FMF - Friends of Minerals Forum, discussion and message board

[GneissWare]

I ran across this fluorite from the Cave-in-Rock District, Illinois, and must admit that my knowledge of crystallography is getting rusty. I can't quite figure out the form, and the phantom is a total mystery. Some cubic faces are evident, as is what seems to be a twin suture line. The phantom is aligned with some faces, but is totally missing from others. It isn't broken off matrix, it is rehealed on the base. I'm curious if anyone can explain the form and phantom morphology.
Bob

[Herman van Dennebroek]

Hallo GneissWare,

The cleavage of fluorite is parallel to the actahedral form. All the lustry faces on your crystal are cleavage planes. It looks like someone tried to make a octahedral "crystal".

On the cube planes you can see all kinds of dislocation. This shows that the crystalgrow was continuing on differant spots on the cubic face. This kind of crystal grow gives steps lt on the cubic plane.

The collor change from yellow to purple is very common for fluorite from Cave in Rock, but I also see it on fluorites from Wölsendorg, Germany. In the Mineralogical Record or in Lapis is written about the collor change. I can't remeber wich magazin and when, but perhaps that one of the other members know about the article.
Vissable is anyhow that the color change folows the octahedral crystalform.

I hope this information will help you a little bit.

[Maxilos]

It is very good possible that someone tried to make a fluorite octahedron out of a cubic one. Since this person was not that good in cleaving fluorite, he (or she) made som small cracks. I have a site where you can read about it:

https://www.inlandlapidary.com/user_area/fakes.asp
(link normalized by FMF)

just scroll down and look for "Fluorite Octahedron.

Mark

[GneissWare]

I studied the faces carefully, and also showed them to a Fluorite collector, and we both are of the opinion the smooth faces are crystal faces and not cleavages. (I will try to get a few better photos along the plane of the faces.) Also, the symmetry and edge geometry seems a bit too perfect for a cleavage attempt. But, we might be wrong.

[Jesse Fisher]

Octahedral faces are very rare on Illinois fluorite. I am almost certain that the "smooth" faces on the specimen are cleaveages. A lot of Illinois fluorite was turned into cleaveage octahedrons and this is surely the result of such an attempt.

[trtlman]

I am far from being a mineral expert and I don't know if it is my place to be commenting but I have looked at this many times and several sides are rough looking, the other sides are smooth and shiny, shouldn't all the sides match in appearance unless the other sides where cut or cleaved