FMF - Friends of Minerals Forum, discussion and message board

[Ru Smith]

Here is a 2 cm interpenetrant fluorite twin from Cambokeels, Co Durham, showing growth zoning. What does such zoning tell us about the evolution of the precipitating solution? How much time is involved? Who else has interesting examples of this?

[am mizunaka]

.
The Berbes fluorite has subtle zoning present.
The Llamas fluorite is much more distinct.

[VRigatti]

Denton Mine, Hardin County, Illinois USA

[cascaillou]

General cause of purple color in fluorite:
https://gemologyproject.com/wiki/index.php?title=Causes_of_color
(link normalized by FMF)

[Ru Smith]

Thanks guys, these are all beautiful images.

Here is a subtle zoning in various shades of green from the West Pastures Mine, Weardale.

[Jesse Fisher]

Fluorite crystals from the Rogerley Mine occasionally show a sharply defined yellow color zone, which is always restricted to one corner of the cube. Why it is not symmetrical with the cube center is a mystery to me. These zones also vary in the intensity of UV fluorescence from the green section of the crystal.

[Ru Smith]

How amazing, Jesse. The UV image seems to show distinct cubic edges to the yellow zone. It would be good to think up some possible explanations to test, but it's not obvious what to suggest.

[Ru Smith]

It's a pity there don't seem to be any more growth bands here to either confirm or deny the idea that the yellow portion is oldest followed by growth to the right and down, but not to the left and up in the view presented in your photo. The Blackdene specimen on the "elongate" thread shows an early core left behind in one corner and I see the same in a transparent Blue Circle crystal, with the early purple core left behind in one corner of the final green cube.

[Jesse Fisher]

On many of them there is a thin outer layer of green over they yellow zone at the cube face. If the yellow zones were earlier than the green, the crystal would have had to nuclear in suspension and grow downward toward contact with the matrix. If this were the case, one would expect to find the occasional "floater" crystal showing no point of attachment. These are virtually unknown from the Rogerley (or other North Pennines mines) to my experience.

[Ru Smith]

Completely good point, Jesse. My couple of examples (Blackdene, Heights) have the early corner-hugging core on the matrix side of the crystals.

[Ru Smith]

A transparent Hilton penetration twin with warm amber colour and a subtle internal phantom.

[Jesse Fisher]

The LWUV fluorescence of Hilton yellow fluorite is generally fairly week compared to other colors in North Pennines fluorite. Have you checked this one to see if any bands of stronger fluorescence correspond to the purple layers?

[Ru Smith]

hmm. goes to look for UV lamp and fails to find it.

[Ru Smith]

I eventually found the UV lamp and took the following photo (first time I try a UV photo of a mineral). Interesting how the bands in the twin element protruding towards us at the top show so clearly here.

[Ru Smith]

This fluorite from the West Pasture Mine is interesting. There's the usual face-parallel growth zoning, but also an unusual rich green internal zone, roughly 1 mm wide extending from close to the centre of the crystal towards an edge of the cube. It's only visible through the face shown in the photo and the opposite face, but not through the side faces. I wonder how that came to be.