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GeneralIdeal and Non-ideal Quartz Contact Twins

10th Sep 2019 17:08 BSTEric He

00756490015681313806479.jpg
I recently collected a specimen of what I believe to be a Grieserntal-law twin, and have recieved differing opinions on it.
As you can see, the re-entrant angle is clearly formed by the inside face, but however only one pair of edges "lines up" per se.

If this is slightly off from the ideal form of the Grierental-law twin, then is it a twin? If so, what amount of deviance is acceptable to say that it is still a twin?

In King County, seismic activity has often shifted the members of a twin, sometimes making otherwise obvious Japan-law twins slightly "off".

10th Sep 2019 20:43 BSTErik Vercammen Expert

This looks like a La Gardette (or Japanese) twin to me, with an angle of 84°30' between the c-axis of the crystals. Can you make a picture with one of the terminations pointing to the viewer?

10th Sep 2019 20:58 BSTEric He

03692720015681453984509.jpg
Yes, here. It is not a Japan-law twin, the photo's perspective makes it look like one.
See the top? Only one of the re-entrant edges matches.

10th Sep 2019 20:59 BSTMark Holtkamp

Hey Eric, I agree with Erik, this looks more like a Japan law twin. Here is an interesting article about using a laser pointer to test for twinning (page 27) : The edges don't have to line up, as long as they run in the correct directions according to the twin law. If the twin is 'off' enough to be seen or measured by simple means (like a contact goniometer), I would say it is unlikely to be a twin 

10th Sep 2019 21:27 BSTEric He

09268240015681471973698.jpg
I don't think that this is a Japan-law. Let me add another photo, straight-on this time. The angle is almost exactly 76 degrees.
Also, if this twin were a Japan law, you would see faces directly at you in this photo. Instead you see edges, indicating some form of RG.

10th Sep 2019 21:38 BSTMark Holtkamp

Yes after seeing those extra pictures, this is not a Japan law twin.

10th Sep 2019 22:39 BSTRonnie Van Dommelen Expert

Certainly not Japan law.  In that case an edge would pass perpendicular to the notch, rather than a pair of faces.  The rarer quartz twins get confusing (at least to me) very quickly.

There are good drawings here (any other search results by Dr Richards should also be considered highly reliable):

There is also a Mindat pic here (photo 307106).  That surely must also be a twin, but doesn't *seem* to show the parallel faces required (as per article above).  Regardless, that would  However, the angle at which a photo is taken can be very deceiving.  Your last two photos look promising.  I'm not sure how you determine accurately whether the outer two pyramid faces are parallel. Pressing something flat against them (say a credit card) might be a start.  Some sort of goniometry for a large specimen would be better but will be tricky.  You could rig something up with a lazy susan, view tube (toilet paper roll with covered ends except for a small hole), and a distant light.  It would take a while to get the specimen centered on the axis of rotation AND at the right orientation, but would increase confidence.

11th Sep 2019 02:22 BSTEric He

When I press the outer face against a flat countertop, the opposite face appears perfectly parallel. I can put a credit card on top of it and it has nearly no variance.
Also, that twin appears to be not a Reichenstein-Grieserntal twin. RG is on the {1,0,-1,1} plane.
 
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