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Donn C.
Joined: 27 Jul 2014
Posts: 10
Location: Arizona
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Posted: Feb 09, 2017 11:32 Post subject: Negative crystals and their formation |
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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?
Mineral: | Quartz |
Locality: | Arizona, USA | |
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Dimensions: | 2mm |
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11802 Time(s) |
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Mineral: | Quartz |
Locality: | Arizona, USA | |
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Dimensions: | 10mm |
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11752 Time(s) |
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Pierre Joubert
Joined: 09 Mar 2012
Posts: 1605
Location: Western Cape
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Donn C.
Joined: 27 Jul 2014
Posts: 10
Location: Arizona
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Posted: Feb 09, 2017 14:52 Post subject: Re: Negative crystals and their formation |
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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
Site Admin
Joined: 29 Dec 2008
Posts: 830
Location: Northeast Ohio
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Posted: Feb 09, 2017 16:34 Post subject: Re: Negative crystals and their formation |
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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.
Mineral: | Quartz |
Locality: | Illiez Valley, Wallis (Valais), Switzerland | |
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Dimensions: | 5 mm crystal |
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_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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John S. White
Site Admin
Joined: 04 Sep 2006
Posts: 1295
Location: Stewartstown, Pennsylvania, USA
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Posted: Feb 12, 2017 05:58 Post subject: Re: Negative crystals and their formation |
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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.
Mineral: | Quartz |
Locality: | Amatitlán, Municipio Eduardo Neri, Guerrero, Mexico | |
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Close-up photo by Russell Feather |
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Mineral: | Quartz |
Locality: | Amatitlán, Municipio Eduardo Neri, Guerrero, Mexico | |
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11468 Time(s) |
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_________________ John S. White
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John S. White
Site Admin
Joined: 04 Sep 2006
Posts: 1295
Location: Stewartstown, Pennsylvania, USA
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Posted: Feb 12, 2017 06:09 Post subject: Re: Negative crystals and their formation |
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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.
Mineral: | Fluorite |
Locality: | Illinois, USA | |
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Mineral: | Fluorite |
Locality: | Illinois, USA | |
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_________________ John S. White
aka Rondinaire |
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Pete Richards
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Joined: 29 Dec 2008
Posts: 830
Location: Northeast Ohio
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Posted: Feb 12, 2017 14:45 Post subject: Re: Negative crystals and their formation |
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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....
_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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John Jaszczak
Joined: 05 Oct 2010
Posts: 300
Location: Hancock, MI
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Posted: Feb 12, 2017 17:01 Post subject: Re: Negative crystals and their formation |
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Here are a few of my favorites from Merelani Hills, Tanzania.
Mineral: | Quartz |
Locality: | Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania | |
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Dimensions: | 0.75 mm tall negative crystal |
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The host crystal has very similar morphology to the larger of the negative crystals shown here. |
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Mineral: | Fluorapatite |
Locality: | Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania | |
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Description: |
Fluorapatite negative crystals (up to about 400 microns across) with fluid and graphite inclusions. |
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11357 Time(s) |
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Mineral: | Fluorapatite |
Locality: | Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania | |
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Description: |
Negative crystals in fluorapatite (largest about 400 microns) with graphite and fluid inclusions. |
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11367 Time(s) |
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_________________ Adjunct Curator, A. E. Seaman Mineral Museum
Professor of Physics
Michigan Technological University |
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