January 24 Through February 4—TUCSON, ARIZONA: Annual show
Welcome to the GemologyOnline.com Forum
A non-profit Forum for the exchange of gemological ideas
It is currently Thu Mar 28, 2024 5:12 am

All times are UTC - 4 hours




Post new topic Reply to topic  [ 62 posts ]  Go to page Previous  1, 2, 3, 4, 5  Next
Author Message
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sun May 08, 2011 11:21 am 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 21602
Location: San Francisco
In addition to the sources I mentioned in an earlier post, I just checked the Gem-A Diploma Course notes I have from 2008 and looked up Feldspar- Moonstone: (the following is copied directly from page Fel-2)
Quote:
Although mostly potassium aluminum silicate these stone consist of extremely fine intergrown alternating layers of orthoclase (potassium aluminum silicate) and albite (sodium aluminum silicate).
Moonstone is often translucent and can be colorless, white, pink, orange, yellow, green, brown or grey. Moonstone may show a bluish to silvery-white reflection effect caused by light reflecting from microscopic layers of the two types of feldspar. Moonstone can also be cut to display chatoyancy.


And, from Webster's 5th Edition Gems, Their Sources Description and Identification page 208, on Orthoclase quoting directly:
Quote:
The important orthoclase gemstone is moonstone, a gem which is characterized by a beautiful blue schiller shown by the better quality specimens. This lovely sheen, termed adularescnce, is due to a combination of orthoclase and albite- one member of the plagioclase series-arranged in layers. Light reflected from thee layers, if they are not too thick, produces by interference effects, the blue schiller in recent studies called "Rayleigh Scattering". If the layers are thick the sheen is white and the stone is far less attractive.


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sun May 08, 2011 12:15 pm 
Offline
Valued Contributor

Joined: Sat Jan 26, 2008 4:55 pm
Posts: 179
Location: London, UK
Barbra Voltaire wrote:
I just checked the Gem-A Diploma Course notes I have from 2008 and looked up Feldspar- Moonstone: (the following is copied directly from page Fel-2)
Quote:
Although mostly potassium aluminum silicate these stone consist of extremely fine intergrown alternating layers of orthoclase (potassium aluminum silicate) and albite (sodium aluminum silicate).
Moonstone is often translucent and can be colorless, white, pink, orange, yellow, green, brown or grey. Moonstone may show a bluish to silvery-white reflection effect caused by light reflecting from microscopic layers of the two types of feldspar. Moonstone can also be cut to display chatoyancy.


That part is also in the current notes under Feldspar, but if you read the section on 'Scattering' in Chapter 7: Colour then it reads:

Quote:
Scattering is the cause of various effects in gemstones. In simple terms it occurs when light is randomly scattered (or reflected) by particles in a substance. Blue is typically more strongly scattered than red light.... (section on milk experiment etc)... The strength and shade of blue is dependent on the size, arrangements and shape of the particles and this scattering effect is most noticeable when the particles are smaller than 400nm in size. This is also the cause of the blue colour in fine-quality moonstones; the scattering in this material is caused by particles of albite


This is part of the reason why I posed this question here. In the feldspar section it talks about thin-film interference being the cause of the colour and then in the Colour section it talks about scattering.

Brian here is attributing the phenomenon to scattering and now Tim is saying he doesn't want to rule out the interference theory... ](*,) :smt022


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sun May 08, 2011 12:29 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 21602
Location: San Francisco
Quote:
the scattering in this material is caused by particles of albite


Perhaps, "layers of albite", not "particles of albite", would have been more accurate, as these "particles" are not typically, randomly disseminated within the orthoclase moonstone. It appears to be rather widely accepted, albite exists in chemically uniform layers within the orthoclase moonstone. :D


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sun May 08, 2011 4:16 pm 
Offline
Valued Contributor

Joined: Sat Jan 26, 2008 4:55 pm
Posts: 179
Location: London, UK
Barbra Voltaire wrote:
Quote:
the scattering in this material is caused by particles of albite


Perhaps, "layers of albite", not "particles of albite", would have been more accurate, as these "particles" are not typically, randomly disseminated within the orthoclase moonstone. It appears to be rather widely accepted, albite exists in chemically uniform layers within the orthoclase moonstone. :D


I guess it all comes down to the definition of a 'particle'...


