FMF - Friends of Minerals Forum, discussion and message board

[Elise]

Hi,

Duncan wrote me yesterday regarding these types of inclusions - I had mentioned it somewhere on FMF after the Rochester Symposium, but I don't think it ever caught on as a topic so I thought I would try again. I have always found them very intriguing and they were a great source of debate/talk at the Symposium last Spring. I went back to John and Duncan's brief discussion of it from May 2009 in which John posted a beautiful photomicrograph taken by John Koivula of JSW's beryl https://www.mineral-forum.com/message-board/viewtopic.php?p=5505&highlight=#5505 Maybe others could post any pictures they have or John might repost his here.

Pasting in part of what I wrote to Duncan yesterday (this is my lunch break and I don't have time to re-type): "This was hot controversy at the Rochester Mineral Symposium this Spring. I met someone who is involved in investigating the cause (John Rakovan); I'll try to get that information to you when it is published or if I hear more about it.

We (Olaf, Jeff and others) discussed it during breaks when one attendee passed around a hand-sized beautiful kunzite spodumene with a helix running its length which Jeff had also imaged. Many people, including Bill, are of the opinion that it is a type of screw dislocation, but Olaf and others argue that screw dislocations are only sub-microscopic. A model which Bill constructed back when he was teaching mineralogy sits on my desk in his lab and I actually contemplate it whenever I sit there - a hexigon of red marbles inbedded in clay and rising in a helical fashion, really quite pretty.

I tried to get a discussion about this going on FMF, but it didn't catch on. My young friend who brought back all the specimens from Pakistan this past year (I posted some images on FMFof his spinels) had many aquamarines with these helix inclusions, some with several in one crystal, all parallel to the C axis and most starting some distance up from the bottom of the crystal and which break the surface of the pinacoid.

Back to the kunzite -- this relatively large helix broke the crystal face surface at each turn and with my loop I could peer into it at each rotation; the formation was hollow throughout (postscript: in hindsight - I don't know if these were actually more like JSW's specimen where it is not actually a helix but "twisted wings" coming off of a helix which broke the surface - even so, why do they break the surface? is it that in this bladelike monoclinic crystal there just isn't room in the short axis direction?)

There are many published photos of these - There is a beautiful image published in Lithographie No 7 Beryl: a 9 cm aquarmarine beryl from Pakistan with a helix running its length -- and others published elsewhere by inclusionist John Koivula - he has some neat images of these in beryl and topaz in the Photolexicon (CGA 2003, 24, 1 ) underwhich he notes that "What causes a helix to develop in a beryl or topaz crystal during its growth is not completely understood. Such inclusions are always oriented parallel to the c-axis, and appear to be associated with strain zones that run the entire length of the inclusion. Whether the helix caused the strain, or strain caused the helix to develop is still under investigation. Perhaps helical inclusions are the result of strain along linear dislocations in the structure." I interpreted that as meaning a screw dislocation, but maybe that's wrong. In the later F05 issue of G&G, John describes such a feature in a 53.4 mm beryl (pictured) as being "a visual form of growth disturbance propagated through the host crystal along a screw dislocation that develops from a source such as a small solid inclusion or a structural defect, often at or near the base of a crystal during the earliest stages of growth." By this could he mean that the helix itself is not the screw dislocation, but an artifact resulting along it? They are fun to study for sure."

I think Bill's answer to me when I returned from Rochester to tell him about it was more of a question back to me to ponder - as in why couldn't it be a screw dislocation? Today when I told him about what I had written here and showed him the images in the above journals, he qualified that a screw dislocation would be at the unit level and that at the pinacoid we would see a hillock with the SEM. These helical inclusions here are on a macro-scale and he was fascinated that I found them to be hollow and breaking the faces. He is very interested in what people will say and what John Rakovan might be finding.

John, I wondered if you or anyone else had found out John Rakovan's thoughts on this or if he has/will publish anything on it since the Rochester Symposium. I have only personally seen them in beryls and spodumene; other than these and topaz, what other minerals have they been found in?

