FMF - Friends of Minerals Forum, discussion and message board

[Peter Farquhar]

The rutile specimen below (PF-3651) is from Graves Mountain, Georgia, and was once in Rock Currier's collection #3085. This rutile is quite unusual, because it has six sides yet no central cavity.

I am interested in a discussion of whether this rutile specimen exhibits cyclic twinning. Is it appropriate to call this rutile a "sixling"?

[Pete Richards]

I am confident that this is a twin, a hexagonal sixling like the drawing. The main difference is that the pits shown in the drawing have been filled in on the actual specimen.

I can imagine distortions to a single prismatic tetragonal crystal that would cause it to look vaguely like a hexagonal plate, but such distortions would seem highly unlikely to occur. For such distortions to produce such a perfect hexagonal plate, with all faces balanced, is beyond credibility.

It might be possible, with careful examination and use of different lighting directions, to make out the twin boundaries between adjacent individuals on the flat plate on top of the twin, on the basis of differences in reflectivity between the two individuals. This is often possible with Graves Mountain twins, especially if their faces are slightly etched, but may not be possible on such a lustrous surface. Use of polarizing filters might help.

The attached photo is of a simple rutile twin from Graves Mountain twinned according to the same law. The flat face corresponds to the hexagonal face in the specimen that is the subject of this post. The parts of the face belonging to the two individuals are illuminated quite differently, in spite of the fact that they are co-planar.

[Bob Morgan]

I too tried to figure how your rutile would be a single and couldn't find any way that worked. The dimpling indicates fairly strong dissolution. perhaps there was also some redeposition that filled in the center.
In the photo there appeared to be some differences near the outer edges which would also indicate twin boundaries.

[James Catmur]

It may be me, but I see very faint marks that are where the dimple should be

[Peter Farquhar]

Based upon the helpful feedback, I carefully examined the top of the rutile specimen again. This time I experimented with different lighting sources, temperatures, and angles; polarizing filters; and 10x, 20x, and 40x stereo magnification. In the end, I could find no evidence of where a central cavity, pit, or dimple might have been.

Using a different camera, I did take another photo of this rutile specimen (PF-3651) in the same orientation. This photo shows a faint scratch not visible in the earlier photo, but otherwise nothing else is revealing. I wish I had capabilities for photographing magnified images of this specimen.

Thanks again for your comments.

[Bob Morgan]

Examine the outer edges where two crystals meet, like the edge to the upper right. Cyclic twins of rutile often have places where one crystal overlaps the other in ways that don't have similar development on the edges of faces of single crystals.
How about the other side opposite the dimpled surface? If that is a contact surface, perhaps the dissolution would be more likely to fill in the center with redeposition.

Whatever, this is quite special. You and Rock have good taste.

[Pete Richards]

Excellent point, Bob!

[Peter Farquhar]

Rutile eightlings are found in several localities worldwide, but they are somewhat rare from Graves Mountain. Below is a photo of an unusual rutile eightling (PF-3645 ) on matrix from Graves Mountain, Georgia. This thumbnail specimen was acquired in a trade, and came originally from Neal Pfaff of "M. Phantom Minerals" many years ago.

After studying hundreds of rutile eightlings, I found they all share one distinguishing feature -- the “zig-zag” pattern of twinning (in German, “Zickzack”). The “zig-zag” pattern occurs ideally when the vertices on the rutile’s perimeter alternately lie in one of two parallel planes. I explain the reasoning and history behind this particular definition in a forthcoming paper that introduces a taxonomy for classifying different types of rutile eightlings.

Meanwhile, it is important to note that rutile eightlings are not defined here by the number of sides. In other words, it is possible for a rutile eightling to have more (or fewer) than eight sides (e.g., with aggregations of intergrown twinned crystals). Thus, it is the zig-zag twinning that defines a rutile eightling, not the number of sides per se.

For other examples of zig-zag twinning, see the photo of the rutile eightling (PF-2965) from Graves Mountain posted earlier in this thread, or the photos of any of the eightling rosettes from Magnet Cove also posted here.