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GeneralFeldspar Twins

31st Jul 2019 20:58 BSTJobe Giles

04100700015652059677588.jpg
I’ve finally gotten around to photographing these plates of feldspar Twins. I’m not exactly sure what Twins they are, I had originally suspected Manebach Twins but without a visible cross-section I’m not really qualified to determine this. I plan to post these to my photo album but want to be sure I have the correct twin... I have a whole pocket of these and I’m curious if anyone has found Any truth in the supposition that; gems are nearby when you start getting into manebach and baveno Twins?

Here is the first plate, between the Twins (which are tapered inward toward their bases) you can see normal microcline with another odd (to me) feldspar growth that resembles broccoli. It does not appear to be Albite.


Do please share your own feldspar twin photos or favorite photos you have seen.

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07995600015652120879617.jpg

31st Jul 2019 21:05 BSTJobe Giles

Plate 2. This plate has the same Albite and Muscovite on the sides of the Twins but doesn’t have the broccoli like growth on the normal microcline crystals.

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31st Jul 2019 21:29 BSTJobe Giles

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The above formations make me question these Albite formations, if the feldspar twin has been over covered with Albite.

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1st Aug 2019 01:30 BSTSteve Hardinger Expert

Twinning is so common in feldspars that if it was an indicator of a nearby gem pocket, there would be much more tourmaline and beryl in the world.


So without the sarcasm: No I don't think feldspar twinning is a reliable indicator of nearby pockets.

1st Aug 2019 02:22 BSTJobe Giles

Yeah that question is a bit tongue in cheek especially since I’m using the term “gems” very loosely, to include quartz. That being said, I do find that the best most “gemmy” smokies in pockets that have twined feldspars. I had heard the ol’ wives tale and when I went back over my photo records I was surprised to find there was a smidge of truth to it. But yeah, I couldn’t agree more, it’s like the relationship between black sand and placer gold.

1st Aug 2019 13:48 BSTHarold Moritz Expert

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It looks like these K-spar crystals are slightly etched and from the etching pattern, it looks like there is a diagonal line crossing the square outlines, corner to opposite corner, of what I assume are the terminal faces of the crystals facing the camera, especially in this photo: 5DE62C4D-9AE8-42A4-86DA-DC014E37F10C.jpeg The parallel etching pattern appears to switch direction on either side of this line, which would be the Baveno twin plane. Etching of K-spar is very common in pocket crystals, and albite overgrowths as well, especially albite overgrowing the thin layers of exsolved albite inherent in perthitic microcline. The presence of albite is easy to tell by the the polysynthetic twinning laminae present in all plagioclase crystals, which on the surface of crystals appears as striations, but you may need magnification to see it. However, for feldspars in general in this mineral forming environment, if it isn't K-spar, then it is albite, so that is probably what your broccoli and your overgrowths in the last batch of photos are. So examples:


https://www.mindat.org/photo-800245.html

https://www.mindat.org/photo-951547.html

https://www.mindat.org/photo-901750.html

https://www.mindat.org/photo-392145.html

https://www.mindat.org/gallery.php?loc=9660&min=96 albite aggregates


Here is another specimen from an unknown locality with albite overgrowths on etched K-spar:

1st Aug 2019 20:57 BSTJobe Giles

Harold, great info thank you! I know feldspars are super common but I love them. I would say almost if not all of our microcline is perthitic, sometimes more sometimes less. Much of the microcline is also etched and the Albite quite visible. The first two plates in my photos were just weathering out of the pegmatite. We haven’t explored it because we have not seen any evidence of quartz at this spot. The third piece is from a pocket that had no exposed microcline crystals, everything had been covered in thick Albite and the microcline had evidently mostly been etched away. It was a large pocket (8’D x 3’W x 2’H) hosted in solid pegmatite. I had wondered how these Hoppered Albite formations had formed and I wonder if now I have my answer here since the exhibit the same tapered form. On the underside the Albite seems to have grown from thin air as whatever was there has been etched away. I will see if I have a photo handy to show this. Here is the top down and side angle photos. All that being said, these could just be an entirely unrelated form of Albite twinning.
Hoppered Albite 1

Hoppered Albite 2

Hoppered Albites 3

1st Aug 2019 22:42 BSTHarold Moritz Expert

You got me stumped on those albite "squares" (hoppers are technically single crystals not aggegates like these). Something had to form first and I guess it would have been microcline (all microcline is perthitic).

2nd Aug 2019 00:08 BSTScott Rider

I find strange habits of perthitic microcline at Devil's Head, Colorado (DH). In a very large chamber pocket my buddies and I found what we called brain spar, microcline that formed odd shapes and some looked like brains... Anyway, I have had this image in Mindat (has child photo), for a while, its from the same pocket. Just thought it would be a nice addition to this discussion, albeit I cannot answer any questions about the twinning.


I do agree with Jobe re: the finding this type of microcline corresponds to lovely, large gem pockets, at least in DH. The chamber pocket had hundreds of gem smokies with none of the deficiencies one finds in the common smoky from this area (many specimens from DH are dull from iron oxides, have iron oxides on them, or have a milky secondary coating on the outside). The brain spar was inside the pocket as well as in many of the "satellite" pockets we found around the chamber, all of which had gem smokies and fluorites. And in other digs at DH, that are much smaller and that have the crappier quartz, the microcline was dull normal looking crystals with less perthitic attributes (like albite coatings that make the feldspar crystals very lustrous) and much less twinning.


https://www.mindat.org/photo-766343.html

2nd Aug 2019 00:37 BSTJobe Giles

Here is a great thread here on mindat initiated by Norman King and contributed on by the late William van Laer. A very good read!

https://www.mindat.org/forum.php?read,11,313823,313865

2nd Aug 2019 01:08 BSTHarold Moritz Expert

Looking in David London's "Pegmatites" at the pic referenced by Chris (William) van Laer in the other thread, I think my specimen photoed above may be orthoclase overgrowths from the Organ Mountains. I had suspected this from another source. There is some albite in it, but not obvious in this pic, which dont show twinning on the overgrowths.
 
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