By the way, my friend's little store was robbed of over $20 K in specimens over Labor Day weekend, many of them from his Pakistan/China trip last Winter https://www.mineral-forum.com/message-board/viewtopic.php?p=10830&highlight=#10830 I don't know if the beryls with the helices were among those lost, but I will see what he has left and try to perk up his spirits with any insights FMF has on these special inclusions. He had some at the Springfield show I believe; perhaps he had a good weekend and sold them before the break-in.

Best wishes,
Elise
(ok, that did take me longer than lunch to write despite cut and paste effort)

[Duncan Miller]

There are some very good photographs of these helical growth features, in beryl, topaz and spodumene on the GemologyOnline Forum at:
https://gemologyonline.com/Forum/phpBB2/viewtopic.php?f=8&t=12302&sid=edfdd38d23837cfc7f4c3dbe15bc83f0
(link normalized by FMF)

You don't have to be a member to view them. There is also some speculation about their origin. I like the idea that a screw dislocation establishes a stress field, which is relieved by the radial propagation of cracks. It will be interesting to hear what the mineralogy experts on this forum think.

Duncan Miller

[Elise]

Jeff sent me a photo to share with FMF of the kunzite crystal which we examined at the Rochester Symposium; he is also looking forward to hearing what theories people have. It is a spectacular formation and a beautiful portrait of this specimen.
Cheers!
Elise

[mmauthner]

Elise,
We have seen the same type of helical etched channels in spodumene crystals from the recent Big Kahuna pocket at the Oceanview mine.
Attached is a photo of the largest crystal we found. The channel is visible on the right side. Unfortunately I was in a bit of a hurry and did not think to take a close-up photo of the channel "inclusions".

Cheers,
Mark

[Pete Richards]

These spiral inclusions are most amazing and puzzling! Appealing to spiral dislocations is tempting, but there are several problems.

Some research indicate that spiral dislocations are found in the millions per square centimeter - if so, why is there any crystal left?!!!! Why are spiral tunnels so rare? This research, however, has focused on metals and alloys, and perhaps the defect density there is orders of magnitudes greater than that for free-grown natural mineral crystals.

Typical spiral dislocations have offsets of one unit cell - generously about 10 angstroms or 1 millionth of a millimeter. The unit cell dimension should determine the pitch of the screw. There are mechanisms proposed for stacking up steps caused by spiral defects to create larger step sizes, but to get to a visible step size as in these spirals would require stacking up some 2,000,000 steps. This seems implausible - how do you do it?

I have not read this literature for a decade or so, so there may be more recent thinking. If someone is familiar with it, I'd welcome comments.

And if my comments are accurate, they leave me saying that I don't understand why these things happen! So they are wonder-full.

Pete Richards

[Duncan Miller]

Before and after pictures of my beryl specimen. Two faces were crudely polished before I acquired it, so I cut the stone to show the helix more clearly. In fact, there are several helices, and a sawn off piece produced a smaller stone with another helical inclusion. The cut stone is 40 mm long, so you can get some idea of the pitch of the helix. It is a helical fracture (shaped like a "spiral" staircase) with paddle-shaped 'wing' fractures emanating at regular intervals on two sides from the helix. I analysed the outcrop of the main helix on a piece cut off the end of the crystal, using X-ray fluorescence spectroscopy in the SEM, and detected no foreign mineral that could have acted as a core. Perhaps someone more competent than I needs to do finite element models of the stress fields in beryl, spodumene and topaz required to produce these cracks. I guess the different crystallographies would account for the different appearances of these helical features in the different minerals, i.e. the variable pitch and size of the cracks, although they don't seem to be determined by cleavage.

Pete is quite right, that screw dislocations are at the unit level, and it is hard to think of how they could give rise to macroscopically observable helices like these, but it is tempting to invoke them because how else can one initiate a regular helix in single crystals of diverse crystallography? Has anyone found one in quartz yet, and if not, why not?

Duncan

[John S. White]

Elise requested that I post this John Koivula photo of a beryl in my collection that was cut into a gem measuring some 4 cm in length. The portion in the photo is about 1 cm across. The beryl is from Brazil, presumably Minas Gerais, but the specific locality is unknown.

