FMF - Friends of Minerals Forum, discussion and message board

[Scott LaBorde]

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?

[Scott LaBorde]

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.

[Pete Richards]

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.

[Scott LaBorde]

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?

[jimB]

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.

[Peter Megaw]

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).

[John S. White]

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.

[parfaitelumiere]

dauphiné law

[Pete Richards]

[Amir Akhavan]

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
Cheers from Japan, the Land of the Vending Machines

[Scott LaBorde]

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

[Pete Richards]

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 https://www.minsocam.org/msa/collectors_corner/arc/qtztwin.htm
(link normalized by FMF)

[Elise]

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)

[Duncan Miller]

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

[Elise]