FMF - Friends of Minerals Forum, discussion and message board

[Pete Richards]

[alfredo]

Many thanks to Peter for taking the time to explain these things! Much appreciated by those like myself who were weak in crystallography and have much to brush up on.

I was told that these amesite crystals represent "6-fold sector twinning". Is that a type of "cyclic twin"? (Thanks to the photographer, Saul Krotki.)

On another note, I think that many complex and multiple Japan-law twin quartzes disprove the notion that twinning had to start at the onset of crystal growth. When one sees Japan-law twins whose members are very different in size, with the smaller one jutting out halfway up the prism of its larger partner, or even multiple "juniors" at different positions on a larger member, it's hard to see how this could have all formed at the initiation of growth.

[Jordi Fabre]

[Peter Farquhar]

As my first post on FMF, I’d like to join the discussion about rutile crystals and cyclic twinning.

“Cyclic twinning” is typically defined as “the repeated twinning of three or more individual crystals according to the same twin law, but with the twin axes not parallel.” (e.g., see mindat glossary and other sources). On the other hand, such repeated twinning is called “polysynthetic” if the twinning axes are parallel.

It may seem surprising, but loop closure is not a characteristic for cyclic twinning under this definition. While Ford points out in Dana’s Textbook, this “repetition of twinning tends to produce circular forms” under certain conditions, he also acknowledges the zig-zag shaped twins from “repeated geniculations.”

Therefore, the category of “cyclic twins” for rutile includes both complete (closed-looped) and incomplete (open-loop) sixlings, eightlings, and other n-sided specimens, as well as the (non-loop) geniculated crystals that zig and zag in successive directions.

Challenge #1: Cyclic twinning is common in rutile, but polysynthetic twinning is rather uncommon in rutile. Here is an example of both types in one specimen. The Brazilian rutile specimen below illustrates both cyclic twinning (a partial sixling) and polysynthetic twinning (a doublet). Considering the earlier discussion in this thread, how did this crystal form?

Challenge #2: Magnet Cove Arkansas has produced some wonderful rutile eightlings. Below is a specimen shows an eightling that is part of a larger eightling. What do you call this cyclic twinned rutile? How did this crystal form?

Please let me know if you’d like to see more challenges.

Peter Farquhar
Claremont, California
USA

[Dean Allum]

[Peter Farquhar]

The rutile sixling from Atglen, Pennsylvania shown below looks almost identical to the purple 3-D drawing that Pete Richards posted earlier.

Peter Farquhar
Claremont, California
USA

[Pete Richards]

[gemlover]

Great and beautiful specimen. And from Atglen, I lived in both Chester and Lancaster counties in that area.

John

[Peter Farquhar]

Another perspective on cyclic twinning in rutiles:

The photographic plate below is from Louis Pope Gratacap’s famous 1912 book, "A Popular Guide to Minerals." The black-and-white plate nicely illustrates different forms of cyclic twinning with three Pennsylvania rutile specimens. Gratacap selected these rutiles from the Clarence S. Bement collection in the American Museum of Natural History, where Gratacap served as the Curator of Mineralogy in the early 1900s.

In 1907, Gratacap used this same plate for another purpose in a short article he published on mineral photography in the "American Annual of Photography 1908." Gratacap was experimenting at the time with different lighting techniques and camera lens, and chose these rutile specimens to illustrate the many challenges in photographing cyclic twins. But little has changed more than a century later. Wendell Wilson recently commented that rutiles “are one of those minerals that are very difficult to photograph, and the worst of the lot are the cyclic twins …” (Axis, 2005).

With these warnings in mind, I tried to recapture in the 2012 color image shown below the same Parkesburg rutile specimen figured on the right side of Gratacap’s 1907 plate. A side-by-side comparison of these two images reveals the complex challenges in portraying the “geniculated oscillations” that Gratacap wrestled with over a century ago.

Peter Farquhar
Claremont, California
USA

[John Jaszczak]

Nicely done!

[Peter Farquhar]

[Tracy]

[Tracy]

Um...so that's one vote for yes, and one vote for no? I'm really confused.

- Tracy

[Pete Richards]

[Tracy]

Would other photo angles be helpful? I should have time this weekend.

Thanks Pete and Peter! The suggestion of intergrown v-twins makes sense to me.

[Puzzles make me happy. :-)]

- Tracy