[John S. White]

The entire cut stone. Jeff Scovil photo. The stone is actually more green in color than it appears here.

[Elise]

Hi,

I am hoping we might reignite this thread after hearing John Rakovan's presentation at the Rochester Symposium last weekend. Below is the model I mentioned at the start of this thread; I am still contemplating it after a year. While the marbles are tactile, not to mention beautiful in the sun, John's 3-D diagrams and explanation of screw dislocations were much more enlightening; though as he concluded, mysteries remain as to the genesis of the helical inclusions. I am still mulling it over...I wish John had a rewind button so I could hear it once again, this time more slowly! Maybe someone here can give a synopsis. The Symposium was a wonderful event once again and it was so nice to see everyone there (it was a great way to spend my 53rd; thank you all for singing me happy birthday - my face may have been the color of these marbles, but that was just because it made me feel so good)!

Best wishes,
Elise

[michael-thorn]

In these cases, is not the center of the crystal the last to be "locked"? So, assuming a load of impurities, crystallization proceeds with as much impurities as the lattice can accept with a straining effect forcing a dislocation and eventual small radiant fracture. The remainder gets "spewed" upwards a small amount in the limited core space, allowing the previous area to "lock", although with a built-in fracture-inducing strain; the next area having to deal with the bulk of the impurities again...?
Assuming a limited core space for crystallization, it is easy to imagine that, having once been strained, the ensuing continuation of strain-lock and reject, would form the circular stair-step effect of the screw dislocation with the attendant fractures...?
Interesting to speculate that, under the increasing "pressure" to lock in new unit cells in a heavily junk-filled narrowing core, when the discontinuity from impurity got so great as to cause a dislocation, resulting in the stair-step mode postulated above, that the direction of rotation would always follow "right-hand' or "left-hand' spiral according to the crystal's right or left hand type...?

[al mar]

I have seen a few spiral inclusions and this type of inclusions reminds me "Liesang rings"

Can be any scientific reason or only is apearance ?

[michael-thorn]

There is in quartz especially, a spiraling energy effect down the prime axis, similar to DNA. If one allows non-proven non-scientific ideas, there is a significant amount of data about rotating stars-of-david and geometric rotation effects in quartz crystals. There is also studies about magnetically induced rotation effects. i have no real comment, other than to inquire if the crystals are right or left handed, and if the rotation follows that...

[Duncan Miller]

At an atomic level quartz does have a helical structure (see Amir Akvan's website The Quartz Page for an excellent explanation of the structure of quartz) but so far I have not seen any images of these helical inclusions in quartz.

Duncan

[michael-thorn]

I read that quartz has both left and right hand atomic intra=matrix tendencies, with one winning out, but having both always present, as in some twinning laws, or something...???

[Elise]

While looking up something else, I found another report of this helical growth in spodumene, including nice images of the crystal and the formation: Gem News International, "Spodumene from Afghanistan with unusual inclusions", John I. Koivula, G&G, Fall 2008, Vol. 44, No. 3, pp. 278-219. The PDF is available for free online:
https://gia.metapress.com/content/3qu13j8258325651/fulltext.pdf
(link normalized by FMF)

[weaver]

Awesome sample pics.

My only contribution is to the thinking realm of things, I have no spiral inclusions in any of my specimens.

"Is it possible that these spirals appear the same way they do in other parts of nature?"

When I think of "Why some inclusions are spiral?" as opposed to straight, cloudy, et al.
I started thinking- "Is there anything else in nature that is anomalously spiral-shaped?"

The only things I could think of are fireworks, atom-smashing test results, and hurricanes. Yes, there are other spiral/helical shapes in nature that form, but they are harder to analyze their initial growth pattern; eg. DNA. Perhaps looking at how other spirals are formed can lend us clues to this mystery?

~

In fireworks, usually the gunpowder charge is set so the missile fires STRAIGHT into the air. But occasionally, a firework is built poorly, or has some other packing/packaging flaw, and the firework spirals on it's ascent. Usually, this spiraling continues on this spiral path until interrupted (no more propellant, or the firework discharges it's main cache.)

