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Is this a twin?
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Duncan Miller




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PostPosted: May 15, 2019 09:33    Post subject: Re: Is this a twin?  

Hello Pete - Thanks for your very prompt response. The crystal does have natural etching. Here is a photograph showing the opposite prism face. Curvy or straight, or should one use magnification?


QUA338 Quartz, Erongo, Nambia 60x25x20 (2) compressed.jpg
 Mineral: Quartz
 Locality:
Erongo Region, Namibia
 Dimensions: 60x25x20 mm
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QUA338 Quartz, Erongo, Nambia 60x25x20 (2) compressed.jpg


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Bob Morgan




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PostPosted: May 15, 2019 11:38    Post subject: Re: Is this a twin?  

I doubt very much that this is a Brazil law twin. The two faces you propose as x faces are actually s faces. (See Drawing) They look like x faces because they are adjacent to large steep rhombohedral faces (the in between green faces).
With s faces on adjacent corners this is certainly a twin. As Pete said it could be either. Since clear evidence of Brazil Law twinning is lacking, it's more likely a Dauphene Law twin.

I'm sorry I can't get the drawing in this post. FOM doesn't allow the file.
Maybe Pete could tell me how to post a drawing.



FOM 1a.jpg
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FOM 1a.jpg


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Pete Richards
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PostPosted: May 15, 2019 11:50    Post subject: Re: Is this a twin?  

I thought maybe I saw that in the first image you posted, one the left side....

Those look like Dauphiné twin boundaries to me, sorry to say. Dauphiné-law twins are much more common, or much more commonly recognizable, than Brazil-law twins.

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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Bob Morgan




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PostPosted: May 15, 2019 12:01    Post subject: Re: Is this a twin?  

Duncan,
I just saw your second photo. That's definitely the sign of Dauphene Law twinning.
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Duncan Miller




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PostPosted: May 15, 2019 14:47    Post subject: Re: Is this a twin?  

Bob Morgan wrote:
I doubt very much that this is a Brazil law twin. The two faces you propose as x faces are actually s faces. (See Drawing) They look like x faces because they are adjacent to large steep rhombohedral faces (the in between green faces).
With s faces on adjacent corners this is certainly a twin. As Pete said it could be either. Since clear evidence of Brazil Law twinning is lacking, it's more likely a Dauphene Law twin.

Bob, thank you for the drawing, which explains the little triangular faces well. I have several undoubted Dauphiné twin quartz crystals from the Erongo, with the characteristic patchy etching that one can follow across adjacent faces, but assumed these two little faces were x and not s. Wishful thinking!
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Kevin Conroy




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PostPosted: May 15, 2019 14:56    Post subject: Re: Is this a twin?  

Hi Pete,

It's been a LONG time since I sat in a mineralogy class, but for the life of me I can't see a difference between the Dauphiné and Brazil twin drawings. Could you please point out what I should be looking for?

Cheers,
Kevin
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Bob Morgan




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PostPosted: May 15, 2019 16:45    Post subject: Re: Is this a twin?  

Kevin,
You got the point. With only s faces they wouldn't look different. They could be either.

Duncan,
As regards to Dauphene twinning revealed by light etching - it's interesting that when a twin boundary goes around an edge from r to z the etch pattern switches. That's because on one side of the edge it's an r face and on the other a z face.
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Kevin Conroy




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PostPosted: May 15, 2019 20:06    Post subject: Re: Is this a twin?  

Thanks Bob!
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Duncan Miller




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PostPosted: May 16, 2019 02:17    Post subject: Re: Is this a twin?  

Bob Morgan wrote:
As regards to Dauphene twinning revealed by light etching - it's interesting that when a twin boundary goes around an edge from r to z the etch pattern switches. That's because on one side of the edge it's an r face and on the other a z face.

Yes, that is very clear on several of my Erongo smoky quartz specimens. Amir Akhavan has a good photograph of an Erongo specimen showing this on his site The Quartz Page.
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