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Brazil Law?
    
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Scott LaBorde




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PostPosted: Dec 16, 2009 20:57    Post subject: Brazil Law?  

I have been calling this a Brazil law twin, but hopefully not incorrectly. I believe that is an s and x face on the left side and an x face on the right side of the large r face. Am I correct?


bltwin2.JPG
 Description:
Specular hematite included phantom in quartz. Personally collected in Chatham County, NC.
4 x .7 cm
 Viewed:  16150 Time(s)

bltwin2.JPG



bltwin4.JPG
 Description:
I outlined what I believe is the s face in green and the x faces in pink.
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bltwin4.JPG


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Scott LaBorde




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PostPosted: Dec 16, 2009 20:59    Post subject: Re: Brazil Law?  

Here are the faces zoomed in. Amir says that a Brazil Law twin "in which the twinning is reflected in the position of crystal faces are extremely rare". (The Quartz Page) So, this is probably a Dauphiné twin?

Relative to the m faces the x faces are in the upper corners. Perhaps a right handed Dauphiné twin.



bltwin5.JPG
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zoomed
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bltwin5.JPG


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Pete Richards
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PostPosted: Dec 16, 2009 21:41    Post subject: Re: Brazil Law?  

I'm afraid not.... The big (relatively) face on the left certainly appears to be an s face, but the face underneath it, if it's an x face, should have parallel edges, not be wedged shaped. This might be explained by imperfections in the prism face underneath, however, so it might really be an x face. However, under Brazil law twinning the two x-faces should be mirror images, and the one on the left should appear "on" the front prism face, not where it is. Dauphiné twinning would place the left x-face where it is in your pictures.

You might look at the surrounding faces in reflected light. With Dauphiné twinning, you can often detect the twin boundary as a wavy line separating areas with separate luster/striations/textures that demonstrate the presence of the twinning.

The attached drawings of ideal quartz twins are scanned from Frondel (1962) Dana's System of Mineralogy, 7th edition, volume 3.



Quartz twin.jpg
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Quartz twin.jpg



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Scott LaBorde




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PostPosted: Dec 16, 2009 21:55    Post subject: Re: Brazil Law?  

Pete thanks so much for the excellent response and illustration. Do I look at the surrounding m or the r and z faces for the twin boundaries?
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PostPosted: Dec 16, 2009 22:25    Post subject: Re: Brazil Law?  

A wonderful crystal. Proof pudding of Helmuts profound observation "a quartz specimen scratches every itch".

Brazil twin, I don't think so. I not sure of Dauphine either but would like to see it in person.

Thank you for the photo, it must have been a grinning good day collecting.

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PostPosted: Dec 16, 2009 22:38    Post subject: Re: Brazil Law?  

Before John jumps in here, I'm going to suggest that the phantom here beats whatever twinning law might be present...remembering Frondel's comment that an "untwinned quartz crystal may be the rarest thing in nature" (paraphrased).
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PostPosted: Dec 17, 2009 05:16    Post subject: Re: Brazil Law?  

To get back to the original question, I believe it is clear that the face outlined in green is an "s" face, but I have no idea what the other two faces are or even if they are related.
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PostPosted: Dec 17, 2009 07:21    Post subject: Re: Brazil Law?  

dauphiné law
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Pete Richards
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PostPosted: Dec 17, 2009 09:42    Post subject: Re: Brazil Law?  

Scott LaBorde wrote:
Pete thanks so much for the excellent response and illustration. Do I look at the surrounding m or the r and z faces for the twin boundaries?


The Dauphiné twin boundaries are usually easiest to see on the r/z faces, but if the twinned zone extends down onto the prism, it can usually be seen there as well.

Attached is a drawing I made of a (hypothetical) Dauphiné twin with iron oxide coating just one set of the terminal faces. Note that Dauphiné twinning has the effect of placing domains of r-faces into z-faces and vise-versa.



