FMF - Friends of Minerals Forum, discussion and message board

[Ru Smith]

I just took this close-up of a Hilton Mine fluorite (reducing the depth of field to avoid the distraction of the internal transparency of the crystal). Who can decipher which forms result in such complex modification of the corners?

[Roger Warin]

Hello,
The cube is truncated by a hexoctahedral form on every corner.
I add a picture in a future post.
Roger.

[Roger Warin]

You have a beautiful fluorite and these truncations are not so frequent.
The result of calculation with the Miller indices {137} is put on drawing.
These values are a visual interpretation of the picture. To confirm, it is necessary to measure the dihedral angles between the various faces.
So your fluorite has 54 faces, 56 corners and 108 edges in a single euhedral crystal .
Roger.

[Jesse Fisher]

While modifications to the cube other than twinning are not commonly seen on Northern English fluorites, I have seen hexoctahedral corners on a few other specimens, including one that I have from the Boltsburn Mine. Rare but not unknown.

[J Rafael]

Nice sample. It's like fluorites from Emilio Mine
https://www.mindat.org/photo-11832.html

J. Rafael

[Roger Warin]

Speaking fluorite, here is a well-presented.
Roger.

[Ru Smith]

Fantastic guys! This definitely adds to my enjoyment of the crystal.

Roger, that is a wonderful summary. What software do you use for the crystal drawing?

I agree with Jesse that these combinations of cubes with hexoctahedra occur elsewhere in this region. I have examples from Seata, Wetgrooves and Great Sleddale in Yorkshire; Burtree Slitts in Weardale and the Diana Vein specimens from Northumberland that we looked at a couple of days ago also show this form.

Delightful specimen from the Emilio Mine J Rafael. Thanks for showing that.

[Roger Warin]

I use an older version of Dowty’s SHAPE (1995).
Roger.

[Pete Richards]

Cubes with corners modified by hexoctahedra (specifically {137}) are fairly common in fluorite from northern Ohio. Other cubes are often modified by the trapezohedron {113}, often mistaken for a trisoctahedron since three faces occur where each octahedron face would be. See drawings for subtle distinction.

[Ru Smith]

That's excellent Pete. Thank you.

I just looked through the fluorites and found some more examples of cubes with hexoctahedra, but also two specimens showing cube with trapezohedron (both from Yorkshire). Most specimens lack these corner modifications.

I'll take some more photos a bit later, but here's one of each.

[Ru Smith]

and I have found one (just 4 mm across), from Great Sleddale with both hexoctahedron and trapezohedron.

[John Medici]

Thanks for the diagrams, Roger and Pete. I'm still learning. Following are a few northwest Ohio modified fluorites. Let me know if I have wrong modification ID. Thanks.

[Ru Smith]

Well look at this! I can see cube, octahedron, trapezohedron and hexoctahedron. There's another form in there too; look at the very subtle bevels at either side of the trapezohedron face at left. Could that be the trisoctahedron?

How many faces, corners and edges do you reckon, Roger?

[Ru Smith]

Lovely specimens, John.

I hadn't seen the tetrahexahedron (double bevel on cube edges) before. I once had some beautiful Naica specimens showing dodecahedral faces, but they've been lost along the way unfortunately.

These complex crystal forms definitely add to the interest.

[Ru Smith]

Those subtle bevels can't be the trisoctahedron now I look again, but must be another form.