FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

Here's a nicely back-lit shot of a well known spinel twinned fluorite from Naica. Color zoning and sharp domain boundaries again show the typical complexity of the Naica material.

For those who have the nice sharp small blue and purplish blue twins that have come out of Naica over the last few years...get out your handlens and look for elongate curving sulfosalt (probably) inclusions...some get quite baroque!

[Ru Smith]

Wow. Another beauty.

[Ru Smith]

Is this a fair summary of what we've seen so far?: 1) both spinel twins and penetration twins in fluorite involve a 60 degree (180 if you like) rotation around [111]; 2) in fluorite spinel twins this rotation does not occur across a single twin plane, but 3) the two interpenetrating twin members (approximately) share large external (111) octahedral faces (we've seen this in the Naica, Hunza Valley and Erongo examples so far).

[Roger Warin]

Hi,
Wooden models were designed to explain to students the properties of crystals. By the reel in hand we find the symmetry operators (useful for identifying a mineral).
Such a model is useful for understanding the spinel twin on several minerals with cubic symmetry (spinel, fluorite, diamond, sphalerite, galena ...)
This {111} twin law is common either by interpenetration or by contact. Fluorite is often twinned by interpenetration under this law.
Spinel often has the same law, but by contact on a composition plane ({111} twin plane). There are thus two octahedra joined to one another in a face of the octahedron.
Here is such a wooden model to illustrate the spinel twin.
The model was cut parallel to a face (e.g. # 31) to obtain the spinel twin after a 60 ° rotation of the lower part.
Roger.

[Ru Smith]

Thanks, Roger. An excellent model and very good match to the Mogok spinel twin illustrated and discussed above. The fluorite twins are different though in that they lack a single composition plane and show complex interpenetration. Have you seen any research publications on the nature of fluorite spinel twins?

[Jesse Fisher]

I think we understand the geometry of spinel twinning (though nice model!). What is in question is just why spinel-twinned fluorite always seems to have a sutured composition plane rather than a straight one as seen in the twinning of other octahedral crystals such as spinels and diamonds.

Below are a couple more photos of spinel-twinned fluorites from Naica.

[Roger Warin]

Hi,
Pete could certainly better answer than me. But here's how I understand the phenomenon.
A first remark, Nature is more capricious than when we try to explain it. Also to simplify the facts, we take extreme situations.
When we say that there is a composition plane in a contact twin, it is assumed that individuals are far from each other, but they touch anyway.
If the distance between the centers of two crystals (or individuals) decreases, a partial interpenetration occurs. The edifice is deformed from the wooden model.
The distance between the centers may still decrease, resulting in a greater interpenetration.
Ultimately, the two centers overlap and we have the perfect interpenetration, another limit of this evolution.
At the atomic level, the twin plane is the same, the face of a {111} octahedron. It is always the same twin.
The contact twin is a mirror twin plane.
Interpenetrating individuals do have an irregular composition surface. For this reason, the twin is defined in this case by a twin direction axis, e.g. [111] for the spinel law.
[…] is used to define a vector, {…} for a crystallographic form, and (…) for a defined plane.
Roger.

[Ru Smith]

Another excellent Naica specimen, Jesse, once again highlighting the degree of interpenetration and absence of a single composition plane in these twins. Going back to the very first image in this thread, I've taken a moment to colour-code the two members of that Erongo twin.

[Ru Smith]

Back to some real spinel twins, these Peruvian alabandites show spinel twinning of cubeoctahedra, octahedra and some other forms I can't decipher.

[Ru Smith]

Another one with a clear composition plane.

[Ru Smith]

I now have one of the little Naica gems to look at closely and here is a quick batch of photos.

The first thing to note is that these are not spinel twins, but beautiful penetration twins (the usual [111] 60 degree rotation). I wonder where the spinel twin idea came from for these? The flattening?

They are complex crystals with numerous small faces. An untwinned crystal on the back has cube, dodecahedron and trapezoidal icositetrahedron. I think all of these forms are probably present in the twins, plus the etched octahedral faces. As you can see, there is selective etching of faces. In the untwinned crystal on the back the icositetrahedron faces are etched and the cube faces not.

looking inside, there are the curving wires of probable sulphosalt (4th photo) mentioned above by Peter Megaw and also a bubble (5th photo) that wanders up and down its tube as you tilt the crystal.

Quite a lot going on here. Good fun!

[Ru Smith]

By the way, each of the large apparent octahedral faces on the Naica specimen is composed of numerous minute cube corners; there are no reflective surfaces parallel to the octahedron. This is very similar in character to the famous "c-faces" of quartz from Diamantina or amethyst from Four Peaks, Arizona. The dodecahedron and cube faces in the Naica specimen are smooth and lustrous. The trapezoidal icositetrahedron faces are rough.

The attached gem pink fluorite from Pakistan is also not a spinel twin, but rather a penetration twin - also a challenge to identify all the faces on this one, though octahedron, dodecahedron and trapezoidal icositetrahedron faces become clear after turning it around for a while.

Do spinel twins of fluorite exist? I've yet to see one.

[Ru Smith]

Some explanation of the Naica twin.

[Ru Smith]

In the Naica image above O is octahedron, C is cube, D is dodecahedron, and I is icositetrahedron.

By way of contrast, here's a spinel law contact-twin of sphalerite.

[Ru Smith]

Checking Dana's Textbook of Mineralogy (pages 165-166) I see that he distinguishes contact-twins (spinel-twins) from penetration twins in isometric crystals, both with "the twinning axis an octahedral axis". He shows one example of each for Galena; 401 being the contact twin and 404 being the penetration twin (attached figure).

I found some nice examples of the penetration twin form on a specimen of galena from Naica, Mexico.