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Spinel twins
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Ru Smith




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PostPosted: Apr 26, 2013 17:46    Post subject: Spinel twins  

Contemplating the attached fluorite twin, I think it's starting to make sense. All the interfingering domains with angular differences are confusing at first. I see now though that these are cube and octahedral faces of the two cube-octahedral members of the twin and, whereas the symmetry relationship between the two members is roughly a reflection across an octahedral face, there is no simple contact plane (as seen in spinel twins of spinel). The two members of the twin are interpenetrating in a complex way, so this seems to be a penetration twin rather than a simple contact twin. Is that right?

The second and fourth images show the interfingering twin domains on one of the two big octahedral faces (roughly hexagonal in shape due to the adjacent presence of three cube faces). You can also see the difference between the dihedral angles of a) two adjacent octahedral faces (109.47 degrees?) and b) adjacent octahedron and cube faces (125.264 degrees?).

I'd like to see some more images of spinel twins in fluorite. Do any of them have simple contact planes? Any ideas on what conditions promote the formation of spinel twins?



Fluorite Erongo twin 1.jpg
 Description:
Fluorite twin.
Erongo, Namibia
30 mm.
 Viewed:  33151 Time(s)

Fluorite Erongo twin 1.jpg



Fluorite Erongo twin 2.jpg
 Description:
Fluorite twin.
Erongo, Namibia.
30 mm.
 Viewed:  33083 Time(s)

Fluorite Erongo twin 2.jpg



Fluorite Erongo twin 3.jpg
 Description:
Fluorite twin.
Erongo, Namibia.
30 mm.
 Viewed:  33091 Time(s)

Fluorite Erongo twin 3.jpg



Fluorite Erongo twin 4.jpg
 Description:
Fluorite twin.
Erongo, Namibia.
30 mm.
 Viewed:  33139 Time(s)

Fluorite Erongo twin 4.jpg


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mmauthner




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PostPosted: Apr 26, 2013 20:02    Post subject: Re: Spinel twins in fluorite  

The twin composition surface is still parallel to the octahedral face and a plane, making this still a contact twin. Twin laws are by definition either interpenetrant or contact depending on the law's twin element which is either an axis or plane ( though 3D merohedral composition surfaces also exist).
Hope that helps.
Mark
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Ru Smith




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PostPosted: Apr 26, 2013 21:04    Post subject: Re: Spinel twins in fluorite  

Thanks very much, Mark.

I was remembering Hurlbut's comments on mica twins (https://www.mineral-forum.com/message-board/files/mica_penetration_twins_203.jpeg) where he wrote, "However, these twins do not conform to that part of the law that states that the composition plane is (), if we consider composition plane in the usual sense, namely the plane on which the two individuals are united. The contact surface is highly irregular but roughly at right angles to (). It is a penetration rather than a contact twin."

This fluorite seems to show a very similar configuration. Indeed, in three domains shown in the last image the far element of the twin has extended completely through the element of the twin in the foreground. But, as you say, wherever there are flat portions of that highly irregular contact surface they are parallel to the octahedral face.

The wikipedia sentence on penetration twinning is as follows: "In penetration twins the individual crystals have the appearance of passing through each other in a symmetrical manner." In the case of this fluorite and Hurlbut's mica twins, the individual crystals do "pass through each other", but they don't do so "in a symmetrical manner", so I'm happy to accept your definition.
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Enrique Llorens




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PostPosted: Apr 27, 2013 03:13    Post subject: Re: Spinel twins in fluorite  

Spinel twin of fluorite from Mexico.


FLUORITA NAICA J SCOVILL-1.jpg
 Description:
Fluorite
Naica, Mexico
5x2,5 cm
 Viewed:  32962 Time(s)

FLUORITA NAICA J SCOVILL-1.jpg


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Ru Smith




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PostPosted: Apr 27, 2013 09:41    Post subject: Re: Spinel twins in fluorite  

A very beautiful specimen, Enrique.

Can you see the contact between the two individuals in each twin? Is it a single plane, or is it a highly irregular surface with one individual "passing through" the other (as in the Erongo specimen)?
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Ru Smith




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PostPosted: Apr 27, 2013 18:08    Post subject: Re: Spinel twins in fluorite  

Here's another complex fluorite to decipher (from the Houston show just now). What do you see?


Fluorite Smoky Hawk 1.jpg
 Description:
Fluorite.
Fluorite Berry Pocket, Smoky Hawk Mine, Florissant, Teller County, Colorado.
17 mm fluorite group on 5 cm smoky quartz.
 Viewed:  32772 Time(s)

Fluorite Smoky Hawk 1.jpg



Fluorite Smoky Hawk 2.jpg
 Description:
Fluorite.
Fluorite Berry Pocket, Smoky Hawk Mine, Florissant, Teller County, Colorado.
17 mm fluorite group on 5 cm smoky quartz.
 Viewed:  32841 Time(s)

Fluorite Smoky Hawk 2.jpg


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Ru Smith




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PostPosted: Apr 27, 2013 20:06    Post subject: Re: Spinel twins in fluorite  

After looking at this for a while I can see rhombic faces of the dodecahedron, and bigger trapezohedral faces of the "trapezohedron" (icositetrahedron) which are truncated by cube faces (there's one abutting against the smoky quartz).

The contact between the two individual components of the upper twin (look for the re-entrants) is not a simple plane and "wraps around". Looking along one of the axes of 3-fold rotational symmetry it seems to me that a rotation of 60 degrees would overlay the faces.

So, this one is a penetration twin? Just like the well-known English penetration twins, but almost unrecognizable (to my eyes) as a result of the dominance of the "trapezohedron" (trapezoidal icositetrahedron) and dodecahedron.

