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Topic: Is Change in crystal shape possible  (Read 5603 times)

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Offline mehc

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Is Change in crystal shape possible
« on: April 16, 2014, 12:34:50 AM »
Every mineral has specific crystal lattice. If we synthesize the same mineral in laboratory by precipitation reaction, can the crystal lattice be changed by careful control of reaction conditions etc.

That is to say if a mineral's natural crystal is rhombohedral, can we make it orthorhombic in precipitation reaction.

Thanks.
« Last Edit: April 16, 2014, 12:54:36 AM by mehc »

Offline Archer

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Re: Is Change in crystal shape possible
« Reply #1 on: April 16, 2014, 02:47:47 AM »
In short, sometimes.

Some crystals exhibit a property called polymorphism. I have only encountered this with organic molecles.

Usually if you seed a recrystallisation with a certain polymorph this will be the product.

This site might help


http://www.tulane.edu/~sanelson/eens211/twinning.htm
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Re: Is Change in crystal shape possible
« Reply #2 on: April 16, 2014, 02:55:18 AM »
Some crystals exhibit a property called polymorphism. I have only encountered this with organic molecles.

Graphite vs diamond.

Aragonite vs calcite.

To some extent PbI2, although its different forms are simply stable at different temperatures and it reversibly changes on heating/cooling, so they never coexist in a stable way.
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Offline Archer

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Re: Is Change in crystal shape possible
« Reply #3 on: April 16, 2014, 06:11:53 AM »

Graphite vs diamond


I would disagree with this one, these are elemental allotropes not crystal polymorphs.
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Re: Is Change in crystal shape possible
« Reply #4 on: April 16, 2014, 07:06:04 AM »
I would disagree with this one, these are elemental allotropes not crystal polymorphs.

I suppose it depends on how we define them, but in both cases we deal with exactly the same substance and two different crystalline forms. I don't see why it is different from the aragonite vs calcite - same substance, different crystalline forms?

IMHO trying to classify these case separately is more artificial than treating them together. Every crystalline allotrope is a crystal polymorph, not every crystal polymorph is an allotrope.
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Offline Archer

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Re: Is Change in crystal shape possible
« Reply #5 on: April 16, 2014, 01:52:01 PM »
I would disagree with this one, these are elemental allotropes not crystal polymorphs.

I suppose it depends on how we define them, but in both cases we deal with exactly the same substance and two different crystalline forms. I don't see why it is different from the aragonite vs calcite - same substance, different crystalline forms?

IMHO trying to classify these case separately is more artificial than treating them together. Every crystalline allotrope is a crystal polymorph, not every crystal polymorph is an allotrope.

I personally don't agree that allotropes are polymorphs, they are not the same substance, the bonding is different. That was just my opinion though, it would appear that a number of resources contradict with my opinion.


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Offline Corribus

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Re: Is Change in crystal shape possible
« Reply #6 on: April 16, 2014, 02:13:20 PM »
My opinion, FWIW, is that the distinction is superficial. I would classify allotropy as a specific kind of polymorphism.

Another inorganic example of polymorphy that is quite well known is the anatase and rutile forms of titanium dioxide. TiO2 can also form nanotubes and other structures (as can a number of other inorganics).
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Offline Enthalpy

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Re: Is Change in crystal shape possible
« Reply #7 on: April 16, 2014, 05:32:33 PM »
For instance SiC exists as three or more different forms, with varied semiconducting properties
http://www.ioffe.rssi.ru/SVA/NSM/Semicond/
so the crystal must be well-defined to produce semiconductor wafers. To my understanding, this is done at the Czochralski process
http://en.wikipedia.org/wiki/Czochralski_process
just by choosing the proper seed.

I hope that Czochralski adapted to organic compounds would offer the same selectivity to produce the desired crystal (and possibly separate the enantiomers, you know).

Offline mehc

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Re: Is Change in crystal shape possible
« Reply #8 on: April 17, 2014, 12:15:12 AM »
Thanks for all responses.

I asked this question because in analysis of magnesium carbonate (synthetic) by XRD, the XRD report showed it as thallium gallium germanium sulfide and not as magnesium carbonate.

In my opinion the crystal structure of synthetic magnesium carbonate matched to thallium gallium germanium sulfide, and that's why the instrument identified it as that.

Offline Archer

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Re: Is Change in crystal shape possible
« Reply #9 on: April 17, 2014, 02:28:30 AM »
Was is single crystal XRD?
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Offline mehc

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Re: Is Change in crystal shape possible
« Reply #10 on: April 17, 2014, 04:18:13 AM »
No, It is powder XRD.

Offline Archer

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Re: Is Change in crystal shape possible
« Reply #11 on: April 17, 2014, 02:01:38 PM »
No, It is powder XRD.

I have no experience with that technique so I can't be any help I'm afraid.

It does highlight the importance of not relying on any one single technique for identification.
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