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Identity HelpZircon twin

29th May 2026 21:24 UTCChristof Schäfer Manager OP

Can anyone help me determine the twin law for this zircon?

The crystal shape is defined by the (101), (201), (301), and (100) faces.

The c-axes of the two crystals are inclined toward each other in two directions.

On two adjacent sides of the prisms (100), the (301) faces almost coincide. On the opposite side, the prisms (100) form inward angles of approximately 140° on two adjacent sides.

One of these is shown in ther photo.

Questions Answered
Can it scratch glass? : Yes
Can you scratch it with your fingernail? : No
Is it light/heavy for the size? : Heavy

30th May 2026 05:27 UTCHerwig Pelckmans

this article states that only the {101} plane is known as the twin plane for zircon:

https://www.ovid.com/journals/mitod/abstract/10.1093/mictod/qaad071~microscopic-visualization-of-crystal-twinning-in-accessory?redirectionsource=fulltextview

Is that a plausible candidate? Sorry, don't have the time right now to verify myself.

30th May 2026 07:10 UTCMark Holtkamp

This is a twin on {112}. Note that {112} is {101} in the morphological unit cell with a:c = 0.64

30th May 2026 10:35 UTCChristof Schäfer Manager OP

I think that a twin on {112} faces would produce symmetrical—that is, equiangular— triangular faces at the inwarded angle (the notch). In this crystal, the triangular faces are not symmetrical; the three included angels of the triangel can be roughly estimated at 30°, 60°, and 90°.

Are {hkl} twins, i.e., a ditetragonal bipyramid, known for zircon?

30th May 2026 22:08 UTCMark Holtkamp

Hi Christof, {112} is the only twin plane I know of. I also expected the {112} twin to produce equiangular triangles in the notch, but that is not the case. See the crystal drawing I made of the twin, oriented like the twin in your photograph.

If I rotate the twin on my screen I can see the two {301} faces that are almost parallel. The angle between the triangular {100} faces in the notch, measured on my screen, is 143 degrees. Also the inward angles of the triangles that make up the notch are close to your angles (about 30, 60 and 90 degrees). I'd say this very much looks like a {112} twin.

Mark

31st May 2026 12:11 UTCChristof Schäfer Manager OP

Mark,

That works!
Thanks so much for your explanation and your crystal drawing.
Christof

1st Jun 2026 14:46 UTCHerwig Pelckmans

Christof,

Would you mind upgrading this photo (with the necessary data) so it becomes a part of the database that remains useful (and searchable) to all? Right now, if someone is looking for photos of twinned zircon, your photo will not show up in their Photo Search.

And adding the link to this thread in the caption of your photo would make it possible to quickly find the information in this thread, including the drawing made by Mark ...

1st Jun 2026 15:44 UTCChristof Schäfer Manager OP

that was the plan.

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