Dauphiné.jpg
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Dauphiné.jpg



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Amir Akhavan




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PostPosted: Dec 17, 2009 11:58    Post subject: Re: Brazil Law?  

I agree with Pete that the relative positions of the faces is the key. If the small left one was an x-face on a Brazil twin, it would be on the right side of the s-face and the left side of the frontal m-face.
So it's more likely a Daupiné law twin.

However, one should not rely on the shape of a face to determine it, s- and x-faces can look very odd on crystals with a non-normal habit.
To me the left small face could very well be an x-face.

Amir
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PostPosted: Dec 17, 2009 12:35    Post subject: Re: Brazil Law?  

This is the reason I joined this forum. Thank you Pete and Amir for getting me straight. I comprehend what you have said and understand so much more about twinning now. This knowledge of twinning and handedness of crystals has revitalized interest in all my quartz crystal specimens, allowing me to stare, discover, and admire them all over again.

Scott



0.JPG
 Description:
Here's another crystal with what seems like a text book s and x face.

Personally collected
Chatham County, NC
 Viewed:  15922 Time(s)

0.JPG



1.JPG
 Description:
large s face

Personally collected
Chatham County, NC
 Viewed:  15920 Time(s)

1.JPG


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Pete Richards
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PostPosted: Dec 27, 2009 17:36    Post subject: Re: Brazil Law?  

Today I stumbled onto an interesting, if old, paper on twinning on quartz. It deals primarily with the parallel-axis twins i.e. Brazil and Dauphiné and mixes of them. It's in American Mineralogist, but it should not be hard for many of you, even if not trained in mineralogy, to gain useful information from it.

See http://www.minsocam.org/msa/collectors_corner/arc/qtztwin.htm
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PostPosted: Mar 05, 2011 22:52    Post subject: Re: Brazil Law?  

Hi - just another treasure from my Tucson adventure:

The snapshot below shows a slice of chalcedony held up to my computer monitor on which is displayed a photomicrograph I'd taken of one of the transparent areas between crossed polars. These are single quartz crystal inclusions which were cut perpendicular to their c-axis when the lapidary artist fashioned the piece.

There is endless fun to be had with it - a veritable light show under the microscope. I have a hobby of collecting as many variations of optical phenomena as I can find, including optic figures and anomalies. I've been learning how to decipher the left and right twin lamellae in other examples - the Brewster fringes in eye-visible Brazil law twinning make that easier -- I've not been able to do it in this as it is too fine of a pattern (I haven't quit yet). It is thin enough to observe under the petrographic microscope: it is interesting that the very center of each crystal is untwinned and immediately out from that, the twinning starts.

In the paper that Pete recommends above, I wonder what the combination of twinning would produce. Any comments are very welcome, including convincing me otherwise of course - I just think it is really neat!

Cheers!
Elise
(Jordi - please pardon pictures of a butchered specimen, but I am also studying natural and synthetic quartz; relating what I see under the microscope to the original crystal to grasp what is going on. The previous discussion is very interesting and helpful)



falk34a.jpg
 Description:
Chalcedony with single crystal quartz inclusions. In the background: photomicrograph of one of the single crystals as seen between crossed polars. Photo: Elise Skalwold
 Viewed:  13995 Time(s)

falk34a.jpg



caxis.jpg
 Description:
Several views under crossed polars and one without. The inclusions are tentatively thought to be sillimanite based on similarity to known specimens (photo: Elise Skalwold).
 Viewed:  13994 Time(s)

caxis.jpg



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xenolithos




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PostPosted: Mar 09, 2011 01:42    Post subject: Re: Brazil Law?  

Elise, you might like this. A photograph of a basal section through a quartz crystal, showing isolated domains of Brazil law twinning. (The black blob is my finger holding the quartz and polaroid in front of the computer screen.) Slightly closer up the camera captured a perfect uniaxial interference figure.

