FMF - Friends of Minerals Forum, discussion and message board

[Malcolm Southwood]

Ahh Pete,
A senior moment - chalcotrichite indeed!

I'm very interested in this theme of elongated crystals, and would like to explore it a little further. I, too, have examples of these among my Weardale fluorites, and the sometimes-cited explanation of "asymmetry in the growth environment" makes intuitive sense, but doesn't in any way explain the phenomenon.

Geometrically, cubes can tesselate perfectly, to form a "bar" of any size or shape with three axes of symmetry still at 90 degrees to one another. Should we perhaps think of these elongated fluorite crystals as a "stack" of cubes?

Also, Kantor (in the reference I cited in my earlier posting) talks about what he calls the "corner octahedron" model for development of skeletal crystals in the isometric system. I won't begin to attempt an explanation of that here, but it seemed to me not a million miles away from the idea of "stacked cubes", but with the stacking very much ordered by the symmetry requirements of the mineral concerned.

Thanks for your thoughts
mal

[Gail]

Blackdene mine.

[Ru Smith]

Nice.

[Jordi Fabre]

[ian jones]

A very quick picture of another elongated Blackdene fluorite with quartz on the LH side. Never common although for some reason, Blackdene seemed to produce them.

Now off to the airport to get away from the cold here in the UK for a few days.

[Ru Smith]

So that's intriguing. What favoured this growth habit at Blackdene and Boltsburn and not at the other mines in the district (bar rare exceptions)? Enjoy the sun!

[Ru Smith]

[Steve Maslansky]

Here is a example from the Denton Mine of Illinois

[Jordi Fabre]

Double-dealing: elongated and etched! ;-)

[Jesse Fisher]

Ru - the Rogerley "yellow corners" are always on an outside corner of the crystal, never at the base or core. Also, they often occur on crystals that are otherwise fairly equant in shape, and show no suggestion of preferential growth in a particular direction.

It occurred to me that, while twinning is common in North Pennines fluorite, the examples of highly elongate crystals I've seen are usually untwinned. Looking through the collection, I found a few examples of twinned crystals that show a tabular habit, but nothing as extreme as the Boltsburn and Blackdene crystals previously posted. Below is one from the Boltsburn Mine. I also have a few, though much smaller examples from the Rogerley.

[Mark Ost]

Spectacular. My Goodness.

[Malcolm Southwood]

Jordi, thanks for the correction, (thereby sparing future blushes!)

Pete,
I'd like to revisit, for a moment, the screw dislocation idea for these elongated crystals. Radovan (2004, Rocks and Minerals 79 (6) pp 415-417) attributes the development of vicinal faces and growth hillocks in fluorite to spiral growth as a result of such dislocations. Surely this would suggest that the mechanism might not be limited to whisker crystals?

Cheers
mal

[Jordi Fabre]

[Pete Richards]

[Malcolm Southwood]

Thanks Pete,
That's helpful.
mal