FMF - Friends of Minerals Forum, discussion and message board

[Peter Farquhar]

Thanks Pete for explaining that parallel orientations of rutile V-twins may be the result of epitaxy where the host has weathered away. This solves one mystery for me about twinning. Now I have another question.

In general, what are the criteria that distinguish between crystals that are merely intergrown versus ones that are twinned? How does this definition determine whether three or more crystals are repeatedly twinned versus just being intergrown?

Below is another rutile from Diamantina similar to the one Tracy has. I'll ignore for now the one small V-twin stacked on top of two larger V-twins. My question is about the two larger V-twins -- are they simply intergrown (V-V), or are they repeated twins (W)?

Based upon the boundary where they contact each another, it looks like the two larger V twins are simply intergrown, but not repeatedly twinned. I'd like some help sorting this out. Thanks.

Peter Farquhar
Claremont, California
USA

[Pete Richards]

[Peter Farquhar]

Pete, thanks for your help analyzing this latest rutile crystal. As Tracy says, "Puzzles make me happy."

Here are the answers to the questions you raised earlier about the angles of the twins in the last photo of the rutile from Diamantina, Brazil.

[Duncan Miller]

Consider a concise definition of 'cyclic twinning' as given in a basic textbook like Dana's Manual of Mineralogy (18th edition ed. Hurlbut). "Repeated or multiple twins are made up of three or more parts twinned according to the same law. If all the successive composition surfaces are parallel, the resulting group is a polysynthetic twin. If successive composition planes are not parallel, a cyclic twin results."

Now, if in the latter case the twinning proceeds in the same direction each time, either right or left, an open U or a closed loop cyclic twin forms. If the twinning alternates, with the same angular relationship between individuals but orientated alternately right and left, a zigzig shape will result. Here the composition planes appear to be parallel, so in terms of the above definition these would be polysynthetic twins.

If one accepts this simple, purely geometric definition then interesting crystal growth issues like penetration twins vs cyclic twins and single nucleation vs successive nucleation perhaps do not determine whether a particular twinned crystal should be described as cyclic. It simply depends on whether successive composition planes are parallel or not. Isn't cyclic twinning just a special case of polysynthetic twinning? (Please note, I cautiously wrote 'perhaps' here because I am also trying to clarify things myself. I am no expert on twinning.)

Duncan

[Pete Richards]

[Duncan Miller]

Pete thanks for your response.

Perhaps I confused two points.

1. I was thinking about the zigzig twins shown above. Are the composition planes not parallel? They are repeated or multiple twins, but according to the Dana definition they would seem to be polysynthetic twins, despite not being repeated very numerous times as in plagioclase feldspars. I know we tend to think of polysynthetic twinning as characteristically having large numbers of lamellar twins, but isn't this just a matter of number, not geometry?

2. If repeat 'geniculate' twin are polysynthetic (in Dana's sense) then don't cyclical twins differ only in having a consistent twinning handedness? I understand that polysynthetic and cyclical twinning generally are seen as distinct categories of repeat twinning but perhaps they just differ in handedness of the geniculation. I will attached a scanned image from the Dana volume of rutile twins to illustrate what I mean.

On a somewhat different tack, from reading the earlier discussion it appears that genetically there may be two kinds of cyclical twinning, both conforming to Dana's geometric definition. The classic chrysoberyl and aragonite sixling twins may be penetration twins, making them distinct from the 'hollow' wheel cyclic twins of rutile, which have repeated 'geniculation' in the same orientation. Is this not a more fundamental distinction than the diverse rutile examples?

Duncan

[Pete Richards]

[Pete Richards]

I mentioned rutile twinned on {301} some while back in this thread, but did not have an image. I just scanned a slide I had of one of mine - no it's not really cyclic twinning, but it sure is pretty....

[Duncan Miller]

[Elise]

Hi --
Karl's paper is a great one; he always includes great diagrams (colorful too). We also had this earlier thread on cyclic twinning which concluded with a chrysoberyl being used as an example. https://www.mineral-forum.com/message-board/viewtopic.php?p=13372&highlight=#13372
Cheers!
Elise

[Pete Richards]

[Elise]

Hi Pete,

I only remembered it having perked up at the SEM images you posted and Duncan's reference to Karl's alexandrite paper...my short-term memory needs a bread crumb trail ;-)

I have been getting some experience with the SEM, realizing how hard it is to capture such images as those!

Elise

[Dean Allum]

Wow, I was just about post a question about the atomic origins of crystal twinning.
Elise, Do you have a pdf copy of that nanoLetters paper?
Regards,
Dean ALlum

[Peter Farquhar]

So far our discussion of cyclic twinning in rutile has covered sixlings and eightlings with a central cavity. In response to Pete's earlier question, though, I cannot find any cyclic twinned rutiles with a hole clear through the center.

On the other hand, we have not yet discussed cyclic twinned rutiles without a central cavity. Below is another type of rutile eightling -- one without the familiar dimple in the center. This old rutile specimen is from Graves Mountain, Georgia; it can be compared with more common eightlings with central cavities from Magnet Cove, Arkansas as shown earlier in this thread.

Peter Farquhar
Claremont, California
USA

[Ru Smith]

This is a great thread, with some fascinating specimens, perhaps the latest one in particular.

Here attached is a very small, but lovely arsenopyrite cyclic twin from Canada that's fun to see.

- and, to support the comments above on interpenetration in trillings, an unusual Brazilian chrysoberyl in which one of the trilling elements is visible in continuity across the twin (this is a side view).