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sun May 08, 2011 11:22 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1499
The effect you see in moonstone is exactly as I explained... coherent scattering of light from the thin sheets of the lamellae.

When I mentioned particles, all I was saying is that any thin plane sheet of material can be considered an assembly of smaller pieces of the material in close contact. Like a chocolate bar... the whole chocolate bar is made of pieces of chocolate connected together. The reason I brought up this point, as one poster mentioned, is that people typically think of scattering as resulting from particles, but if you assemble the particles into a plane whose thickness mirrors the particle size, then the scattering can result from the plane.

Scattering from separated particles is incoherent, but scattering from a plane is coherent. When your notes say that there is an interference effect, it is quite unfortunate that they don't actually name the effect. The interference effect is not thin film interference. It is another type of interference that arises in coherent light.

A defining characteristic of laser light is that it is coherent. During the first times I saw a spot of laser light on a screen, I saw little black dots in the beam. I thought my eyes were playing tricks on me; the beam must be so bright, I am just dazzled and I am imagining those black dot. So I was amazed when I found out they were real. The phenomenon is called speckle, and it results from interference in coherent light.

It is interesting to note that the wiki article mentions speckle resulting from sunlight scattering off a fingernail. That is another instance of coherent scattering off planes.

And so, just as I show in a drawing on the previous page, the coherent scattering has little black dots mixed into the scattered light.

In that earlier description of coherent scattering, I didn't name the result as an interference phenomenon, because I didn't want anyone to be misled into thinking it was thin film interference, which is normally the only type people have encountered. Just as when I write the word "cube," a geometric shape where all side lengths must be equal, some people will think I must be talking about stretching it out along one side. And so if I write the word interference, someone is bound to think I must be talking about thin film interference.

So... there is no thin film interference. Because of the possible thickness ranges of the lamellae, if thin film interference were important, the result would be iridescence. There is speckle interference in the coherently scattered light, and this particular interference effect causes the "ghostly" phenomenon seen in the light.


Last edited by Brian on Mon May 09, 2011 12:11 am, edited 2 times in total.

Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Sun May 08, 2011 11:34 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 21602
Location: San Francisco
Quote:
The effect you see in moonstone is exactly as I explained... coherent scattering of light from the thin sheets of the lamellae.

And may I add, explained coherently, succinctly, accurately and in far greater detail than I have seen from most other sources. :D Excellent job, Brian.


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 12:00 am 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1499
Thanks Barbra.

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

I was just teasing. That was a really clever post; it made me laugh.

Alas, it has been those family thingys that has prevented my participation in chats.

I will see if sometime in the next few weeks I can participate in a chat.

As for coherent vs. incoherent scattering... hmm, I don't know.


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 4:17 am 
Offline
Moderator
User avatar

Joined: Thu Dec 13, 2007 3:42 pm
Posts: 4091
Location: the Netherlands
Quote:
Brian here is attributing the phenomenon to scattering and now Tim is saying he doesn't want to rule out the interference theory...


hehe... I'd believe Brian if I where you, I'm just citing texts I trust, Brian here is the one who has a thorough comprehension of physics.

I just re-read that bit of text again (Diploma coursenotes version 2003 - Chapter 23 - Causes of colour - Physical optics - Scattering (page 32) and here's what it reads:

Quote:
The reflective effect seen in moonstone, an orthoclase feldspar, is a result of thin-film interference plus the scattering of light from alternate layers of feldspar with slightly differing composition and structure. Together these cause a soft silvery or blueish iridescence. If the intergrowth of layers is on an ultra fine scale, then the iridescence is blue. This effect in moonstone is sometimes called 'adularescence', 'sheen' or 'shiller'; however, the effect is best termed moonstone iridescence.

Pale, transparent labradorite feldspar with multicoloured iridescence is often called 'rainbow moonstone' in the trade; however its iridescent colours are caused mainly by tin-layer interference and it differs from traditional moonstone in composition and appearance.


You can see why I was reluctant to let the thin film interference part go... It's not just that text either btw... I could so easily imagine it... all the ingredients are present... thin layers of material with slightly alternating refractive indices stacked on top of each other. I reckoned it would be hard not to get some thin film interference going on in there... but...

Brian wrote:
Quote:
So... there is no thin film interference. Because of the possible thickness ranges of the lamellae, if thin film interference were important, the result would be iridescence. There is speckle interference in the coherently scattered light, and this particular interference effect causes the "ghostly" phenomenon seen in the light.


