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GeneralFeldspar Twins and hoppered Albites

31st Jul 2019 20:58 UTCEch Noch

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.

31st Jul 2019 21:29 UTCEch Noch

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

1st Aug 2019 01:30 UTCSteve 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 UTCEch Noch

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 UTCHarold Moritz 🌟 Expert

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 UTCEch Noch

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 UTCHarold 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 UTCScott 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 UTCEch Noch

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 UTCHarold 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.

14th Jul 2021 05:53 UTCEch Noch

Harold would you or anyone else be able to point me in the direction of someone knowledgeable and willing to work up an article on these Albite pseudomorphs or epimorphs? Hopefully someone who specializes in feldspars.

14th Jul 2021 06:29 UTCLuís Martins 🌟

It looks like hoppered growth. Why do you say it's a pseudomorph/epimorph?

14th Jul 2021 10:57 UTCHarold Moritz 🌟 Expert

Ed, I don't know who to refer you to, sorry.

14th Jul 2021 14:51 UTCTony Albini

Larry Venezia of the Boston area is a feldspar collector.

14th Jul 2021 15:35 UTCEch Noch

My hypothesis is based upon observations of the undersides Of the hoppers and similarities to the other feldspar twins as well as the etched away feldspar that has left behind the skeletal Albite structure. Let’s me get a photo to show you what I mean.

14th Jul 2021 15:36 UTCEch Noch

Thank you Tony!

14th Jul 2021 17:37 UTCEch Noch

Luís, here is an example from Southern California that to me indicates that the hoppers May indeed be pseudo or epimorphs after feldspar twins. Here the feldspar is far less etched and some perthitic structure is still visible.

14th Jul 2021 17:38 UTCEch Noch

Photo 2,

14th Jul 2021 17:40 UTCEch Noch

I believe all three specimens were collected by the rose family.

14th Jul 2021 19:36 UTCErik Vercammen Expert

For me, it looks like an oriented overgrowth of albite on microcline.

14th Jul 2021 21:11 UTCEch Noch

Oriented to what? The feldspar baveno twins?

15th Jul 2021 09:13 UTCErik Vercammen Expert

Albite and microcline have the same structure => if the microcline growth is ended and albite is deposited, it aligns on the structure of the microcline.

14th Jul 2021 21:16 UTCEch Noch

Rare hoppered Albite
https://youtu.be/RRwVRolc10I

Albite exhibiting hopper like growth with Muscovite
https://youtu.be/vPknNOvzwnA

Partial cluster of Albite Hoppers exhibiting growth structure
https://youtu.be/ZW6ywiRp4_Q

Hoppered Albite with Muscovite
https://youtu.be/lLKwz2hsYT8

Albite Hopper with green Muscovite

https://youtu.be/-UqVDTNESEk

Albite exhibiting Hopper like growth
https://youtu.be/XXi47okq8l0

Epitaxial Albite on baveno twin with Muscovite part 2
https://youtu.be/Df3YF7mX4Ms

Tightly packed Albite hoppers with Muscovite
https://youtu.be/b_U2cvuF-uc

16th Jul 2021 13:59 UTCDana Morong

A very interesting reference is by Joseph V. Smith (1974): FELDSPAR MINERALS, in Three Volumes. Volume 1 is Crystal Structure and Physical Properties, Volume 2 is Chemical and Textural Properties (has a lot about growth, defects, twins, perthites, intergrowths), and I don't know what is in Volume 3 as I have not seen it.

29th Sep 2021 19:02 UTCEch Noch

Thank you I will look into that!

15th Jul 2021 19:38 UTCHarold Moritz 🌟 Expert

The albite overgrowths align with some of the albite layers in the perthitic microcline. I see this all the time in pocket microcline crystals. Has nothing to do with microcline twinning.

30th Sep 2021 23:33 UTCDon Windeler

Interesting to see these from southern California. When I saw prior photos I was sure they were from the MT No. 1 pit in Plumas County, CA (https://www.mindat.org/loc-203943.html ). I haven't collected there personally, but a lot of Bay Area Mineralogist members have, especially Chuck Trantham (RIP). Some of the feldspars there show similar hoppered growth patterns, e.g. this one.

Unfortunately I don't have much to add on the science behind what conditions drive this formation.

Cheers,

3rd Oct 2021 15:10 UTCEch Noch

Woah Don! That’s a fantastic representation of this growth form! Absolutely beautiful!

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