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Elise
Joined: 22 Dec 2009
Posts: 243
Location: New York State
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 Posted: Mar 10, 2011 17:47 Post subject: Re: Brazil Law? |
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I think that is such a neat trick that I had to try it too....
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| 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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Elise Skalwold |
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marvsT/Nminerals
Joined: 06 Nov 2008
Posts: 68
Location: Juda, Wisconsin
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| Posted: Mar 11, 2011 14:10 Post subject: Re: Brazil Law? |
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| 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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Member: Badger Lapidary and Geological Society
Great things can be found in small packages! |
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Pete Richards
Site Admin
Joined: 29 Dec 2008
Posts: 845
Location: Northeast Ohio
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| Posted: Mar 11, 2011 15:19 Post subject: Re: Brazil Law? |
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| 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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Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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Duncan Miller
Joined: 25 Apr 2009
Posts: 138
Location: South Africa
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| Posted: Mar 14, 2011 09:27 Post subject: Re: Brazil Law? |
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| 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
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