FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hello,
How can we explain this dispersion of the color centers in the shape of a cross perpendicular to a ternary axis of this fluorite?
I expected the edges (of a dodecahedron perhaps) to focus the color centers of this cleaved fluorite.

[Bergur_E_Sigurdarson]

I've seen similar to this in Amethyst/Quartz, but never Fluorite.

[SteveB]

Ok, I’m going to sound stupid on this, I think the main structural molecules use less energy and have chemical stability (time) to form their purest crystals without any impurities. Result is a clear crystal in the middle and towards the outside the main chemicals are reduced so form with impurities which is where the colour comes from. I’ve done experiments with those crystal growing kits for kids trying to grow large crystals by leaving them to grow in a sealed container. The coloured solution darkened over months and I emptied it when a crystal was touching the surface. I was surprised the crystals that had formed were glass clear with no hint of colour. Just don't ask why many quartz varieties are clear at the tips instead :)

[Matt_Zukowski]

Are you looking right down into the cleavage plane? If so you are looking down a three-fold axis.

You say this view is looking perpendicular to a three-fold axis (perpendicular to an octahedral face). In my mind's eye it seems like all the three-fold axes of a cube are perpendicular. If so, you are looking down another three-fold axis.

[Bob Morgan]

I went to mindat and looked at other images of these stars. One seemed to indicate that your orientation is turned around and that this may be the cleaved corner of a cube which is the more likely growth form for that locality - temperature of formation being low. This would be buttressed by your magnified image which looks like evidence of hexoctahedral structures like often found on corners of cubes.

[Roger Warin]

Hello,
In a fluorite crystal, there are 4 quaternary axes, 3 ternary axes, etc.
The direction of a 3-fold axis is [111]. This direction coincides with the optical axis of the camera.
The cleavage is perpendicular to this direction. After several cleavages, it gives the fluorite cleavage octahedron {111}.
In fact, the (rare) star is only visible in one direction. The star is therefore not repeated by the operators of symmetry. It is likely following a growth direction. On other cleavages, I estimated that the star underlined 3 edges of an internal dodecahedron, but I have no experimental proof to affirm it.
We never cleaved a fluorite cube at Seilles, because any automorph form is rare in Seilles quarry. They are often shapeless but often monocrystalline volumes which are thus transformed into pretty octahedra, after cleavage.
The interior of the stone may be of gem quality. Pretty gems have been cut.
Roger.

[Riccardo Modanesi]

Hi to everybody!
I also have a fluorite with this "Mercedes" cross. Some publications gave it the very improper name "trigonal", while everybody knows fluorite crystallizes cubic, not trigonal.
Greetings from Italy by Riccardo.

[Bob Morgan]

I still think this star in the octahedral cleavage comes from cleavage across cube edges. Ether would look the same except for the orientation of the star. If your image were rotated 180 degrees the star would be right for cube edges instead of dodecahedral. The images of the clear cleavages with violet stars would fit that way too. One of them is actually in that orientation.
My preference for the cube option comes from the lack of dodecahedral crystals reported from Belgium or other similar deposits of low temperature formation.
Finally, there appears to be widening of the spokes leading into the center of the star, which would be accounted for by there having been hexoctahedral faces at the corners of the cube like often found on fluorite crystals from many localities of that general region.

[John S. White]

No addition to the discussion but just sharing a photo of a (111) cleavage in a Cave in Rock, Hardin Co., Illinois fluorite in my collection. 4.5 cm across.

[Roger Warin]

Hello,
Here are some cleavages of Seilles fluorite.
It is not common to find multiple stars in the same specimen.
In this one, the identical orientation of the 4 crosses shows that the crystal lattice is unique. These are parallel growths.