FMF - Friends of Minerals Forum, discussion and message board

[colin robinson]

Here is a piece from a location near Stanhope. In more than 45 years of field collecting Weardale fluorites this is probably one of the more extreme examples. Although it is damaged (inevitable with dump material) the green/purple zoning is quite striking.

[Ru Smith]

A lovely and striking piece, Colin.

I wonder what the explanation is? Perhaps intermittent deposition of radioactive material on surfaces during crystal development? (as discussed for "Blue John" by Galwey et al. 1979 here:
https://www.minersoc.org/pages/Archive-MM/Volume_43/43-326-243.pdf )
(link normalized by FMF)

Conclusions from that paper are attached.

[Ru Smith]

This is an attractive zoned fluorite twin, blue/green to purple, from Blackdene Mine.

[nurbo]

Hi Ru,
What do you make of this? Its got me baffled. I just got it a couple of weeks ago and Ive been thinking about how it formed ever since. Any thoughts on it?

[Ru Smith]

This reminds me of Jesse's yellow-cornered green fluorite, nurbo.

The only explanation I can think of is that parallel growth (or near-parallel since High Skears specimens often show subtly curved faces) resulted in a step at the corner of the composite crystal and subsequent precipitation, in crystallographic continuity, then "healed" the step.

The attached photos show some examples of such steps at the corners/edges of composite crystals from the High Skears Mine.

[nurbo]

Thanks Ru,
I had pondered that but I couldnt think of a reason why the purple cube would stop forming when it reached the extent of the first crystals boundaries. Unless theres some kind of almost epitactic relationship or a very unlikely coincidence.

[prcantos]

Hi, Ru! I am delighted with your crystallographical threads! I have some questions concerning these two fluorites before in this thread:

https://www.mineral-forum.com/message-board/viewtopic.php?p=32953#32953

https://www.mineral-forum.com/message-board/viewtopic.php?p=31064#31064

Why can we say that they are indeed twins, or twinned crystals? They both show evident lack of global symmetry. Doesn't this reveal that their crystalline lattices don't fit, which is required for twinning?

Regards!!

[Jesse Fisher]

They certainly look like classic penetration twins [111} to me.

[prcantos]

Yes, they look like so. But do notice that {111} axis doesn't fit the rotation axis. There is a translation beyond the {111}-rotation, and this leads to a lack of symmetry!

[Ru Smith]

The nice thing about your crystal, nurbo, is that the growth banding clearly shows that the purple corner is a late fill of an earlier step. The next increment of growth would wrap around the whole "healed" crystal. Jesse describes ( if I remember right) just such a late "wrap" around his yellow-cornered green crystals from Rogerley.

Thanks Pablo. The angular relations look good on that penetration twin for a 60 degree rotation around [111]. As usual, different faces grow at different rates and we end up with something less ideal than Josele's wooden model, but the angles are good. The closest natural examples to the ideal I have seen have been in a diamond and a loparite.

[James Catmur]

I have always been fascinated by this specimen. Not only do the fluorite crystals have a complex form, but also the phantom forms a cross within each crystal, which goes through the center of the edge of each crystal.

[Ru Smith]

Looks like a nice hexoctahedron showing on the edges, James. Do you have some more photos to show the shape of the phantom?

[James Catmur]

Ru

Not at the moment, but let me see what I can do

James

[James Catmur]

I had a go with some image correction to enhance a new photo - the color is not right but you can now see that color band / zoning in both crystals. In person it forms a 100, 010, 001 cross with a band perpendicular to each face, in the center of the face.

[Ru Smith]

I just saw an article on Sectoral Zoning by John Rakovan that looks very relevant for your specimen, James.