Duncan



Quartz slice (001).jpg
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Quartz slice (001).jpg



Uniaxial interference figure.jpg
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Uniaxial interference figure.jpg


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PostPosted: Mar 10, 2011 10:54    Post subject: Re: Brazil Law?  

xenolithos wrote:
A photograph of a basal section through a quartz crystal, showing isolated domains of Brazil law twinning. (The black blob is my finger holding the quartz and polaroid in front of the computer screen.) Slightly closer up the camera captured a perfect uniaxial interference figure.

I think that is a neat trick Duncan! Why does the optic figure resolve without a conoscope or curved surface on the specimen....a function of the camera lens? I found my monitor is polarized diagonally; my iPod screen is useless because even though I have a very useful flashlight "Ap" on it which is bright enough to shine through small crystals, the protective sheet I applied to save it from scratches is very birefringent and so useless with filters (as it had been suggested in latest G&G). Rather than posting any more sliced and diced specimens, I threw some up on a page I've been working on - there are probably errors, but I can only work on it when I have time here and there -- any comments, insights or corrections are welcome:
http://www.nordskip.com/ametrine.html
(link normalized by FMF)

Cheers!
Elise

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Elise




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PostPosted: Mar 10, 2011 17:47    Post subject: Re: Brazil Law?  

I think that is such a neat trick that I had to try it too....


monitor.jpg
 Description:
Ametrine quartz crystal sliced perpendicular to the c axis and cut in a hexagonal shape. The amethyst sections show Brazil law twinning and dark Brewster fringes when held in crossed polarized light, between the computer monitor and a polarizing filter.
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monitor.jpg



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marvsT/Nminerals




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PostPosted: Mar 11, 2011 14:10    Post subject: Re: Brazil Law?  

Pete Richards wrote:
Today I stumbled onto an interesting, if old, paper on twinning on quartz. It deals primarily with the parallel-axis twins i.e. Brazil and Dauphiné and mixes of them. It's in American Mineralogist, but it should not be hard for many of you, even if not trained in mineralogy, to gain useful information from it.


The article from the American Mineralogist mentions etch figures. When a quartz crystal has natural etch figures on alternate "M" faces only, and none on the "R" or "Z" faces is it possible evidence of Dauphine law twinning? These crystals seem to be off of Fisher Mountain near Mount Ida, Arkansas.

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PostPosted: Mar 11, 2011 15:19    Post subject: Re: Brazil Law?  

marvsT/Nminerals wrote:
The article from the American Mineralogist mentions etch figures. When a quartz crystal has natural etch figures on alternate "M" faces only, and none on the "R" or "Z" faces is it possible evidence of Dauphine law twinning? These crystals seem to be off of Fisher Mountain near Mount Ida, Arkansas.


I think the answer is no. The M faces, the prism faces, are all part of the same form and so all should have the same etch pits. However, according to the American Mineralogist article, these etch pits may not be symmetric. If so, because of the symmetry of quartz, they should point up on one M face and down on the adjacent M faces, alternating around the crystal. If the crystal has Dauphiné twinning, this pattern would reverse itself on opposite sides of the twin boundary. So the mere presence of etch pits on adjacent M faces would not prove twinning, but twinning might be revealed by the details of their orientation.

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PostPosted: Mar 14, 2011 09:27    Post subject: Re: Brazil Law?  

Elise wrote:
Why does the optic figure resolve without a conoscope or curved surface on the specimen....a function of the camera lens?


Hi Elise,

I have no idea why it works - just discovered it by accident. The interference figure appears close up with my digital point-and-shoot, and not with the close up macro lens on my old SLR. At the same magnification that just produces diffuse interference colours. It must have to do with the lens design of the digital camera. Perhaps someone like Olaf Medenbach could help us here.

Here is another cute picture, taken the same way, with the interference colours in one twin showing up twinning in a flattish cleavage rhomb of calcite.

Duncan



Twinning in calcite.jpg
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Twinning in calcite.jpg


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