I had never seen this before. Anyone else?
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Ru Smith




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PostPosted: Apr 27, 2013 23:29    Post subject: Re: Spinel twins in fluorite  

Back to spinel twins, here's a galena showing the same juxtaposition of octahedron and cube faces as seen in the Erongo twin.


Galena Bulgaria.jpg
 Description:
Galena spinel twin.
Borieva Mine, Rhodope Mountains, Bulgaria.
37 mm specimen.
 Viewed:  32731 Time(s)

Galena Bulgaria.jpg


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Roger Warin




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PostPosted: Apr 28, 2013 08:09    Post subject: Re: Spinel twins in fluorite  

Spinel twin in diamond.
Here is a diamond crystal twinned on {111} (spinel law).
Roger.



Diamant#2d_R.jpg
 Description:
Diamond
venezuela
Spinel twin on {111} (octahedron face)
 Viewed:  32644 Time(s)

Diamant#2d_R.jpg



Diamant#2e_R.jpg
 Description:
Diamond
Venezuela
Same crystal
 Viewed:  32658 Time(s)

Diamant#2e_R.jpg


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PostPosted: Apr 28, 2013 10:59    Post subject: Re: Spinel twins  

Here is an interesting fluorite spinel twin from Dalnegorsk, Russia. It is 4 cm in the widest dimension. The bottom member of the twin is very transparent but the top member is not, which I find very unusual.


P1010797.JPG
 Description:
 Viewed:  32631 Time(s)

P1010797.JPG



P1010796.JPG
 Description:
 Viewed:  32652 Time(s)

P1010796.JPG



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Ru Smith




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PostPosted: Apr 28, 2013 11:09    Post subject: Re: Spinel twins  

Thanks, Roger. Your diamond is a nice example of a simple spinel twin where one individual is spun 180 degrees (just 60 also works) around [111] relative to the other on a (111) twin plane (or we just reflect across that plane). There's a side view of such a twin in spinel attached, which I like because you can see the reflection, but also easily imagine rotating the top part 60 degrees to reform the octahedron. I read somewhere that Be ions substitute for Mg ions along the (111) twin boundaries in these Mogok spinels, promoting exaggerated growth along the twin boundary.

So, are such twins, with a discrete twin plane between the individuals, ever seen in fluorite? or do fluorite spinel twins always show the complex intergrowth of the two individuals seen in the Erongo specimen above?



Burma spinel side.jpg
 Description:
Spinel twin of spinel.
Pinpyit, near Mogok, Burma/Myanmar.
7 mm.
 Viewed:  32621 Time(s)

Burma spinel side.jpg


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Ru Smith




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PostPosted: Apr 28, 2013 11:13    Post subject: Re: Spinel twins  

That's a really interesting twin, John. Just octahedra this time, so a bit less mind-scrambling? What do you think is going on in this one? The two members were not (fully) coeval?
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Ru Smith




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PostPosted: Apr 28, 2013 11:15    Post subject: Re: Spinel twins  

.. though I see some cube faces too. Are the angles right? - are we sure this is a twin?
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Jesse Fisher




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PostPosted: Apr 28, 2013 14:01    Post subject: Re: Spinel twins  

I can not recall ever seeing a spinel-twinned fluorite having a simple contact plane as seen in twinned spinels. I always assumed that it was due to the presence of cube and/or dodecahedral faces on the fluorite, but we all know the old saying about assumptions. Would be interesting to know the cause.

Below are photos of a spinel twin fluorite from Pakistan, which shows this fairly nicely.



F299r.jpg
 Description:
Fluorite (spinel twin)
Chumar Bakhoor, Hunza Valley, Northern Areas, Pakistan
10x8x2.5 cm overall size
 Viewed:  32519 Time(s)

F299r.jpg



F299-7670r.jpg
 Description:
Fluorite (spinel Twin)
Chumar Bakhoor, Hunza Valley, Northern Areas, Pakistan
10x8x2.5 cm overall size
edge-on view showing twin contact.
 Viewed:  32541 Time(s)

F299-7670r.jpg


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Ru Smith




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PostPosted: Apr 28, 2013 14:26    Post subject: Re: Spinel twins  

Wonderful crystal, Jesse.

- and quite like the Erongo example in clearly showing the complex boundary between the two twin members.
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PostPosted: Apr 28, 2013 18:13    Post subject: Re: Spinel twins  

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!



Naica spinel twin web.jpg
 Description:
Fluorite (spinel twin)
Torino-Tehuacan Chimney, Naica Mine, Chihuahua, Mexico
minature
Lauren Megaw specimen'
Jeff Scovil photograph
 Viewed:  32377 Time(s)

Naica spinel twin web.jpg



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Ru Smith




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PostPosted: Apr 28, 2013 18:23    Post subject: Re: Spinel twins  

Wow. Another beauty.
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Ru Smith




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PostPosted: Apr 28, 2013 22:49    Post subject: Re: Spinel twins  

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).
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PostPosted: Apr 30, 2013 04:22    Post subject: Re: Spinel twins  

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.



Spinel-tw-modele-1_9505_R.jpg
 Description:
Wooden model: octahedron
 Viewed:  32076 Time(s)

Spinel-tw-modele-1_9505_R.jpg



Spinel-tw-modele-2_9506_R.jpg
 Description:
wooden model, after a partial rotation around [111]
 Viewed:  32069 Time(s)

Spinel-tw-modele-2_9506_R.jpg



Spinel-tw-modele-3_9507_R.jpg
 Description:
wooden model, after a 60° rotation.
 Viewed:  32049 Time(s)

Spinel-tw-modele-3_9507_R.jpg


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Ru Smith




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PostPosted: Apr 30, 2013 19:08    Post subject: Re: Spinel twins  

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?
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