FMF - Friends of Minerals Forum, discussion and message board

[Ru Smith]

So, I think I've found an actual spinel twin of fluorite. It's from Naica and involves cube-dodecahedral crystals. The twin plane is parallel to an octahedron face, but there are no octahedral faces expressed, so the twin looks very different from the flattened pseudohexagonal penetration twins. Nice!

[Ru Smith]

Another view. The trace of the twin plane is in the reentrant just beneath the quartz to the right and is shown again by the reentrant on the left. A cube face of the upper member of the twin is facing us, with dodecahedron faces above to the left and above to the right. The twin axis passes through the triple junction of these two dodecahedron faces and the third one just out of sight at the back.

[Ru Smith]

found this in Chang, Howie & Zussman (1996).

[crazy.stone]

beautiful especially color. pretty nice.

[Ru Smith]

Thanks crazy stone.

There are lots of interesting colours here, but this thread is mostly about shapes - stimulating spatial mental puzzles!

[Ru Smith]

Here is a very large one of the Erongo pseudohexagonal penetration twins.

[Ru Smith]

Here I've added some backlighting. The cube faces are naturally colour-coded in dark purple, making the twin easier to decipher.

[Ru Smith]

This is approximately what's going on with the interpenetrating twin elements. The blue-shaded element has purple cube-faces adjacent to the NW, NE and S ("N" assumed to be up) edges of the front shared hexagonal octahedron face. The unshaded element has purple cube faces adjacent to the SW, N and SE edges of the shared frontal octahedron face. The pattern is similar on the reverse side of this floater, but I'm too lazy to draw it.

[Josele]

This is a very interesting thread. Ru's twinned crystals collection is a truly treasure and explanations and drawings are very clear and instructive, thanks for your effort.

Concerning spinel and penetration twin in fluorite, seems that is not easy to establish a border line between them. Theoretically spinel twins have a contact plane interface but actually this surface is not plane and occupies a 3D space, sometimes very intrincated. Where is the difference with penetration twins? In the fact that penetration twins cross to the other side? In the degree of intrincation? Could be that they are the same thing? Could be that this is only a morphological classification easily applicable in ideal geometric forms but perhaps is not valid in some wayward species as fluorite?
After all, mystery of twins is how they manage to match both crystal lattices and this miracle occurs in both cases...

[Ru Smith]

Many thanks Josele.

Nice model of the penetrating cubes. I still haven’t seen the ideal example of this in nature, though a fluorite found by Arthur Scoble in the Blackdene Mine comes close.

I didn’t know quite what to expect from the so-called spinel twins in fluorite and was surprised by that first Erongo one.

It is true that all twins shown above by various people in this thread share one property: rotation of 60 or 180 degrees (whichever you prefer) around [111]. "The twinning axis is an octahedral axis" as Dana puts it. So, if we are “lumpers”, sure, these all belong in one big family.

Now there is then a clear class to be seen in the sample of specimens shown above in which the contact between twin elements forms a continuous trace around the twin, representing the intersection of a plane perpendicular to [111] with the outside of the twin. How much deviation from a plane there is inside the crystal is not accessible to us without some dissection or perhaps tomography. The view from the outside, however, suggests a very close approximation to a plane. The examples showing this above are: spinel, sphalerite, alabandite, diamond, Dana’s drawing of a galena spinel contact-twin, a wooden model, and the single example of a fluorite crystal showing this that I could find (the cube-dodecahedron from Naica). "This is called the spinel twin because it is so common for the mineral spinel" - and numerous similar statements in the various textbooks.

The other end-member would be your wooden model and the classic English fluorite interpenetrating cube twins, which entirely lack the twin boundary perpendicular to [111]. The pseudohexagonal fluorite twins also lack this boundary between twin elements and each element penetrates completely through the other. These twins look a bit different from the interpenetrating cubes since there’s an octahedral face bounding top and bottom of the twin, shared between the two twin elements on both sides (let’s say “top and bottom”) of each twin. This was a revelation to me on seeing the first Erongo specimen and the Naica and Chumar Bakhoor examples shown above confirmed the motif (Jesse by the way clicked to this very quickly). The delightful fluorite “berry” from Colorado showed one more variation on this theme, with octahedron faces missing and the combination of cube, dodecahedron and icositetrahedron making the crystal shape tend towards a sphere.

So far we have seen two discrete classes: one with a (near-) planar boundary between the twin elements (not one of my examples illustrated above shows a deviation from a plane that is visible to the eye - can't speak for Roger's diamond); twins in the other class all lack this planar boundary and show interpenetration between the two twin elements. Presence or absence of octahedral faces makes a large difference in the appearance of this second family of penetration twins.

The remaining distinctive motif (belonging to the larger family in which "the twinning axis is an octahedral axis") in the sample of specimens shown in the thread is Dana’s penetration twin of galena (his illustration 404). One element of these twins is rotated around [111], but does not penetrate out to the octahedral faces through which the twinning axis passes. I’m guessing that large areas of the boundary between elements in such twins would be roughly perpendicular to [111], so, in this sense, these could possibly be considered transitional.

[Ru Smith]

Meanwhile, inspired by Josele's wooden model, goes to fridge to look for model-making materials. Carrots or cheese?

[Ru Smith]

Cheese.

[Ru Smith]

The penetration twin lacked something in the execution, but the cube and cube-dodecahedral spinel contact twins came out well.

[prcantos]

Really surprising idea! These are 'gourmet twinnings'...

These examples show that twinning may be studied using abstract geometry which doesn't depend on the crystalline structure.

[Peter Farquhar]

What a great idea for learning about twinning while enjoying a pleasant snack. The opportunities seem endless. Do you recommend any particular cheese?