In atom-smashing, we see that [quarks, anti-quarks, gluons] sometimes travel straight, but often times create spiral patterns. I think this happens if they bounce off of something. I do not know more about this.

In hurricanes and galaxies: We see spiral shapes that are born from fluid dynamics, or other similar chaotic phenomena. These spirals dissipate over time based on external influences, gravity, loss of momentum, etc.


~

The fireworks and hurricane/galaxy examples show spirals in an open environment. Meaning: External forces can affect the outcome. Whereas, in a crystalline environment, fewer external forces come into play- so the inclusion is "stuck" repeating it's spiral shape. The mineral is practically a closed system (provided that IT has the stability to form over time), so there would be less to "knock it off course" of it's spiral growth. Perhaps at the time of the inclusion's INITIAL formation, some conditions were correct for the spiraling to begin?



Flights of fancy can be fun, until they take a spiral-shaped nose dive!

Thanks again for the pics, and the words!

[John S. White]

With respect to spirals in nature don't forget narwhal tusks and some goat horns. Of course many sea animal shells are spiralled as well.

[cascaillou]

Let's note that such srew dislocation figures have also been seen in emerald and in topaz

[Duncan Miller]

Some time ago Elise Skalwold kindly sent me some references to helical crack growth and also the programme of the 2011 Rochester Symposium. Now I have had the time to give some more thought to the helical growth features described and illustrated in this thread. I am going to take the liberty of reproducing below the abstract of John Rakovan and John White's contribution to the Rochester Symposium, because I think it neatly summarises what is known about these helical cracks and proposes a feasible mechanism for their formation.

I also will attach a photograph of a helical dislocation, on a microscopic scale (copied from Hull & Bacon 1984:65). Perhaps with time, in a hot crystal, such a dislocation may expand, like a slowly unwinding spring, to macroscopic size. The resulting strained volume around the dislocation would be vulnerable to preferential etching, as proposed by Rakovan & White, to produce a hollow helix like that described at the top of this thread by Elise.

The regularly spaced, slightly twisted blade-like cracks emanating from the turns of the helix in both John White's and my beryl specimens have been puzzling me, because they don't appear to be open etch features. Perhaps the answer lies in the fact that beryl has strongly anisotropic thermal expansion. When heated from 0- 800*C, it expands linearly in the a-axis direction, but the c-axis contracts in the range 0-300*C and thereafter expands (McColm1983:14). The reverse would the true on cooling from 800*C, as happens to a natural crystal after formation, so below 300*C the crystal would expand in the c-axis direction. This would add strain to any pre-existing helically strained region in the crystal, which could be relieved by regularly spaced cracking emanating from the helical core.

Topaz and spodumene also have markedly anisotropic co-efficients of thermal expansion, which may account for any laterals cracks associated with helical cracks in these materials. What remains puzzling to me is that although helical dislocations have been reported in a wide variety of materials of differing crystallography, as far as I know macroscopic examples have been reported only in beryl, spodumene and topaz. What is also puzzling is that they appear only parallel to the c-axes of these minerals, which don't even share the same crystal system.

I think that these helical growth/etch features, and the macroscopic growth hillocks seen on many crystal faces (see Rakovan 2004), really should not be called 'screw dislocations' even if one or more screw dislocations originally played some part in their formation. Helical dislocations are not pure screw dislocations, and most dislocations have a mixed edge/screw characteristic, occur in high density and on a microscopic scale.

References:
Hull, D. & Bacon, D.J. 1984. Introduction to dislocations. Oxford: Pergamon Press.
McColm, I.J. 1983. Ceramic science. Glasgow: Leonard Hill.
Rakovan, J. 2004. Growth hillock. Rocks & Minerals 79:415-417.

