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Negative crystals and their formation
  
  Index -> Minerals and Mineralogy
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Donn C.




Joined: 27 Jul 2014
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Location: Arizona

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PostPosted: Feb 09, 2017 11:32    Post subject: Negative crystals and their formation  

Not sure if this is the right place for this question but here it is. I ran across a negative crystal in one of the quartz crystals that I collected. I had never seen one before and it took me a while to find out that it was called a negative crystal. I am trying to find out more about why during the formation of the quartz crystal itself that it would form an internal void in the shape of a crystal, with all the facets, faces and terminations of a crystal. In this case the negative crystal also does not have the normal pointed termination of a quartz crystal but I do not know enough about negative crystals to know if they should form as typical quartz crystals or not. Can anyone explain the why/how of a negative crystal formation?


crystal bubble 2.jpg
 Mineral: Quartz
 Locality:
Arizona, USA
 Dimensions: 2mm
 Description:
 Viewed:  916 Time(s)

crystal bubble 2.jpg



DSC_0009.JPG
 Mineral: Quartz
 Locality:
Arizona, USA
 Dimensions: 10mm
 Description:
 Viewed:  904 Time(s)

DSC_0009.JPG


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Pierre Joubert




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PostPosted: Feb 09, 2017 14:24    Post subject: Re: Negative crystals and their formation  

Hi Donn. Nice to see a negative crystal inclusion from a different locality than Brandberg. Take a look at a few photos I took of some of our specimens:
http://www.mineral-forum.com/message-board/viewtopic.php?t=3621&postdays=0&postorder=asc&start=40

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Donn C.




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PostPosted: Feb 09, 2017 14:52    Post subject: Re: Negative crystals and their formation  

Yes, your negative crystal photos look very much like the negative crystals that I have in other crystals I have collected from the same Arizona location.
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Pete Richards
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PostPosted: Feb 09, 2017 16:34    Post subject: Re: Negative crystals and their formation  

Donn C. wrote:
... Can anyone explain the why/how of a negative crystal formation?


My understanding is that negative crystals start as fairly normal fluid inclusions, which are usually rather irregular in shape. A crystal which is not growing is however not static. Atoms are constantly detaching from the surface and re-attaching, even when there is no net accumulation of crystal mass. Statistically, this process promotes movement of atoms from areas of high surface energy to areas of lower surface energy. This process is well shown during the aging of snowflakes in a snow bank, which gradually lose their points and become more granular.

The same process takes place in the fluid inclusion. Atoms detach from the crystal walls of the inclusion and re-attach elsewhere, statistically preferring low energy crystal faces (the ones we see on the outside of the crystal) to randomly oriented higher-energy surfaces. Gradually the shape of the inclusion changes from its original shape to one more like a crystal. Sunagawa points out that the equilibrium form of a negative crystal can included rounded surfaces as well as flat crystal faces.

Remembering that the rates of chemical reactions approximately double for every 10°C of increased temperature, this kind of process proceeds much more rapidly at the temperatures at which quartz is forming than it does at room temperature (where the rate is so slow as to be essentially 0).

Fluid inclusions which form from fluids trapped when the crystal is forming (what I will call primary inclusions) have longer to make this adjustment and at higher temperatures than at least some inclusions that form when an essentially complete crystal fractures and the crack heals, and as a consequence these secondary inclusions are more often irregular, but in principal given enough time in a warm enough enviornment, any fluid inclusion's boundary could come to look like the external morphology of the crystal that holds it - i.e. a negative crystal.

Below is a quartz crystal with a large inclusion. It has a somewhat crystal-like outline - tending toward polygonal rather than irregularly curved, but it is very flat, and a great deal of quartz would have to migrate in order for the volume to become more equant and crystal-like.



Quartz 1.JPG
 Mineral: Quartz
 Locality:
Illiez Valley, Wallis (Valais), Switzerland
 Dimensions: 5 mm crystal
 Description:
 Viewed:  814 Time(s)

Quartz 1.JPG



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Collecting and studying crystals with intersting habits, twinning, and epitaxy
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John S. White
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PostPosted: Feb 12, 2017 05:58    Post subject: Re: Negative crystals and their formation  

One of the very best localities for quartz with negative crystals is Amatitlan, Mexico. I have seen quite a few from there. Here is one from my collection. The crystal is 4 cm tall but only a part of it is seen in the close-up photo.


Quartz - Mexico SC-99 6-4-20 (with negative xl incl.) Feather.jpg
 Mineral: Quartz
 Locality:
Amatitlán, Municipio Zumpango del Río, Guerrero, Mexico
 Description:
Close-up photo by Russell Feather
 Viewed:  601 Time(s)

Quartz - Mexico SC-99 6-4-20 (with negative xl incl.) Feather.jpg



Quartz - Mexico SC-99 6-4-20.JPG
 Mineral: Quartz
 Locality:
Amatitlán, Municipio Zumpango del Río, Guerrero, Mexico
 Description:
 Viewed:  589 Time(s)

Quartz - Mexico SC-99 6-4-20.JPG



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John S. White
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PostPosted: Feb 12, 2017 06:09    Post subject: Re: Negative crystals and their formation  

Another negative crystal with a definite box shape and a liquid bubble that touches the sides of the box. It was a lucky find in a cleavage of fluorite. In the photo of the entire piece the bubble can be seen above the 3.2 mm mark on the scale. The photo of the bubble is by Russell Feather.


Fluorite - Illinois (wbubble) 4-12-3.JPG
 Mineral: Fluorite
 Locality:
Illinois, USA
 Description:
 Viewed:  597 Time(s)

Fluorite - Illinois (wbubble) 4-12-3.JPG



Fluorite - Illinois (wbubble) 4-12-3 Feather photo.jpg
 Mineral: Fluorite
 Locality:
Illinois, USA
 Description:
 Viewed:  593 Time(s)

Fluorite - Illinois (wbubble) 4-12-3 Feather photo.jpg



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Pete Richards
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PostPosted: Feb 12, 2017 14:45    Post subject: Re: Negative crystals and their formation  

A real collector's treasure would be to find a crystal enclosing negative crystal of a different habit. This could happen, for example, if the external environment changes after the fluid inclusion is trapped, and the change favors a different habit. This would have to be a chemical change, not a change in something like temperature or pressure. Since the fluid inclusion is trapped inside, the equilibrium shape of the negative crystal would not respond to the chemical change in the external environment.

Fluorite might be a good candidate mineral....

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John Jaszczak




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PostPosted: Feb 12, 2017 17:01    Post subject: Re: Negative crystals and their formation  

Here are a few of my favorites from Merelani Hills, Tanzania.


685-12_med.jpg
 Mineral: Quartz
 Locality:
Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania
 Dimensions: 0.75 mm tall negative crystal
 Description:
The host crystal has very similar morphology to the larger of the negative crystals shown here.
 Viewed:  494 Time(s)

685-12_med.jpg



508-02_med.jpg
 Mineral: Fluorapatite
 Locality:
Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania
 Description:
Fluorapatite negative crystals (up to about 400 microns across) with fluid and graphite inclusions.
 Viewed:  493 Time(s)

508-02_med.jpg



508-22_med.jpg
 Mineral: Fluorapatite
 Locality:
Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania
 Description:
Negative crystals in fluorapatite (largest about 400 microns) with graphite and fluid inclusions.
 Viewed:  492 Time(s)

508-22_med.jpg



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