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Fluorite or beryl?

Posted by Nick Gilly  
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Nick Gilly December 30, 2017 03:48PM
Not mine but for sale on a well-known auction site as a tabular spinel twin of fluorite. Is this not a tabular beryl crystal though?:







Looks more like hexagonal symmetry to me, and I'm guessing it might be a Pingwu specimen.

Thoughts please?



Edited 2 time(s). Last edit at 12/30/2017 03:50PM by Nick Gilly.
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Reiner Mielke December 30, 2017 04:29PM
Looks much like a fluorite I once had from Naica, Mexico. Same color as yours with this form: https://www.mindat.org/photo-254169.html
Should be very easy to tell the difference with a hardness test.



Edited 1 time(s). Last edit at 12/30/2017 04:34PM by Reiner Mielke.
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Reiner Mielke December 30, 2017 04:32PM
Here is another one similiar to yours : https://www.mindat.org/photo-705538.html
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Dana Slaughter December 30, 2017 04:33PM
I've had them from Naica as well that are virtually identical--a quick hardness test on the underside would quickly discern whether beryl or fluorite. The staining on the piece would more align with the Pingwu beryl specimens however.



Edited 1 time(s). Last edit at 12/30/2017 04:35PM by Dana Slaughter.
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Owen Lewis December 30, 2017 05:01PM
I'd say not fluorite - but not enough to go on to say beryl.

What's the SG? The SG of fluorite is sufficiently uncommon as to make a a reasonable call on that hard information only.

A trick that should work. Hold the crystal right up to an eye - as though it was a spectacle lense. Look through it and, if possible out of a window too, so you are looking at objects at least several tens of feet away. Rotate the crystal about its three spatial axes. If the crystal is cubic in system, however you rotate the crystal, you will always see objects with single, clear edges, If the crystal is anisotropic (i.e. not cubic), at certain points in rotation you will see single edges and at others double edges - or else you may see objects with double edges (double vision) only.

Trigonal, hexagonal and tetragonal crystals show a single image of objects when viewed in this way and straight down the crystal's c-axis but there will be double images when viewing is along any other line through the crystal. Given the form of the specimen, if you get that result beryl is a likely (but not the only) call.

Have fun :-)
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Nick Gilly December 30, 2017 05:06PM
Thanks for the replies. As I say, I don't have access to the specimen, but it just looked very unusual for a fluorite. Unfortunately it is also way out of my price league, otherwise I would probably buy it.

Actually Rainer, that second photo of the colourless fluorite reminds me of the tabular lens-shaped phenakite crystals from Brazil. Fascinating.



Edited 1 time(s). Last edit at 12/30/2017 05:17PM by Nick Gilly.
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Matt Neuzil December 30, 2017 05:37PM
There are a couple twins from that area posted on mindat.

The seller has plenty of beryl listed. I wouldn't have reason to question it. It's likely they know the difference and might have even done a test themselves.
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Erik Vercammen December 31, 2017 03:16PM
Based on the picture, apatite is also possible, but hardness will help.



Edited 1 time(s). Last edit at 12/31/2017 03:16PM by Erik Vercammen.
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