role of the scattering in the stone is to produce the ghostly blue light emanating from all regions of the stone. If you bump up the number of scatterers and increase the thickness of the stone, the ghostly blue would become more well-defined (just as denser and thicker atmosphere down at sea level makes bluer sky, compared to up on Mt. Everest).

And look at that. You probably thought... why is he bothering with these non-gem exam questions. Yet those questions connect directly to the last question you asked. Subtle, I am. ;)

Oh and one last thing. Interference could also cause the blue light. The information on hand doesn't prove the colloids exist, nor does it prove that the interlayers are aligned for interference. So it is a toss-up whether one or the other is the dominant cause, or whether both contribute equally.


Last edited by Brian on Thu May 05, 2011 5:52 pm, edited 1 time in total.

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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 5:51 pm 
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Quote:
"Colloidal particles" is secret code for "very small bits of stuff usually on the nanometer to hundreds of nanometer size,"


:D

Subtle you are...

Quote:
For further information on the physics of the phenomenon,see:
"Rayleigh Scattering", George Rossman and E. Fritsch, G&G, Summer 1988


Perhaps Rossman & Fritsch mention the identity of the colloidal particles? I don't have that issue and have ditched my subscription so no online access for me... :?


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 6:05 pm 
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ah you added something...

Quote:
Oh and one last thing. Interference could also cause the blue light. So with the information on hand, to me it is a toss-up whether one or the other is the dominant cause, or whether both contribute equally.


I guess the thin film interference isn't destructive to the other colors but is constructive to the higher frequencies, just intensifying the blue. After all, moonstone appears white/clear, instead of dark like common labradorite which gets its wild colors from thin film interference alone. That phenomenon got a name of its own: labradorescense.

Does that sound plausible?

Building on that: when the blue is intensified by constructive interference, could scattering by those colloidal particles then give the stone its ghostly character? So... could the scattering be the cause of the appearance rather than the color?

I'm lead to think the scattering doesn't play a big role in the blue because I have seen various doublets of slices of moonstone with quartz domes above 'm (no colloidal particles in that quartz) and they are blue as...


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 6:10 pm 
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Wow! =D> Thank you so, so much for the great answers - and for explaining things in such a simple manner that I think I might just have got it all this time!

I'm rather glad that Tim got there before me on the test questions and so I've got out of having to do them, but very useful for explaining the whole scattering effect.

I'd read that the higher and thicker the cabochon the more concentrated the blue colour but the smaller the field.

Does the issue of coherent versus incoherent scattering come into play here at all or is it not relevant?

(Is anyone doing revision classes for the FGA Diploma exams this year? [-o< )


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Thu May 05, 2011 7:50 pm 
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pandora wrote:
I'm rather glad that Tim got there before me on the test questions and so I've got out of having to do them, but very useful for explaining the whole scattering effect.

That's ok, Tim is used to the pop quizzes.

Tim wrote:
I'm lead to think the scattering doesn't play a big role in the blue because I have seen various doublets of slices of moonstone with quartz domes above 'm (no colloidal particles in that quartz) and they are blue as...

pandora wrote:
I'd read that the higher and thicker the cabochon the more concentrated the blue colour but the smaller the field.

pandora wrote:
Does the issue of coherent versus incoherent scattering come into play here at all or is it not relevant?

:smt102

Tim wrote:
I guess the thin film interference isn't destructive to the other colors but is constructive to the higher frequencies, just intensifying the blue.

Not quite. The thin film interference is destructive to all other colors reflecting off the pair of surfaces. But not all light going into and coming back out again has to result from reflection off those two layers.

The pretty picture of blue glass with orange light shining through on the wikipedia topic of Rayleigh scattering gives me an idea. If you shine white light through the stone and out comes red-orange-yellow, then maybe scattering. If you shine white light through it and out comes white, then maybe thin film. But as my luck has been lately with devising tests, probably you'll see white with just a hint of orange.


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 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Fri May 06, 2011 12:23 am 
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Getting back to the cause of adularescence in feldspars, I think a few terms have to be defined in order to fully understand WHY the layering causes the phenomenon.

Regular stacking of alternating layers cause diffraction effects producing PURE spectral colors because the layers have different indices of refraction and different chemical composition. This is not to be confused with iridescence which is a combination of several spectral colors.

These alternating layers are known as exsolution lamellae.
The colors created by the exsolution lamellae are dependent on their thickness, coupled with their indices of refraction.

Diffraction can only occur within a relatively narrow range: 100-400nm. When the irregular components in a gem are outside of this range, visible light can not be diffracted, BUT these irregularities can cause scattering.

Scattering is a process where light entering a stone is deflected in different directions. This scattering can create interesting optical phenomenon.

When scattering centers are irregularly distributed and smaller than the wavelength of visible light it is called Rayleigh Scattering.

Essentially, violet and blue produce the most energetic radiations and are scattered much more strongly than the other components of the spectrum (16X more efficiently than red). This results in the less energetic components, red and orange, passing through the stone as transmitted light.

Violet and blue light is vibrantly scattered and can be observed perpendicular to the incident beam.

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?
Quote:
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.

:D


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