FMF - Friends of Minerals Forum, discussion and message board

[Elise]

Hi Duncan!
Great fun that you picked this thread back up! The Rochester Mineralogical Symposium is coming up very soon - helical fans John White and John Rakovan will both be there and Bill will be nearby -- it would be a wonderful time to sit and brainstorm this fascinating formation. There is still time to book a flight ;-) or we can put you on SKYPE! Speaking of which, getting on a plane now headed for the Sinkankas Symposium...We'll have fun discussing this thread on the trip and we will get back with any " high altitude, bright ideas", if any occur!
Cheers!
Elise

[mjbainbridge]

Hi Everyone,

Elise brought this fascinating thread to my attention after I posted this photo of a screw-dislocated natrolite on Facebook. I must admit, most of this discussion is way over my head, but I'm happy to be able to contribute, and looking forward to learning more about it!

BTW, I showed this to John Rakovan in Tucson when I got it, and he's interested in studying it, so I'll post any new information that comes out of it.

Michael

[Duncan Miller]

When I listed the minerals in which these helical growth features\screw dislocations (?) had been observed, I forgot to mention natrolite. Thank you for reminding me. I have seen several in natrolite specimens from a quarry near Hardap Dam, Mariental, Namibia.
Duncan

[marco campos-venuti]

Another example in tourmaline from Madagascar

[marco campos-venuti]

I would like to restart this interesting topic.
We use the term spiral dislocation because we have in mind the screw dislocation studied in crystal growth. We realize, speaking in this thread, that the scale is so different that we are probably in front of a different thing. We can then call it just fractures.
First we better we try to analyze these spiral structures. We have 2 different structures presented in the pictures in this thread.
1- a spiral fracture aligned along the long axis of the crystal (as in the Aquamarine of François Lietard )
2- many “en echelon” fractures (as in the John Koivula photo)
I found spirals in chalcedony and in opals. I have interpreted them as conchoidal fractures formed by contraction stress during loss of water in the free surface of small opal ponds. (see picture of chalcedony)
I reproduced this at home using egg white (see picture) that is a similar gel material.
What is the relation of these spirals in amorphous material with spirals in crystals, I don't know. Maybe there is no relation.
But the second style of fractures looks to me like a more clear stress pattern, due to geological deformation or simply to the contact of two different materials. As is the case of this picture of a quartz with lithyophilite inclusions. I'm not sure about the inclusions, they could be a mica. In this case the stress is confined at the boundary between quartz and the edge of the lithyophilite crystal. It is not rare that the inclusion crystals induce fractures in the included crystal. In this case the fractures are “en echelon”, maybe because the inclusion is flat.
I can't bring any conclusions, but I hope someone help me to reopen this topic,
marco

[Elise]

[Conny Forsberg]

That is a really nice one Elise!

[marco campos-venuti]

Wow Elise, that piece make me dream.
I remembered that I have an other spiral in a quartz, of a quite different kind, but of similarly obscure origin. It is a biterminated quartz crystal from the lodo quarry in Brazil that has a "lodo spiral" inside, visible from both sides.
A have an idea in mind, but I wait for your interpretation.

[Elise]

Marco,

That quartz specimen is now second only to my other favorite on FMF, that of Vic Rzonca's find: https://www.mineral-forum.com/message-board/viewtopic.php?p=25510#25510

I have no idea what growth mechanism could cause such a structure to form and await your thoughts - what a special piece it is! Thank you so much for sharing this with us! I sent a link to Si & Ann Frazier, John Koivula, John White and other quartz enthusiasts friends, so they might enjoy the discussion as well.

Elise

[Elise]

[marco campos-venuti]

The world of crystals is full of oddities, but the spiral is always an attraction for any people. The spiral is not on the C axis, but at 15-20 degrees. It is clearly not casual, because the spiral is composed of 2 branches with the same pattern. They make around 2 turns in a clockwise direction. The spiral grows with the growing crystal and reaches the surface in 2 points at 2 faces of the rhombohedron. It is clearly tridimensional. The spiral is made of small crystals, most of them look like white rhombohedral dolomite (or feldspar) mixed with some positive quartz crystals, and few yellow needles of a mineral different from rutile.
I will try to upload a video or to link to it if I learn how to do it.

[marco campos-venuti]

I uploaded a video of the crystal on youtube:
https://www.youtube.com/watch?v=ybcAAWND1D0

[jimsfault]

this is so fantastic.
that would be amazing to try and photograph! (photomicrograph)

[Martin Rich]

Sorry Marco for my silly statement, but is this a petrified hurricane? :)

Martin

[Stretch]

I was asked to post my aqua with an interesting spiral top and spiral inclusion. Not sure if they are associated or not, it seems to be more than coincidental but I honestly do not know. One thing I find strange about this piece is that half way down where the spiral jettisons out, you can see where it was trying to form the termination. So I don't know if this is an etching with a healing or if the spiral formation is a growth anomaly sprouting out the top in a 2nd generation of strange growth. Certainly an interesting piece that sparks a lot of interesting thought. I would love to hear from those that know more about how this might have been formed. Or even wild proposals of how this might happen. Anything in the interest of learning about this strange one is welcomed. I hope you enjoy it as much as I do :)
Regards,
Stretch

[Elise]

Stretch - Thanks so much for adding these pictures of this extraordinary specimen to this thread (I had asked him to). I'm looking forward to any ideas members might have regarding what is happening in this specimen and how the formation at the top relates, if at all, to the inclusion. Are there any close-ups of the inclusion itself?

Best wishes,
Elise

[Duncan Miller]

Stretch - That is really an astonishing beryl specimen. Would it be possible to post a video of it on a rotating turntable?

[marco campos-venuti]

I found an other strange spiral growth. This is an aragonite specimen from an old collection, labeled as coming from Stratoni, Chalkidiki, Greece.
No idea why it is spiral shaped,
marco

[Michael Shaw]

Hi Marco,

Fascinating aragonite. I wonder if the spiral growth could be the result of the aragonite replacing a gastropod.

Michael

[marco campos-venuti]

Hi Michael,
I was thinking the same. For the size and geometry it can be a Cerithioidea (Telescopium, family Potamididae). They are amphibious gastropod molluscs that live in brackish waters and are common in the fossil record. But its spiral is sinistral, that is quite unusual for this genera.
marco