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Possible Belowda-law twin of quartz
  
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eric8he




Joined: 13 Oct 2019
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Location: Seattle, Washington


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PostPosted: Oct 13, 2019 13:32    Post subject: Possible Belowda-law twin of quartz  

Hey guys, I'm excited to join this forum.
I collected this interesting specimen about a month ago in a King County, Washington location. I believe it to be a quartz twin of some sort. Unlike most queries I've seen about quartz "twins", these two crystals line up on the same plane and both exhibit both s- and x-faces.
The angle appears to be between around 50-60 degrees which makes me believe that this is a Belowda twin.

A Japan-law twin was found in the same pocket if that helps.



IMG_2803.JPG
 Mineral: Quartz
 Locality:
King County, Washington, USA
 Dimensions: 16x23x15 mm
 Description:
 Viewed:  2345 Time(s)

IMG_2803.JPG



IMG_2808.JPG
 Mineral: Quartz
 Locality:
King County, Washington, USA
 Dimensions: 16x23x15 mm
 Description:
Top-down view of the specimen.
 Viewed:  2354 Time(s)

IMG_2808.JPG



IMG_2804.JPG
 Mineral: Quartz
 Locality:
King County, Washington, USA
 Dimensions: 16x23x15 mm
 Description:
View of s- and x- faces of one of the crystals
 Viewed:  2344 Time(s)

IMG_2804.JPG


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Bob Morgan




Joined: 18 Jan 2018
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Location: Savannah, Georgia


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PostPosted: Oct 13, 2019 15:44    Post subject: Re: Possible Belowda-law twin of quartz  

If this is indeed a twin on the 302 with a 55o 23’ c-axis angle, it will be the third I'm aware of. The first was brought from Brasil years a go by Richard Gaines and written up by Frondel. The second was found a couple of years ago by James Zigras in Arkansas.
It should not be called a Bolowda Law twin, because that law is a beta quartz law. No name has been assigned to this law. I think it would most appropriately be designated the Frondel Law, since Frondel first described it.

Your specimen appears to lack a feature that the other two show - exaggerated growth in the twin notch where opposing prism faces have had greater layer stimulation and stacking - but that doesn't rule it out as one of those twins.

It is hard to confirm the angle comparing the twin to a drawing or goniometer set to the c-axis angle. A far better check can be from setting a goniometer or using a drawing to the complementary angle of 125 degrees and comparing alignment with striations across the twin boundary.

I look forward to more information of this exciting find.

By the way, having found a Japan Law twin in the same pocket is interesting. That is particularly true of most Grieserntal Law twins.
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