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Duncan Miller
Joined: 25 Apr 2009
Posts: 138
Location: South Africa
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 Posted: May 15, 2019 09:33 Post subject: Re: Is this a twin? |
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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?
| Mineral: | Quartz |
| Locality: | | Erongo Region, Namibia |  |
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| Dimensions: | 60x25x20 mm |
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Bob Morgan
Joined: 18 Jan 2018
Posts: 233
Location: Savannah, Georgia
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| Posted: May 15, 2019 11:38 Post subject: Re: Is this a twin? |
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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.
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Pete Richards
Site Admin
Joined: 29 Dec 2008
Posts: 833
Location: Northeast Ohio
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| Posted: May 15, 2019 11:50 Post subject: Re: Is this a twin? |
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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
Joined: 18 Jan 2018
Posts: 233
Location: Savannah, Georgia
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| Posted: May 15, 2019 12:01 Post subject: Re: Is this a twin? |
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Duncan,
I just saw your second photo. That's definitely the sign of Dauphene Law twinning.
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Duncan Miller
Joined: 25 Apr 2009
Posts: 138
Location: South Africa
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| Posted: May 15, 2019 14:47 Post subject: Re: Is this a twin? |
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| 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
Joined: 03 Dec 2018
Posts: 78
Location: Saint Louis, Missouri
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| Posted: May 15, 2019 14:56 Post subject: Re: Is this a twin? |
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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
Joined: 18 Jan 2018
Posts: 233
Location: Savannah, Georgia
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| Posted: May 15, 2019 16:45 Post subject: Re: Is this a twin? |
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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
Joined: 03 Dec 2018
Posts: 78
Location: Saint Louis, Missouri
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| Posted: May 15, 2019 20:06 Post subject: Re: Is this a twin? |
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| Thanks Bob!
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Duncan Miller
Joined: 25 Apr 2009
Posts: 138
Location: South Africa
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| Posted: May 16, 2019 02:17 Post subject: Re: Is this a twin? |
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| 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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