The bold sentence is the key here I guess. At least it was what made my brain click...


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 8:06 am 
Offline
Valued Contributor

Joined: Sat Jan 26, 2008 4:55 pm
Posts: 179
Location: London, UK
Looks like they've changed their minds with the new notes.

I'm currently rewriting the section of my project that talks about the cause of adularescence - I just hope that I don't get marked as wrong for having the info I have learnt here, but I'd rather be right and lose the marks...

I shall have to add 'Brian the Physics Professor' to my bibliography! :D


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 8:53 am 
Offline
Valued Contributor

Joined: Sat Jan 26, 2008 4:55 pm
Posts: 179
Location: London, UK
Could someone check this for me and see if it sounds right:

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.

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.

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.

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.

[-o<


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 11:28 am 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 21602
Location: San Francisco
Seriously?

Would Gem-A would expect an answer like this if a student were asked to explain adularescence in moonstone. Geeze, I hope they don't ask to compare and contrast phenomenon seen in moonstone, labradorite and opal. :| Half the student body would just jump off a bridge.

And, by the way you've completely left out the physics involved with Rayleigh scattering. :wink:

PS. I've noticed that many gemological texts use the term "interference" but I think they use it incorrectly from the standpoint of a correct definition within physics, as I was earlier.


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 2:24 pm 
Offline
Site Admin

Joined: Thu Jan 05, 2006 4:27 pm
Posts: 1750
Quote:
PS. I've noticed that many gemological texts use the term "interference" but I think they use it incorrectly from the standpoint of a correct definition within physics, as I was earlier.


There was a report in the Gem-A's recent journal of gemmology which discussed misused terminology especially with regards to the refractometer, but stating cases where Peter Read, Anderson et al actually got it a wee bit wrong.

I don't think the Gem-A would mark you down for not knowing the physics behind Rayleigh scattering...Though if you were trying for the Tully or some other prize it wouldn't do you any harm to mention it

Personaly I think this depth of knowledge is in advance of basic educational requirements and should probably come under the "you never stop learning even after you're qualified" coursework.

Kudos to pandora for opening this particular box of delights in her student studies though =D>


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 2:48 pm 
Offline
Site Admin
User avatar

Joined: Sun Oct 16, 2005 12:22 pm
Posts: 21602
Location: San Francisco
Quote:
Kudos to pandora for opening this particular box of delights in her student studies though =D>


Agreed.
Please feel free to delve into any other topic Pandora. This was one of the most interesting threads I can recall in the recent past. :D


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 5:40 pm 
Offline
Gold Member
User avatar

Joined: Mon Apr 16, 2007 12:05 am
Posts: 1499
pandora wrote:
Could someone check this for me and see if it sounds right:

No need to tell everything you know. I would suggest you pare down your answer to one sentence. Twenty-five words or less.


Top
 Profile  
Reply with quote  
 Post subject: Re: Cause of adularescence in Moonstone
PostPosted: Mon May 09, 2011 7:41 pm 
Offline
Valued Contributor

Joined: Sat Jan 26, 2008 4:55 pm
Posts: 179
Location: London, UK
Aw, thank you :oops: , no doubt something will crop up during revision - I'm going through past papers right now so I'm sure they'll throw up some goodies!

I should have probably explained... there is now a 20% course-work component to the FGA Diploma (it counts towards the overall grade but not towards medals etc). One of the assignments is a 1500 word project on one of a number of topics. I chose a study of the Meetiyagoda moonstone mines which covered the general lithology of Sri Lanka, moonstone itself and a report on the actual mines.

As I was limited to 1500 words (not including footnotes :twisted:) I had to super-condense each section - hence why I couldn't properly go into Rayleigh Scattering.

I've never been much good at just accepting course notes if I don't totally get something. It's a bit like when I was learning to drive I had to find out exactly how the car and the gears and everything worked before I was able to make sense of the actual driving part. :roll: Some parts of the course I am happy to just rote learn - mainly the bits that don't actually interest me that much (diamonds) - but others I want more than a two line explanation that I have suspicions isn't the whole story.

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.


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 62 posts ]  Go to page Previous  1, 2, 3, 4, 5  Next

All times are UTC - 4 hours


Who is online

Users browsing this forum: No registered users and 26 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
Gemology Style ported to phpBB3 by Christian Bullock