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Fluorite crystallography?
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Bob Morgan




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PostPosted: Dec 04, 2023 12:19    Post subject: Re: Fluorite crystallography?  

The picture you feature under your name is cube-octahedral.
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PostPosted: Dec 04, 2023 12:23    Post subject: Re: Fluorite crystallography?  

The picture I referenced is under Carles Millan's name.
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Johan Kjellman




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PostPosted: Dec 12, 2023 17:58    Post subject: Re: Fluorite crystallography?  

Jesse Fisher wrote:
Another Naica fluorite, which according to Pete's analysis would be cube-dodechahedral habit. From the find circa 2005.


I 100 % agree with Pete's analysis of how to differentiate between the two combinations.
But in the case of this (Jesse's) fluorite it appears to me to be a cube-octahedron.

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PostPosted: Dec 12, 2023 18:03    Post subject: Re: Fluorite crystallography?  

Yes.
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Pete Richards
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PostPosted: Dec 12, 2023 20:52    Post subject: Re: Fluorite crystallography?  

I am not sure how to interpret Jesse's fluorite. I cannot see any view that is unambiguous. It appears that the cube faces are clear, maybe slightly pebbled, and that the other faces are frosted.

In the bottom-most fluorite in his specimen, the lower left corner of the forward-facing cube face points to the edge between a pair of the other faces, beyond which is a face that is just barely visible because it is nearly perpendicular to the view. This face holds the answer. If it is a cube face, then the habit is cube-octahedral; if it is one of the other, frosted faces, the habit is cube-dodecahedral.

The crystal on the upper left also helps a little. It has a cube face oriented forward and slightly down. Above and to the right of this face are two of the other faces. To the top-right of these is another face, sloping backwards. It is hard to see clearly, but it appears to me to be frosted. If so, this crystal also indicates a cube-dodecahedral habit; if instead this is a cube face, then the habit is cube-octahedral.

In my view, and with considerable uncertainty, the habit of this one is, like the other examples, cube-dodecahedral - (100)(110). Perhaps Jesse will look at his specimen from all angles and let us know!

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PostPosted: Dec 12, 2023 22:29    Post subject: Re: Fluorite crystallography?  

Pete,
Points well stated.
I looked at photos in Mindat of fluorite on sphalerite. Nearly all were cube- dodecahedral that could be discerned clearly.
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PostPosted: Dec 13, 2023 02:57    Post subject: Re: Fluorite crystallography?  

I agree again - and withdraw my claim.
And Pete, you point directly at what I now think so many of us, although experienced, keep on misinterpreting these combos. We are all fast and confident in identifying the first prominent cube face. Then a "second face" a bit too quickly identified as a cube and, voila, you have a cube-octahedron.
Maybe we all should slow down and look for a third alternative cube face. I (think I) see them in both crystals you described on Jesse's specimen.The thing is, and this may be part of the problem, these cube faces seem relatively smaller than the obvious large one.

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Pete Richards
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PostPosted: Dec 13, 2023 09:42    Post subject: Re: Fluorite crystallography?  

Johan Kjellman wrote:
(snip)
The thing is, and this may be part of the problem, these cube faces seem relatively smaller than the obvious large one.


Yes, definitely. These crystals show substantial deviations from ideally developed morphology, where all faces of a given form would have the same size and shape. Several of the dodecahedral faces appear to have nice three-fold symmetry, which is a false impression, but possible since these faces have six edges. With a little distortion, they can present as alternating long and short edges, and voila, pseudo-three-fold symmetry. The angles between the edges would be wrong for three-fold symmetry, but this is very hard to judge when a face is tilted.

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