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38th Rochester Mineralogical Symposium 14-17 April 2011 UNUSUAL SPIRAL INCLUSIONS IN CRYSTALS. J. Rakovan1 and J. S. White2. 1Department of Geology Miami University, Oxford, OH 45056; 2Kustos, P.O. Box 332 Stewartstown, PA 17363.
Numerous crystals have been observed by the authors that exhibit interesting spiral inclusions; some quite spectacular. These include beryl, topaz, and spodumene. Many of these inclusions are visible without magnification, with lengths up to many centimeters and widths in millimeters. In certain cases, most common in emerald, the morphology is that of a double helix (Vuillet and Rotlewicz 2001). Some authors have referred to these as screw dislocations, however, the lateral dimensions and morphology of these inclusions is atypical for screw dislocations. Although dislocations can have lengths on the order of centimeters their lateral dimensions are usually in the Angstrom or nanometer range. Thus, optical detection is usually not possible. Furthermore, most screw dislocations are straight features. Although they are commonly observed to change orientation along their length, generally due to movement (creep) as a result of strain, screw dislocations do not generally form a helical morphology. There are, however optically observable manifestations of screw dislocations in crystals. The microtopography on crystal faces may exhibit growth spirals and hillocks that are the result of a growth mechanism associated with screw dislocations. Another example is the preferential etching along dislocations. Etch pits and the less common etch channels can be macroscopic signs of dislocations (Authier and Zarka 1977). The lateral dimensions of these features can be many orders magnitude larger than the associated dislocations, thus making them optically visible. Although less common, dislocations with a helical morphology have been observed, and an extensive physics and materials science literature about these exists. Observations of helical dislocations minerals, including fluorite, cuprite, olivine, garnet and sapphire, have been published (Amelinckx et al. 1957; Caslavsky, and Gazzara 1971; Veblen and Post 1983). The mechanisms of their formation are debated, however, most authors propose that helical dislocations form from movement, climb and glide, of previously straight screw dislocations due to thermal or compositional stresses (Weertman 1957). Microscopic observations of spiral inclusions in several samples (Fig. 1 and 2A) indicate that these are hollow and may be partially filled with some other solid phase. In the case of the hollow spiral in the topaz pictured in Fig. 2A there is a very fine and detailed microtopography on its inner surface. Precise 30 repetition is observed not only in the helical repeat but also in the microtopography on adjacent segments of the spiral. Although speculative, the authors propose that the observed, macroscopic spirals are the result of natural post growth etching or decoration, coeval with growth, of helical dislocations and the strained region of the crystal structure around them. If the dislocation offset (Burgers vector) and associated strain is large then etching may occur only along the defect with little affect to the rest of the crystal.
References:
Amelinckx, S., Bontinck, W., Dekeyser, W., and Seitz, F. 1957. On the formation and properties of helical dislocations. Philosophical Magazine, 8th Series, 2, 355-378.
Authier, A. and Zarka, A. 1977. Observation of growth defects in spodumene crystals by X-ray topography. Physics and Chemistry of Minerals, 1:15-26.
Caslavsky, J. L., and Gazzara, C. P. 1971. The observation of helical dislocations in sapphire. Journal of Materials Science, 6:1139-1141.
Veblen, D. R. and Post, J. E. 1983. A TEM study of fibrous cuprite (chalcotrichite): microstructures and growth mechanisms. American Mineralogist, 68:790-803.
Vuillet, p., and Rotlewicz, J. 2001. Les Inclusions en Double Hélice dans les Émeraudes de Colombie [Double helix inclusions in Colombian emeralds]. Revue de Gemmologie AFG, 143:15-19.
Weertman, J. 1957. Helical Dislocations. Physical Review, 107: 1259-1261.

[Jordi Fabre]

Just to make known to everyone that as we did some time ago in the Spanish side, we will create here a new section named: "Featured columns of FMF" (if someone has a better name, please let me know via PM) where we will save the articles that in the opinion of the FMF's administrators deserves a special space with a different organization than the rest of the Forum, a kind of organization a little bit similar to a magazine's style. The topic "Inclusions of spiral or helical growth and screw dislocations" will be the number 0 of this new section considering its quality and interest. Of course several other topics will be in the new section "Featured columns of FMF" but after reading the last post of this thread I could not resist the temptation to announce now, here, the establishment of that new section.

Thank you to all participants in this thread. Great job!