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EducationBow-tie stilbites: single crystals??

16th Oct 2018 22:03 UTCŁukasz Kruszewski Expert

Hello!

There is an idea that the bow-tie stilbites are in fact SINGLE CRYSTALS..... My imagination is incapable of realizing what kind of crystallographic laws would allow to describe such aggregate-like "monocrystals" as... monocrystals.... To me they are evident bow-tie aggregates but... I may be wrong...

Anyone?

Cheers,

Luke Kruszewski

16th Oct 2018 22:19 UTCBenjamin Oelkers

In the (usual) sense of the word, i.e. being adequately described by a single set of unit cell, orientation and space group, the stilbite bow-ties can definitely not be single crystals. The orientation of crystal faces changes slowly going along the seemingly curved faces, and you need a new orientation of the unit cell for each of these small steps, resulting in quite a large number of different crystals (or crystal domains).

Maybe the proposal originally was that the whole bow-tie aggregate stems from a single seed crystal? That does at least seem quite reasonable to me.

16th Oct 2018 22:35 UTCŁukasz Kruszewski Expert

Hi Benjamin. Thanks! This is very interesting. The suggestion was that some serious structure defects are engaged, but I personally do not know such defects. My idea was that - maybe - there is some superstructure involved or sth.... But exactly no known set of symmetry elements would allow it to be a single crystal, in my opinion too.

16th Oct 2018 22:40 UTCŁukasz Kruszewski Expert

Btw, I was just instructed to compare with the gwindel variety of quartz... and there is what is found in the mindat page:

"A high number of straight screw dislocations cause a uniform so-called Eshelby twist (Cordier and Heidelbach, 2013; Eshelby, 1953) that is typical for slow growing crystals. This slow growth is in line with the occurrence of gwindels in certain alpine-type fissures.

Because of the often multiple tips and the strong development of sutures on the crystal surface, gwindels are often described as lateral stacks of quartz crystals. This is not correct: each gwindel starts as a single crystal, and only the overgrowth with quartz of a macromosaic structure causes the impression of a composite specimen (Kuzmina et al, 1987; Dolino and Bastie, 2009)."

17th Oct 2018 00:18 UTCBenjamin Oelkers

Hmm, I think this comes down to the definition of a (single) crystal. From a microscopic perspective, having 3D ordering at (formally arbitrarily) large distances is what makes a crystal a crystal. As such, there will always be a more or less distinct transition from "sufficiently ordered" structures that would be considered (mono)crystalline to "insufficiently ordered" structures. The latter could then be further broken down into aggregates of multiple crystals and amorphous matter. Because diffraction experiments are routinely used to study crystal structures, one major determining factor for practical purposes is the existence of sharp peaks in the diffraction pattern.

In this case, there will be sufficient disorder in the crystal(s) that such a diffraction pattern might become less useful - typically a descriptor of non single crystalline matter. On the other hand is such a behaviour also known for, e.g., mica structures with lots of stacking variants.

The original stilbite example reminds me of a tree with different branches that divide into smaller and smaller twigs. Sure, the whole thing (except for the outmost parts and leaves) is of the same composition and started from a uniform seed. But would you say the tree is of the same "structure" as it was at a younger age? (Echinocactus grusonii would fit that description, for example, as it mainly gets bigger.) I would say the complexity grows with each level of branching, and as such additional information is needed to describe it. I'm no expert on such order-disorder transitions, but surely someone will have invented a useful, but convoluted term that fits our stilbite better than the old "single crystal"! ;-)

17th Oct 2018 01:31 UTCWayne Corwin

How many "C" axis does it have?

17th Oct 2018 07:01 UTCŁukasz Kruszewski Expert

Benjamin - by the PXRD pattern blur did you mean Scherrer broadening or rather an introduction of many additional reflections (I'd expect the latter)? I thought about single-xl XRD but this would need a well-oriented fragment to show non-single-crystallinity... But, me also no expert in the OD issues.... I actually never heard about these Eschelby twists. Indeed, maybe it can simply be described using the less sophisticated crystallography tools?

Regarding the tree - this just exactly reminds me many spray-forming or skeletal aggregates of many crystals...

Wayne - exactly... I think your questions makes the point...

17th Oct 2018 09:42 UTCBenjamin Oelkers

I would assume that the PXRD looks rather normal, given that the percentage of somehow disordered matter will be relatively small and that the different orientations of the unit cells are unimportant here. Just like with any polycrystalline sample, really.

In single crystal XRD, however, I would assume to see myriads of very small peaks to the point of not having any useful reflexes at all - also just like with any highly polycrystalline sample. And because of that, I find it problematic to call it a single crystal!

Even if the stilbite were to have only one c axis, which the single crystal XRD would easily show (the pattern would look "nice" from only one direction), that alone also does not make it a single crystal in the narrower meaning of the word, I think. It would still lack the defining characteristic that you can predict every other part of the crystal starting from any given point in the crystal, i.e., a (virtually) fully ordered crystal lattice.

17th Oct 2018 10:02 UTCVolker Betz 🌟 Expert

Hi,

This is a question which touches the lack of nomenklature definiton. Distored crystals are widespred in minerals.Stacking faults cause theortical plane faces and straight axis to be curved, uneven and non linear. If you are looking into small distances (fractions of a millimeter) the distortion is minimal and you may accept a single crystal definiton. If the distance is longer (cm range) you can see the accumulation of all the minimal distortions. But the grade of distortion over distance is the same. So to my understanding a stibite bow-tie

ist just a distored crystal. Most crystals in nature are imperfect and distored.

Volker

17th Oct 2018 16:09 UTCBenjamin Oelkers

It just occurred to me that the situation is also likely different depending on what language you use. In German you can easily distinguish between "Einkristall" (single crystal as a technical term, roughly translated "one-crystal") and "ein Kristall" (a single crystal, as in: not two or three; roughly translated "one crystal"). That makes the problem easy for me to solve in my native language:

Nicht jeder einzelne Kristall ist ein Einkristall.

However, this sentence is a little less unambiguous in English:

Not every single crystal is a single crystal.

31st Oct 2018 08:15 UTCJohan Kjellman Expert

In Munich I picked up a piece of the quite recent epidote bowties from Baluchistan.

I find one in the image gallery:

https://www.mindat.org/photo-842810.html

so I can think of stellerite/stilbite, cavansite and epidote, certainly there are several more.

obviously not single crystals, but there is certainly some crystallographic "mechanism" behind that I am not familiar with.

twinning and quartz gwindels come to mind - "polysynthetic twinning around a point"?

did anyone give it a thought - would be interesting to hear what you have to say.

I suppose - Lukasz - your question once again, somewhat rephrased.

cheers

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